Phenotypic Integration of the Cervical Vertebrae in the Hominoidea (Primates)

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Catalina I. Villamil Dickinson College

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Recommended Citation Villamil, Catalina I., "Phenotypic Integration of the Cervical Vertebrae in the Hominoidea (Primates)" (2018). Dickinson College Faculty Publications. Paper 887. https://scholar.dickinson.edu/faculty_publications/887

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Title: Phenotypic integration of the cervical vertebrae in the Hominoidea (Primates)

Short running title: Cervical vertebrae integration in apes

Catalina I. Villamil

Department of Anthropology, Dickinson College, PO Box 1773, Carlisle, PA, 17013

Center for the Study of Human Origins, Department of Anthropology, New York University,

25 Waverly Place, New York, NY 10003, USA

New York Consortium in Evolutionary Primatology, New York, New York, 10024, USA

Corresponding author:

E-mail address: [email protected] (C.I. Villamil)

Author contributions: The author is responsible for all aspects of the work including study design, data collection, data analysis, and manuscript preparation.

Acknowledgements

I would like to thank Susan C. Antón, Terry Harrison, Scott A. Williams, Cliff Jolly, Mark

Grabowski, Emily R. Middleton, Miriam Zelditch, and two anonymous reviewers for helpful

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/evo.13433.

comments and suggestions throughout this process. I would also like to thank Eileen Westwig at the

American Museum of Natural History; Darrin Lunde at the National Museum of Natural History;

William Stanley and Rebecca Banasiak at the Field Museum; Judith Chupasko and Mark Omura at the Harvard Museum for Comparative Zoology; Hellen Opolot at the Uganda National Council for

Science and Technology; Peace, Masili Godfrey, Duncan Kange, Jackson Batte, and Drs. William

Buwembo and Erisa Mwaka at Makerere College; Brendon Billings, Jason Hemingway, and Daigo

Shangase at the University of Witwatersrand; Terry Walschaerts, Karine Maurice, and Drs. Patrick

Semal and Georges Lenglet at the Royal Belgian Institute of Natural Sciences; Emmanuel Gilissen and Wim Wendelen at the Royal Museum of Central Africa,Dr. Inbal Livne at the Powell-Cotton

Museum; Dr. Daniel Antoine at the British Museum; and others for providing or facilitating access to collections in their care. Equipment for this project was provided by the Center for the Study of

Human Origins at New York University. Funding for this project was provided by the Wenner-Gren

Foundation (Gr. 9110), a New York University Global Research Institute Fellowship, and the New

York University Antonina S. Ranieri Fund. Funding for publication was provided by Dickinson

College. The funding bodies had no role in any aspect of the study design, data collection, analysis, or interpretation.

Data archiving

DOI: https://doi.org/10.5061/dryad.4q5k6

Abstract

Phenotypic integration and modularity represent important factors influencing evolutionary change. The mammalian cervical vertebral column is particularly interesting in regards to integration and modularity because it is highly constrained to seven elements, despite widely

variable morphology. Previous research has found a common pattern of integration among quadrupedal mammals, but integration patterns also evolve in response to locomotor selective pressures like those associated with hominin bipedalism. Here, I test patterns of covariation in the cervical vertebrae of three hominoid primates (Hylobates, Pan, Homo) who engage in upright postures and locomotion. Patterns of integration in the hominoid cervical vertebrae correspond generally to those previously found in other mammals, suggesting that integration in this region is highly conserved, even among taxa that engage in novel positional behaviors.

These integration patterns reflect underlying developmental as well as functional modules.

The strong integration between vertebrae suggests that the functional morphology of the cervical vertebral column should be considered as a whole, rather than in individual vertebrae. Taxa that display highly derived morphologies in the cervical vertebrae are likely exploiting these integration patterns, rather than reorganizing them. Future work on vertebrates without cervical vertebral number constraints will further clarify the evolution of integration in this region.

Keywords: cervical vertebrae, phenotypic integration, modularity, covariation, primates

Introduction

Phenotypic integration and modularity are thought to be important factors influencing anatomical evolution. Integration refers to covariation between traits and structures that is due to genetic, developmental, and functional (e.g. joint-joint contacts) relationships between those traits

(Olson and Miller 1958; Cheverud 1982, 1996; Zelditch 1988; Rolian et al. 2009; Rolian 2014). Traits that are highly integrated with one another to the exclusion of other traits form modules (Wagner

1996), which can evolve independently of other sets of traits (Winther 2001, Schlosser 2002, Beldade

and Brakefield 2003). These relationships can be the result of relatively simple pleiotropy or of more complex environmental and developmental interactions (Klingenberg 2004, Rolian 2014). Integration and modularity can facilitate or constrain evolution, as integrated modules may be “funneled” or canalized into particular evolutionary trajectories (Cheverud 1996; Wager 1996, 1997; Gibson and

Wagner 2000; Hansen and Houle 2004; Hallgrimsson et al. 2007; Goswami and Polly 2010; Goswami et al. 2014; Rolian, 2014). Because phenotypic integration may result from developmental interactions, both integration and modularity can further be understood as emergent properties of developmental systems (Hallgrimsson et al. 2007, Rolian 2014). As a result, studies of integration can bridge the gap between evo-devo, which focuses on immediate developmental mechanisms as agents of evolutionary change, and population-level patterns of variation (Reno et al. 2008, Rolian 2014).

Patterns of integration are highly conserved across mammals generally (Marroig and

Cheverud 2001; Ackermann 2002, 2004; Marroig et al. 2009; Porto et al. 2009; Goswami and Polly

2010; Singh et al. 2012; Haber 2015, 2016). In fact, they are probably highly conserved among all species because developmental-genetic systems are themselves highly conserved across vertebrates

(Acampora et al. 1989, Gaunt 1994, Burke et al. 1995, Wellik 2007, Böhmer et al. 2015). However, pattern and magnitude of integration can evolve, which can be an important mechanism for subsequent evolutionary change in morphology (Olsen and Miller 1958, Cheverud 1982, Hansen and

Houle 2004, Goswami and Polly 2010, Marroig et al. 2009, Porto et al. 2009, De Oliveira et al. 2009,

Melo and Marroig 2015). Further, some features of adult integration patterns likely result from the direct mechanical effects of function (Zelditch 1988, 1989; Wagner and Schwenk 2000; Ackermann

2005).

Cervical vertebrae (C1 throughC7) are of particular interest for studies of phenotypic integration and modularity because vertebrae represent a set of repetitive units and because, in mammals, the number of cervical vertebrae is highly conserved (Schultz 1961, Galis 1999, Galis et al.

2006, Buchholtz et al. 2012, Buchholtz 2014). All mammals have seven cervical vertebrae, except

manatees (Trichechus) and sloths (Bradypus, Choloepus) (Schultz 1961). Outside these taxa, variation in number or morphology of the cervical vertebrae is tied to increased frequencies of cancer and death during early life because any variation in the number of cervical vertebrae is likely due to a serious disruption of Hox gene expression (Galis 1999, Galis et al. 2006) and other developmental processes

(Buchholtz 2012). Cervical vertebral abnormalities are particularly frequent in populations on the verge of extinction (e.g. van der Geer and Galis 2017). The developmental underpinnings of cervical vertebral number are also tightly linked to important soft tissue features such as diaphragm placement, which may have led to the evolution of a fixed number of cervical vertebrae (Buchholtz et al. 2012).

Thoracic, lumbar, sacral, and caudal vertebrae are not nearly as conserved in mammals (Schultz 1961,

Buchholtz 2012).

The cervical vertebrae are developmentally and functionally linked with the basicranium and with the thoracic vertebrae in two distinct transitional regions. Unlike other regions of the cranium, the occipital portion of the cranial base is developmentally derived from somites, like the rest of the vertebral column (Hayek 1927, Reiter 1944). Each cervical vertebra is formed from two separate somites—the caudal portion of one somite fuses with the cranial portion of the next somite to form a prevertebra, which then develops into a vertebra (Kessel et al. 1990). In some non-mammalian vertebrates, the most caudal occipital somite and the most cranial vertebral somite form a proatlas

(Hayek 1922). The occipital and C1 through C5 are all patterned to some degree by the expression of

Hoxb-1 and Hoxa-1 (Figure 1A; Kessel and Gruss 1991) and the Hox expression patterns governing the occipital somites are especially highly conserved among tetrapods (Burke et al. 1995). The C2 dens is formed from parts of the C1 somite (Kessel et al. 1990), and the development of C2 further displays a unique pattern of Hox expression that is itself transitional, as C2 shares Hoxb-4 and Hoxa-4 expression with all cervical vertebrae except C1, but does not share Hoxb-5 or Hoxa-5 with vertebrae

C3 through C7 (Figure 1A; Kessel and Gruss 1991). In mice and other model taxa, a C3 through C5 module corresponds to a shared suite of Hox patterning genes (Kessel and Gruss 1991, Buchholtz et

al. 2012) and cervical vertebral morphology can be mapped directly onto the expression of those Hox genes (Johnson and O‟Higgins 1996). The boundary between the cervical and thoracic vertebrae is expressly regulated by Hoxc-6 across tetrapods (Burke et al. 1995).

The Hox genes shaping cervical vertebral morphology and number are further regulated by and interacting with other genes, such as Jmjd3, which regulates Hox expression (Naruse et al. 2017), and the Polycomb genes and ActRIIB and Gdf11, which regulate anterior-posterior and left-right patterning (van der Lugt et al. 1994, Alkema et al. 1995, Yu et al. 1995, Akasaka et al. 1996,

Schumacher et al. 1996, Oh et al. 2002). More specifically within the vertebral column, Pax1 and

Pax9 are directly tied to the development of vertebral bodies but not neural arches (Wallin et al. 1994,

Peters et al. 1999) and Uncx4.1 is tied to the development of pedicles and transverse processes

(Leitges et al. 2000). Other quantitative trait loci are tied to vertebral features such as spinal curvature, vertebral spine length, vertebral body size, and fusion of vertebral elements (Kenney-Hunt et al. 2008,

Negrín-Báez et al. 2015, Howes et al. 2017). Such loci are also tied to the internal bone structure of vertebrae (e.g. Bouxsein et al. 2003) and vertebrae number (e.g. Mikawa et al. 2007) in other regions of the skeleton. However, quantitative expression of Hox genes remains the primary determinant of cervical identity (Kappen 2016), which means that Hox expression patterns can be used to identify potential modules within the cervical vertebral column.

The most well defined Hox-based (gene expression) module is C3 through C5, with substantial overlaps in Hox expression in the region encompassing the cranial base through C5, and distinct gene expression patterns at C1, C2, C6 and C7 (Figure 1B; Kessel and Gruss 1991, Buchholtz et al. 2012), whereas the functional modules are more sharply defined cranial base/C1-C2, C3 through

C5, C6 through T1/2 (Figure 1C; Graf et al. 1995b). More recent work utilizing anatomical network analysis to identify muscular-skeletal units found a similar organization of the neck into a cranial base/C1 module, a C2 through C4/C3 module, and a C4/C5 through C7/thoracic module, with shifting boundaries in some mammalian groups (Arnold et al. 2017b). Three recent studies have shown that

phenotypic integration patterns of the cervical vertebral column are both highly conserved and highly predictable in felids and canids (Arnold et al. 2016, Randau and Goswami 2017a, b), generally in line with the predicted genetic and functional modules (Kessel et al. 1990, Kessel and Gruss 1991,

Buchholtz et al. 2012) as well as the functional modules determined from live animal studies (Vidal et al. 1986; Graf et al. 1995a, b). Randau and Goswami (2017a) report a generalized cervical module with a C6 through T2 transitional zone and no clear distinction of C1-C2. Although they did not include vertebrae C3 and C5, they estimate that there would not be a distinguishable C3 through C5 module (Randau and Goswami, 2017a). They also found that generally vertebrae can be separated into vertebral body and vertebral spine modules, but that this was not the case for the C4 (Randau and

Goswami 2017a, b). Arnold et al. (2016) did not include C1-C2, but found that C6 and C7 appear to form a different module than C3 through C5. They also found that the vertebral spine and vertebral body are highly integrated, so that cervical vertebrae appear to be formed of a single module, rather than multiple modules (Arnold et al. 2016). Randau and Goswami (2017a, b) thus suggest a module that possibly includes C1 through C5, with a C6-C7 transitional zone to the thoracic vertebrae, and ventral and dorsal aspects to each cervical vertebra, while Arnold et al. (2016) suggest a C3-C4-C5 module and a C6-C7 module, with each vertebra composed of a single unit rather than ventral and dorsal aspects. Randau and Goswami (2017a) further suggest that development and function are interacting in shaping apparent integration patterns in the cervical vertebral column.

Despite these findings of similar integration patterns in multiple taxa, we know that cervical vertebrae display a great deal of functional variation across mammals (Walker 1970, Van Sittert et al.

2010, Buchholtz et al. 2012). Canids and felids are representative of some mammal groups, but as primarily quadrupedal mammals, they do not fully encapsulate the diversity of mammal locomotor behaviors, which includes such extremes as swimming, flying, and upright suspension. Recent research has shown that the evolution of integration patterns in the limbs and limb girdles is linked to locomotor behavior in primates, in particular (Rolian 2009, 2010; Grabowski et al. 2011; Grabowski

and Roseman 2015). It is possible that despite the highly conserved nature of cervical vertebral numbers, integration and modularity patterns in the cervical vertebrae may have changed substantially among mammals that engage in non-quadrupedal locomotor behaviors. Primates, and hominoids in particular, include taxa that are primarily upright, and represent a useful comparative group to test the extent to which integration patterns in the cervical vertebrae may evolve in response to divergent selection pressures.

Although there are no clear differences in head and neck carriage at rest among species with different postures, quadrupeds reorient their cervical vertebral column during locomotion while humans, and presumably other upright mammals, do not (Vidal et al. 1986; Graf et al. 1995a, b; Strait and Ross 1999). This will inevitably result in different weight-bearing patterns during locomotion.

Some researchers have further hypothesized that upright posture leads to reduced muscular action on the cranium and cervical vertebrae (Adams and Moore 1975, Demes 1985), which experience different strains and forces in different postures (Pal and Routal 1986, Pal et al. 2001, Nalley and

Grider-Potter 2015). These differences in cervical vertebral function, and the selective pressures which likely act on these differences, present a possible mechanism by which integration patterns could have evolved. Here, I test the hypothesis that hominoids (humans, chimpanzees, and gibbons) have experienced a reorganization of integration and modularity patterns in the cervical vertebrae relative to each other and to other mammals, and that this reorganization is linked to differences in locomotion and posture between them.

Methods

Sample

The sample consists of 120 recent Homo sapiens, 112 Pan troglodytes, and 92 Hylobates lar.

The human sample is equally distributed between recent East Africans, recent Zulu, and medieval

Sudanese. The 112 Pan troglodytes represent the three subspecies of Pan as follows: 59 P. t.

troglodytes, 29 P. t. schweinfurthii, 3 P. t. verus, and 21 of unknown subspecies designation. The 92

Hylobates lar individuals belong to either H. l. lar (90) or H. l. entelloides (2). The distribution of males and females is approximately equal in each taxon.

The three taxa used in this study are descended from ancestors that were adapted to orthograde (upright) postures (Crompton et al. 2008), but have themselves evolved to exploit a wide variety of locomotor and positional „spaces‟. These three taxa represent three main locomotor modes: bipedalism, quadrupedalism, and brachiation. Homo sapiens is characterized by obligate orthograde

(upright) bipedalism with little to no use of the forelimb in locomotion in adulthood. Pan troglodytes is characterized by relatively more pronograde (torso parallel to the substrate) postures and quadrupedal knuckle-walking, and does not differ substantially from baboons in its use of different locomotor behaviors, except for its more frequent use of one-armed hanging and vertical climbing

(Hunt 1992). Although vertical climbing is the second most common form of locomotion in chimpanzees and is substantially more frequent in chimpanzees than in baboons, when chimpanzees vertical climb they do so in a way that is kinematically similar to baboon (Hunt 1992). On the other hand, suspensory behaviors may be more important during infancy and juvenility (Sarringhaus et al.

2014). Hylobates lar is characterized by orthograde forelimb suspension and brachiation, with some vertical climbing, infrequent pronograde behavior, and little or no quadrupedal walking or standing

(Nowak and Reichard 2016).

Measurements

I collected three-dimensional coordinate data on the cranium of each individual using a

Microscribe G digitizer (Solution Technologies, Inc.). Landmarks encompass the morphology of the cranial base (CB) and were used to extract linear data, which were combined with additional caliper data for analyses (Figure 2, Table 1). These encompass the morphology of the cranial base, including the occipital and temporal bones, and include measurements described by Dean and Wood (1981) that were shown to vary among the hominoids, including basioccipital length and aspects of petrous

temporal bone morphology. I also collected linear measurements on each of the seven cervical vertebrae (C1 through C7; Figure 2, Table 1) using a Mitutoyo digital caliper accurate to 0.02mm.

These measurements are adapted from previous studies (Sanders 1995, Nalley 2013, Middleton 2015) and encompass important aspects of vertebral morphology that vary between different taxa. These measurements focus primarily on the vertebral body and neural arch, which serve important load- bearing functions (Pal and Routal 1986, Pal and Sherk 1988), but also include the transverse processes and spinous process. These processes serve as attachment sites for muscles associated with head carriage, posture, and locomotion, including the scalene muscles, the transversospinales muscles, splenius cervicis and capitis, obliquus capitis superior and inferior, rectus capitis anterior and posterior, and trapezius. For analyses of entire elements, the cranial base is represented by 13 measurements, C1 is represented by eight measurements, C2 is represented by 11 measurements, and

C3 through C7 are each represented by nine measurements. The number of measurements per element is the same across the sample species. A subset of these measurements was used for analyses of only the ventral or dorsal portions of the vertebrae. Raw data will be available on PRIMO

(http://primo.nycep.org) one year after publication.

It is important to note that the orientation and angle of the articular facets and of the spinous process (Pal et al. 2001, Nalley 2015) and of the transverse processes (Nalley 2013) were not considered here, and that these components of shape may be relatively independent of other aspects of morphology. These were not included because when angular and linear data are combined for integration analyses, the results are nonsensical, such as estimates of covariation of over 100%

(Middleton, pers. comm.). This is likely because angular data are ratio data rather than continuous variables. I also did not include in my analyses thickness of the spinous process or of the lamina, which appear to have a strong functional signal (Nalley 2013), as many of the specimens I used had soft tissue or breakage obscuring these features, especially in Hylobates. The presence or absence of transverse foramina, which have been suggested to vary with locomotor behavior in primates, were

also not included as they cannot be measured for the purposes of this study (Rios et al. 2014). It is possible that these aspects of cervical vertebral morphology vary independently of the features tested here, but that remains to be tested.

Data analysis

In order to quantify magnitudes of integration, I calculated the mean integration (ī) statistic proposed by Hansen and Houle (2008). The ī statistic simulates the response of a set of traits to selection and quantifies the degree to which evolution of those traits is limited given a specific covariation relationship between them. A high value for ī indicates that traits are highly integrated, and thus unlikely to evolve independently. In addition to ī, I calculated the relative variance of eigenvalues (VE) as a measure of integration (Pavlicev et al. 2009). The VE measures the dispersion of morphological variation among traits. A higher VE indicates that variation is more concentrated among some traits, which means integration is higher. The VE is standardized relative to trait number, and so is comparable across matrices (Pavlicev et al. 2009). I also calculated mean evolvability (ē) and mean conditional evolvability ( ̅) (Hansen and Houle 2008). Mean evolvability quantifies the ability of a set of traits to evolve in response to selection, irrespective of the covariation between those traits. A high value for ē means traits are able to evolve to a large degree, but that evolution may be correlated. Mean conditional evolvability quantifies the ability of a set of traits to evolve in response to selection, when constrained by their covariation relationships. A high value for ̅ means traits are able to evolve to a large degree because constraints are low. Although Hansen and Houle (2008) propose additional statistics to quantify integration and evolvability, these require extremely large sample sizes in order to capture the population mean (Grabowski and Porto 2017) and are not included here.

For all four statistics, data are first adjusted for sex, geographic (Homo), or subspecies (Pan,

Hylobates) related variation using MANOVA. The residuals from this adjustment are then used to construct a variance/covariance (VCV) matrix. Phenotypic VCV matrices generally reflect underlying

genetic covariation matrices (Cheverud 1988). For the VE statistic, the adjusted VCV matrix is used to construct a correlation matrix. The VE statistic is then calculated directly from this correlation matrix as follows, where λi represents an eigenvalue within a correlation matrix, N represents the number of traits, and N-1 represents the maximum eigenvalue variance for a given number of traits:

∑

The adjusted VCV matrix is also used for the calculation of ī, ē, and ̅, for which the VCV matrix is first standardized by the mean of each trait. I then generated a beta (β) vector representing a series of selection pressures. This vector was generated in R 3.3.1 (R Core Team 2016) using the rnorm function to choose a series of random numbers with a mean of zero and a standard deviation of one, following Grabowski et al. (2011). This beta (β) vector is then used in the following equations from Hansen and Houle (2008), where P represents the phenotypic VCV matrix:

Randomized beta vectors are generated 1000 times and each integration statistic generated each time. The mean and standard error are calculated from this distribution (Marroig et al. 2009), which is intended to simulate a variety of possible selective regimes. The ī, ē, and ̅ statistics represent the average response to those regimes.

Correlation matrices estimated from small samples typically reflect the true population value starting at sample sizes of around 40 (Ackermann 2009). However, following Grabowski and Porto

(2017), I estimate that the ī, ē, and ̅ statistics, in combination with the correlation values and trait number used here, require sample sizes of between 80 and 140 in order to capture the true population values. Conditional evolvability, in particular, is estimated to require extremely large sample sizes beyond 140 and to display high levels of inaccuracy. Because such large sample sizes are required, and ī, ē, and ̅ statistics have high margins of error even at large sample sizes (Grabowski and Porto

2017), I calculated 99% confidence intervals for each statistic using a standard deviation estimated from the standard error:

√

I calculated integration variables for the cranial base and each vertebra individually and as one-on-one relationships, so that a C1-C3 comparison includes only vertebrae C1 and C3, and so on. I determined statistical significance of differences between species or elements using the confidence intervals for each variable value, where taxa or elements differed significantly if their confidence intervals did not overlap. The sample sizes used here are generally adequate to capture the true value of ī, ē, and ̅ within this confidence interval (Grabowski and Porto 2017). For analyses of the ventral

(vertebral body) and dorsal (neural arch) portions of the cervical vertebrae, I excluded the cranial base. I assessed ī, VE, ē, and ̅ between the ventral and dorsal portions of a single vertebra (e.g.

integration between the neural arch and vertebral body of C5) and between homologous portions of pairs of vertebrae (e.g. integration between the neural arches of C3 and C4) following Randau and

Goswami‟s (2017a, b) divisions.

In addition, because the number of measurements (traits) for each element (e.g. C1 versus C7) differs, and trait number may play a role in estimates of ī, ē, and ̅ (Grabowski, pers. comm.), I ran a short simulation to determine the general role of trait number in these variables. I used the Homo sapiens C2 data and calculated ī, ē, and ̅ for each possible trait number between two and 20 traits.

The analysis was performed at least five times per trait number, with the specific traits picked randomly at each iteration. For mean integration, the adjusted R2 for trait number was 0.8102 (p <

0.001), with a higher number of traits associated with higher integration. However, I also performed a regression of mean integration to sample size and reanalyzed the residuals of this regression relative to trait number, in which case the adjusted R2 decreased to 0.0698 (p = 0.003). This suggests that, in this simulation, trait number is linked to mean integration only because it reduces the sample size.

There was no substantial role of trait number in evolvability (adjusted R2=0.0070, p=0.648). For conditional evolvability, the adjusted R2 for trait number was 0.554 (p < 0.001), with a higher number of traits associated with lower conditional evolvability. I performed the same procedure as for mean integration, performing a regression of mean conditional evolvability to sample size and reanalyzing these residuals relative to trait number. In this case, the adjusted R2 decreased to 0.0263 (p = 0.047).

Because of these weak relationships between trait number, mean integration, mean evolvability, and mean conditional evolvability, and the general similarities in sample size across taxa, I am confident that the results presented here, which include large confidence intervals, can be generally interpreted in a straightforward manner.

Results

Cranial base and the cervical vertebrae

In general, the cranial base is more strongly integrated within itself than it is with any single vertebra (Tables 2-3, Figures 3-4). The mean integration (ī) statistic suggests that vertebrae C2 (the axis) and C7 are the most weakly integrated elements of the cervical vertebral column, whereas C1 is the most strongly integrated, regardless of the trait set used (Tables 2, Figure 3). These results generally hold true using the VE statistic, where C1 is strongly integrated in Hylobates and Homo, and C2 and C7 are generally more weakly integrated than the other vertebrae (Table 3, Figure 4). In all three taxa, the cranial base (CB) is highly integrated with the seven cervical vertebrae and with C1 in particular, but this pattern is clearer using the mean integration statistic (Table 2, Figure 3). The relationships between C3-C4, C3-C5, and C4-C5 are consistently the strongest across taxa and across measures of integration (Tables 2-3, Figures 3-4). Across taxa, C3 generally displays decreasing integration (ī) and VE with C5, C6, and C7. Both C4 and C5 follow similar patterns for non-adjacent, more caudal vertebrae. When ī is compared between taxa, values are generally similar at each level and are not statistically distinguishable. When VE is compared between taxa, Hylobates tends to have significantly lower integration than both Homo and Pan for vertebrae C2 through C3 (Table 3). VE is also significantly higher in Pan than in either Homo or Hylobates at CB-C5, CB-C7, C1-C5, C4-C5,

C4-C7, C5-C6, and C5-C7.

In all three taxa, evolvability (ē) is highest at C1 and in the C1 relationships to all other vertebrae, and lowest at C2 and in the C2 relationships to all other vertebrae (Table 4, Figure 5). The ē of C7 is also relatively low compared to other individual vertebrae. The differences in ē between C1 and C2 are significant between all taxa, but the differences between C1 and C7 are significant in

Homo but not in the other taxa (Table 4). Other differences are not significant. The ē of CB-C1 relationship is also relatively high compared to other cranial base relationships. Evolvability is not statistically distinguishable between taxa at any level.

For all three taxa, the conditional evolvability ( ̅) is highest for the individual vertebrae and lower for intervertebral relationships (Table 5, Figure 6). The ̅ is particularly low for C3-C4, C4-C5,

and C5-C6. These differences are significant only for some comparisons and are not consistently significant in any of the taxa. In all three taxa, ̅ is consistently lower in the cranial base than in any of the vertebrae, and C1, C2, C3, C6, and C7 tend to have higher ̅ than C4 or C5 (Figure 6). However, margins of error and confidence ranges are large for ̅, and there are not consistently any significant differences between the cranial base and any of the vertebrae in the three taxa.

Ventral (vertebral body) and dorsal (neural arch) portions of the cervical vertebrae

Magnitudes of ī are remarkably similar across taxa and the intervertebral relationships are significantly stronger for the dorsal aspects of the vertebrae (Table 6, Figure 7). In all three taxa, C2 and especially C7 tend to have the lowest ī between ventral and dorsal elements. With VE, Hylobates displays significantly higher VE in the ventral C1-C2 relationship than either Pan or Homo, and

Homo and Pan tend to have significantly higher VE in the ventral intervertebral relationships of C3,

C4, C5, and C6 (Table 7, Figure 8). In the intervertebral relationships of the dorsal aspect of each vertebra, Pan tends to have higher VE. In all three taxa, in both ī and VE, C1 and C2 tend to have the weakest relationships with other vertebrae. In all three taxa, C3-C4, C3-C5, and C4-C5 consistently display high ī and high VE in the ventral aspect, as do adjacent vertebrae C5-C6 and C6-C7. In

Hylobates and Pan, C4-C5 and C5-C6 consistently display high ī and high VE in the dorsal aspect, but this relationship is weaker in Homo.

C1 consistently displays the highest ē in all its relationships, although these differences are not generally significant (Table 8, Figure 9). However, ē is consistently higher in all intervertebral relationships in the dorsal aspect of the vertebrae. Intervertebral relationships follow a similar patter for ̅, with C1 displaying the highest ̅ in all its relationships, both among ventral and dorsal aspects

(Table 9, Figure 10). The strength of ̅ is similar in the ventral and dorsal aspects of the vertebrae, and so is its pattern across vertebrae. Differences between vertebrae are generally not significant, but the ventral aspects of C3-C4, C3-C5, and C4-C5 may display particularly low ̅ (Figure 10).

Discussion

Humans, chimpanzees, and gibbons do not differ substantially in patterns or magnitudes of integration, either from each other or from other mammals. Instead, the modules identified here follow the developmental and functional modules that distinguish cranial, middle, and caudal regions of the cervical vertebrae (Vidal et al. 1986; Kessel et al. 1990; Kessel and Gruss 1991; Graf et al.

1995a, b; Buchholtz et al. 2012). These modules are generally consistent with those found by Arnold et al. (2016) and Randau and Goswami (2017a, b) for quadrupedal felids and canids, indicating that the hominoids follow the general mammalian pattern of cervical vertebral integration despite more frequent or exclusive use of upright posture and locomotion. Differences between the three hominoid taxa are rarely significant, and not replicated across different measures of integration. Together, these results indicate that there is no shift in patterns or magnitudes of integration among the cranial base and cervical vertebrae associated with the upright posture or locomotion.

Patterns of integration in the hominoid head and neck

I found the following general modular pattern: CB-C1, C2, C3-C4-C5, C6-C7. Like Arnold et al. (2016) and Randau and Goswami (2017a), I found strong support for a middle cervical module consisting of C3-C4-C5, and corresponding to the exclusively shared expression of Hoxb5 and Hoxa5

(Kessel and Gruss 1991). Based on VE, Hylobates and Pan may display a more extended module containing C3 through C6, but this pattern is not statistically significant in Hylobates, and may be driven by particularly high integration in C5 in Pan (Table 3, Figure 4). The C3-C4, C4-C5, and C5-

C6 relationships also show particularly low conditional evolvability compared to other relationships between adjacent vertebrae (Figure 6), suggesting that this middle module of the cervical region is subject to relatively greater constraints than other modules. Although Randau and Goswami (2017a) suggest that C1-C2 are not clearly differentiated from C3-C5, my results disagree with these conclusions. Overall, my results suggest that a C3-C4-C5 module is a highly conserved mammalian pattern.

In all taxa, C6 appears transitional between C5 and C7, and it is relatively strongly integrated with both these vertebrae individually but not with the C3-C4-C5 module generally. This may be a result of the mechanical interactions between C5 and C6, which are adjacent to each other, or the intermediate Hox expression patterns displayed by C6 (Figure 1; Kessel and Gruss 1991). Notably, despite its integration to C5, the morphology of the C6 vertebra is highly distinctive in primates, with extended, bifurcated transverse processes. The C6 vertebra also displays morphological peculiarities in other taxa (Arnold et al. 2016). These findings, in which C6 serves as a transitional vertebra, are consistent with Randau and Goswami‟s (2017a) findings of a module encompassing the caudal cervical vertebrae and the first several thoracic vertebrae.

The cranial base is strongly integrated with the cervical vertebrae in general and with C1 and

C3, C4, and C5 in particular, although only when using the mean integration statistic (Figures 3-4).

These cranial base-vertebrae relationships have only moderate evolvability and low conditional evolvability relative to other inter-element relationships, further indicating that these relationships are fairly constrained (Figures 5-6). This integration relationship of the cranial base to the cervical vertebrae also corresponds to the Hox expression patterns described by Kessel and Gruss (1991;

Figure 1), but this integration pattern may also be linked to the mechanical interactions between the skeletal elements of the region and the spinal cord.

Despite its link to the cranial base, C1 is consistently the most evolvable vertebrae, and has high evolvability and conditional evolvability, despite being highly integrated with other cervical vertebrae. This may reflect its highly specialized function as a transition between the cranial base and the other cervical vertebrae. Further, C1 is the only vertebra that retains a ventral arch, rather than a vertebral body (Kessel et al. 1990), which may free it from the functional requirements of weight bearing and articulation that affect other cervical vertebrae.

When cervical vertebral ventral and dorsal regions are considered separately, the neural arches appear to form a C2-C3 module and a C4-C5-C6 module. This neural arch pattern may

represent a boundary shift from the modules represented by the whole vertebrae, where a distinct C2 vertebra is now shifted caudally into a C2-C3 region, and a C3-C4-C5 module is now shifted caudally into a C4-C5-C6 region. This provides further support for identifying C6 as a transitional vertebra between the typical cervical vertebrae and a C7-thoracic region. Further, the vertebral bodies may display a somewhat weaker modular organization, as adjacent vertebrae all display relatively strong integration with each other. This is consistent with the generally lower evolvability of the vertebral bodies compared to the neural arches, suggesting the vertebral bodies are constrained by their larger weight-bearing function. The neural arches also have greater potential to evolve than the vertebral bodies or the cervical vertebrae as a whole, but the conditional evolvability of the neural arches and vertebral bodies is similar (Tables 8-9, Figures 9-10). Together, this suggests that the neural arches are likely evolving to a greater degree than the vertebral bodies, but they are evolving together in similar ways, rather than as independent vertebrae. These results are in agreement with Randau and

Goswami‟s (2017a,b) finding of separation between ventral and dorsal regions of each cervical vertebra, but contradict those of Arnold et al. (2016).

Interestingly, C2 and C7 consistently display the lowest magnitudes of integration in all three taxa (Table 2, Figure 3), as well as low evolvability (Tables 4, 7; Figure 5) but average or high conditional evolvability (Table 5, Figure 6). Unlike Randau and Goswami (2017a) I find C2 to be particularly distinct from other vertebrae. Arnold et al. (2017a) also found that, in mammals, C2 and

C7 follow similar allometric trajectories, which differ from those of the rest of the cervical vertebrae.

Both C2 and C7 are transitional vertebrae, with C2 serving as a transition between the C1 and the typical cervical vertebrae, and C7 serving as a transition between the cervical column and the thoracic vertebrae. The discrepancies in the apparent modules between the whole vertebrae (C2, C3-C4-C5,

C6-C7) and the neural arches (C2-C3, C4-C5-C6, C7) further highlight the transitional nature of C2 and C7, which belong to different modules when their ventral and dorsal elements are considered separately. In humans C2 and C7 experience different forms of force transmission than C3 through

C6, as compression forces diffuse through the laminae in both C2 and C7 (Pal and Routal 1996) and this differential force transmission may represent a mammal-wide pattern during conserved neck postures (Vidal et al. 1986; Graf et al. 1995a,b). Hox expression patterns in the vertebrate neck typically precede the evolution of the mammalian cervical pattern (Gaunt 1994, Buchholtz 2014,

Böhmer et al. 2015), and it is possible that the developmentally and morphologically transitional nature of C2 and C7 resulted in these specific patterns of differential force transmission. Reduced magnitudes of integration in these two vertebrae may reflect or facilitate their special transitional functions .

Evolutionary implications

My results suggest that unlike other regions of the skeleton (e.g. Rolian 2009, Rolian et al.

2010, Grabowski et al. 2011), the cervical vertebrae are constrained in their patterns of integration even in the face of fairly significant shifts in functional pressures. In this sense, phenotypic integration is in agreement with genetic and developmental studies that have found that variation in cervical vertebral number reflects fundamental developmental and metabolic abnormalities that result in early death in most mammals (Galis 1999; Galis et al. 2006; Buchholtz et al. 2012; Buchholtz 2012, 2014).

It is possible that morphologically extreme taxa, like giraffes (Badlangana et al. 2009, Gunji and Endo

2016) or cetaceans (Buchholtz 2007), achieve some kind of change in patterns and magnitudes of integration along with subtle shifts in the position of transitional segments (Van Sittert et al. 2010,

Gunji and Endo 2016). However, Arnold et al. (2017a), in a study of allometry, did not find evidence of these changes. These taxa may instead have taken advantage of the different integration patterns present in the neural arches and vertebral bodies to shift the functional boundaries of the cervical vertebral region without significantly altering the underlying developmental relationships. Sloths, which display eight or nine cervical vertebrae, contrary to most mammals, may also display novel patterns and magnitudes of integration despite retaining typical mammalian Hox expression patterns in the cervical region (Arnold et al. 2017a).

My results further suggest a way in which the study of phenotypic integration in adults can be used to estimate developmental patterning in taxa, such as humans, for which it is impossible to perform the kind of knockout experiments commonly performed in mice (e.g. Kessel and Gruss

1991). Cheverud (1988) similarly concluded that phenotypic correlation patterns generally reflect genetic correlation matrices, but there are persistent concerns that the study of phenotypic integration captures only functional covariation and not the underlying genetic and developmental underpinnings of morphology (Zelditch 1987). The results from this study show that, at least for some systems, morphology based estimates of patterns of integration adequately represent true genetic and developmental pleiotropy.

Implications for the hominin fossil record

The highly integrated nature of the cervical vertebrae suggests that the most fruitful means of investigating functional aspects of cervical morphology in mammals is by investigating patterns of morphological change across the cervical column, rather than by identifying specific morphological features on individual vertebrae. Randau et al. (2016) reached a similar conclusion in a study of felid vertebral morphology. This is particularly true of C3 through C7, as C1 and C2 may be evolving relatively independently, and C1 especially may be evolving to a greater degree than the rest of the cervical vertebrae. Generally, the neural arches are also capable of evolving morphologically to a greater degree than the vertebral bodies, and the two regions are likely following slightly different evolutionary trajectories. As a result, the morphology of the neural arches may provide a greater functional or phylogenetic signal than the cervical vertebral bodies. This is consistent with cursory observations of cervical vertebral morphology, in which the morphology of cervical vertebral bodies is generally conserved.

The lack of locomotion- or posture-related variation in patterns of integration and evolvability in the cervical vertebral column and the strong integration between the cranial base and the cervical vertebrae suggest that evolution in cervical vertebral morphology may be driven by evolution of the

cranium. Other evidence, such as the similarity of head and neck postures in many mammals (Vidal et al. 1986; Graf et al. 1995a, b), and the general dearth of traits linked to locomotion and posture in the cervical vertebral region (Nalley 2013; Nalley and Grider-Potter 2015; Grider-Potter et al. 2016; but see Ankel 1970, 1972; Fenton and Crerar 1984; Manfreda et al. 2006) relative to other skeletal regions (e.g. Shapiro 1991, 1993; Grider-Potter et al. 2016) also suggests that locomotion and posture are relatively weak selection pressures in the neck of most mammals and particularly in primates.

Instead, it seems possible that many aspects of cervical vertebral morphology are being driven by the demands of craniofacial morphology, whether these are related to head posture or other functions of the craniofacial skeleton (e.g. Antón and Galobart 1999, Salesa et al. 2005). As shown here, the cervical vertebrae are highly integrated with the basicranium (see also Mizoguchi 1995) and the basicranium in turn is highly integrated with the facial skeleton (e.g. Lieberman et al. 2000, Gkantidis and Halazonetis 2011, Bastir and Rosas 2016, Villamil 2017). Variation in the size and position of the mandible has also been linked to changes to the relationships between the cranium and cervical vertebrae (Urbanowicz 1991, Yoo and An 2009). Together, these results suggest that evolutionary changes in the cranium are driving many of the morphological changes we observed in the cervical vertebral columns.

Other researchers (Gómez-Olivencia et al. 2013, Meyer 2016, Meyer et al. 2017) have reached similar conclusions based on fossil evidence. Many features of fossil hominin cervical morphology remain primitive until fairly late in hominin evolution, despite the early adoption of bipedalism (Nalley 2013, Gommery 2006; Gómez-Olivencia et al. 2007, 2013). For example, many fossil hominins that are fully bipedal, such as KNM-WT 15000r (Homo erectus) retain relatively long vertebral spines and short transverse processes relative to modern humans (Nalley 2013). Another early member of genus Homo, KNM-ER 164c (Homo sp.), retains relatively thick laminar cross- sectional areas in C7 when compared to modern humans (Nalley 2013). Neanderthals display vertebral spines that are more horizontally oriented than in modern humans (Gómez-Olivencia et al.

2013). These peculiarities can be explained if cervical vertebral morphology is being driven by craniofacial morphology, which changes substantially in Homo sapiens relative to earlier hominins, rather than locomotion or posture per sé (Gómez-Olivencia et al. 2013, Meyer et al. 2017).

Conclusion

In summary, patterns of phenotypic integration in the head and neck in hominoids are strongly developmentally constrained by Hox expression patterns despite a radical shift in posture and locomotion in these taxa. These results, in conjunction with previous studies, suggest a mammal-wide pattern of integration and modularity in the cervical vertebral column.

References

Acampora D, D‟Esposito M, Faiella A, Pannese M, Migliaccio E, Morelli F, Stornaiuolo A, Nigro V,

Simeone A, Boncinelli E. The human HOX gene family. Nucleic Acids Res. 1989;17(24):10385-

10402.

Ackermann RR, Cheverud JM. Detecting genetic drift versus selection in human evolution. Proc Natl

Acad Sci. 2004;101(52):17946-17951.

Ackermann RR. Ontogenetic integration of the hominoid face. J Hum Evol. 2005;48: 175-197.

Ackermann RR. Patterns of covariation in the hominoid craniofacial skeleton, implications for paleoanthropological models. J Hum Evol. 2002; 43:167-187.

Ackermann RR. Morphological integration and the interpretation of fossil hominin diversity. Evol

Biol 2009; 36(1):149-156.

Adams LM, Moore WJ. Biomechanical appraisal of some skeletal features associated with balance and posture in the Hominoidea. Acta Anat. 1975;92:580-594.

Akasaka T, Kanno M, Balling R, Mieza MA, Taniguchi M, Koseki H. 1996. A role for mel-18, a

Polycomb group-related vertebrate gene, during the anteroposterior specification of the axial skeleton.

Development 1996;122(5):1513-1522.

Alkema MJ, van der Lugt NMT, Bobeldijk RC, Berns C, van Lohuizen M. Transformation of axial skeleton due to overexpression of bmi-1 in transgenic mice. Nature 1995;374-724-727.

Ankel F. Einführung in die Primatenkunde (6). Stuttgart: Gustav Fischer Verlag; 1970.

Ankel F. Vertebral morphology of fossil and extant primates. In: Tuttle R, editor. The functional and evolutionary biology of primates, part IV: postcranial morphology (pp. 223-240). Chicago: Aldine-

Atherton; 1972.

Antón M, Galobart A. Neck function and predatory behavior in the scimitar toothed cat Homotherium latidens (Owen). J Vert Paleontol. 1999;4:771-784.

Arnold P, Amson E, Fischer MS. Differential scaling patterns of vertebrae and the evolution of neck length in mammals. Evolution. 2017a;71:1587-1599.

Arnold P, Esteve-Altava B, Fischer MS. Musculoskeletal networks reveal topological disparity in mammalianneck evolution. BMC Evol Biol 2017b;17:251.

Arnold P, Forterre F, Lang J, Fischer MS. Morphological disparity, conservatism, and integration in the canine lower cervical spine: insights into mammalian neck function and regionalization. Mamm

Biol. 2016;81:153-162.

Badlangana NL, Adams JW, Manger PR. The giraffe (Giraffa camelopardalis) cervical vertebral column: a heuristic example in understanding evolutionary processes? Zool J Linn Soc-Lond.

2009;155:736-757.

Bastir M, Rosas A. Cranial base topology and basic trends in the facial evolution of Homo. J Hum

Evol. 2016;91:26-35.

Beldade P, Brakefield PM. The difficulty of agreeing about constraints. Evol Dev. 2003;5(2):119-120.

Böhmer C, Rauhut OWM, Wörheide G. Correlation between Hox code and vertebral morphology in archosaurs. Proc R Soc B. 2015;282:20150077.

Bouxsein ML, Uchiyama T, Rosen CJ, Shultz KJ, Donahue LR, Turner CH, Sen S, Churchill GA,

Müller R, Beamer WG. Mapping quantitative trait loci for vertebral trabecular bone volume fraction and microarchitecture in mice. J Bone Miner Res 2004;19:587-599.

Buchholtz EA, Bailin HG, Laves SA, Yang JT, Chan MY, Drozd LE. Fixed cervical count and the origin of the mammalian diaphragm. Evol Dev. 2012;14(5):399-411.

Buchholtz EA, Stepien CC. Anatomical transformation in mammals: developmental origin of cervical anatomy in tree sloths. Evol Dev. 2009;11(1):69-79.

Buchholtz EA. Crossing the frontier: a hypothesis for the origins of meristic constraints in mammalian axial patterning. Zoology. 2014;117:64-69.

Buchholtz EA. Flexibility and constraint: patterning the axial skeleton in mammals. In: Asher RJ,

Müller J, editors, From clone to bone: the synergy of morphological and molecular tools in palaeobiology. Cambridge: Cambridge University Press; 2012. p. 230-253.

Buchholtz EA. Modular evolution of the Cetacean vertebral column. Evol Dev. 2007;9(3):278-289.

Burke AC, Nelson CE, Morgan BA, Tabin C. Hox genes and the evolution of vertebrate axial morphology. Development. 1995;121: 333-346.

Burke AC, Nelson CE, Morgan BA, Tabin C. Hox genes and the evolution of vertebrate axial morphology. Development. 1995;121:333-346.

Cheverud JM. A comparison of genetic and phenotypic correlations. Evolution. 1988 42(5):958-968.

Cheverud JM. Developmental integration and the evolution of pleiotropy. Amer Zool. 1996; 36:44-

50.

Cheverud JM. Phenotypic, genetic, and environmental morphological integration in the cranium.

Evolution. 1982; 36(3):499-516.

Crompton RH, Vereecke EE, Thorpe SKS. Locomotion and posture from the common hominoid ancestor to fully modern hominins, with special reference to the last common panin/hominin ancestor.

J Anat. 2008;212:501-543.

De Oliveira FB, Porto A, Marroig G. Covariance structure in the skull of Catarrhini, a case of pattern stasis and magnitude evolution. J Hum Evol. 2009; 56:417-430.

Dean MC, Wood BA. Metrical analysis of the basicranium of extant hominoids and Australopithecus.

Am J Phys Anthropol. 1981;54:63-71.

Demes B. Biomechanics of the primate skull base. Advances in anatomy: embryology and cell biology 94. New York: Springer-Verlag; 1985.

Fenton MB, Crerar LM. Cervical vertebrae in relation to roosting posture in bats. J Mamm.

1984;65(30:395-403.

Galis F, Van Dooren TJM, Feuth JD, Metz JAT, Witkam A, Ruinard S, Steigenga MJ, Wijnaendts

LCD. Extreme selection in humans against homeotic transformations of cervical vertebrae. Evolution.

2006;60(12):2643-2654.

Galis F. Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and cancer. J Exp Zool (Mol Dev Evol). 1999;285:19-26.

Gaunt SJ. Conservation in the Hox code during morphological evolution. Int J Dev Biol.

1994;38:549-552.

Gibson G, Wagner G. Canalization in evolutionary genetics: a stabilizing theory? BioEssays.

2000;22:372-380.

Gkantidis N, Halazonetis DJ. Morphological integration between the cranial base and the face in children and adults. J Anat. 2011;218:426-438.

Gómez-Olivencia A, Been E, Arsuaga JL, Stock JT. The Neandertal vertebral column 1: the cervical spine. J Hum Evol. 2013;64:608-630.

Gómez-Olivencia A, Carretero JM, Arsuaga JL, Rodríguez-García L, García-González, R, Martínez I.

Metric and morphological study of the upper cervical spine from the Sima de los Huesos site (Sierra de Atapuerca, Burgos, Spain). J Hum Evol. 2007;53:6-25.

Gommery D. Evolution of the vertebral column in Miocene hominoids and Plio-Pleistocene hominids.

In: Ishida H, Tuttle R, Pickford M, Ogihara N, Nakatsukasa M, editors. Human origins and environmental backgrounds. New York: Springer; 2006. p. 31-43.

Goswami A, Polly PD. The influence of modularity on cranial morphological disparity in Carnivora and Primates (Mammalia). PLOS. 2010;5(3):e9517.

Goswami A, Smaers JB, Soligo C, Polly PD. The macroevolutionary consequences of phenotypic integration: from development to deep time. Phil Trans R Soc. 2014;369:20130254.

Grabowski M, Porto A. How many more? Sample size determination in studies of morphological integration and evolvability. Methods Ecol Evol. 2017;8(5):592-603.

Grabowski MW, Polk JD, Roseman CC. Divergent patterns of integration and reduced constraint on the human hip and the origins of bipedalism. Evolution. 2011:65(5);1336-1356.

Grabowski MW, Roseman CC. Complex and changing patterns of natural selection explain evolution of the human hip. J Hum Evol. 2015;85:94-110.

Graf W, de Waele C, Vidal PP. Functional anatomy of the head-neck movement system in quadrupedal and bipedal mammals. J Anat. 1995a;186:55-74.

Graf W, de Waele C, Vidal, PP, Wang DH, Evinger C. The orientation of the cervical vertebral column in unrestrained awake animals. Brain Behav Evol. 1995b; 45:209-231.

Grider-Potter N, Goto R, Nakano Y. The effects of posture on neck and trunk musculature in

Hylobates lar and Macaca fuscata. Am J Phys Anthropol. 2016;S159:161.

Gunji M, Endo H. Functional cervicothoracic boundary modified by anatomical shifts in the neck of giraffes. R Soc Oen Sci. 2016;3:150604.

Haber A. Phenotypic integration and morphological diversification in the ruminant skull. Am

Naturalist. 2016;187(5):576-591.

Haber A. The evolution of morphological integration in the ruminant skull. Evol Biol. 2015;42(1):99-

114.

Hallgrimsson B, Lieberman DE, Young NM, Parsons T, Wat S. Evolution of covariance in the mammalian skull. In Novartis Foundation, Tinkering: the microevolution of development. Chichester:

John Wiley & Sons, Ltd.; 2007. p. 164-190.

Hansen TF, Houle D. 2004. Evolvability, stabilizing selection, and the problem of stasis. In: Pigliucci

M, Preston K, editors. Phenotypic integration: studying the ecology and evolution of complex phenotypes. New York: Oxford University Press; 2004. p. 130-153.

Hansen TF, Houle D. Measuring and comparing evolvability and constraint in multivariate characters.

J Evol Biol. 2008;21(5):1201-1219.

Hayek H. Über den Proatlas und über die Entwicklung der Kopfgelenke beim Menschen und bei einigen Säugetieren. Sitzungsberichte d Akad d Wiss. 1922;31:25-60.

Hayek H. Untersuchungen über Epistropheus, Atlas und Hinterhauptsbein. Jährbuch für Morphologie und mikroskopische Anatomie. 1927;58:269-347.

Howes TR, Summers BR, Kingsley DM. Dorsal spine evolution in threespine sticklebacks via a splicing change in MSX2A. BMC Biology 2017;15:115.

Hunt KD. Positional behavior of Pan troglodytes in the Mahale Mountains and Gombe Stream

National Parks, Tanzania. Am J Phys Anthropol. 1992;87:83-105.

Johnson DR, O‟Higgins DR. Is there a link between changes in the vertebral “hoxcode” and the shape of vertebrae? A quantitative study of shape change in the cervical vertebral column of mice. J Theor

Biol. 1996;183(1):89-93.

Kappen C. Developmental patterning as a quantitative trait: genetic modulation of the Hoxb6 mutant skeletal phenotype. PLOS ONE 2016;11(1):e0146019.

Kenney-Hunt JP, Wang B, Norgard EA, Fawcett G, Falk D, Pletscher LS, Jarvis JP, Roseman C,

Wolf J, Cheverud JM. Pleiotropic patterns of quantitative trait loci for 70 murine skeletal traits.

Genetics 2008;178:2275-2288.

Kessel M, Gruss P. 1991. Homeotic transformations of murine vertebrae and concomitant alteration of Hox codes induced by retinoic acid. Cell , 67, 89-104.

Kessel M, Balling R, Gruss P. 1990. Variations of cervical vertebrae after expression of a Hox-1.1 transgene in mice. Cell 61:301-308.

Klingenberg CP. Integration, modules, and development: molecules to morphology to evolution. In:

Pigliucci M, Preston K, editors. Phenotypic integration: studying the ecology and evolution of complex phenotypes. New York: Oxford University Press; 2004. p. 213-230.

Leitges M, Neidhardt L, Haenig B, Hermann BG, Kispert A. The paired homeobox gene Uncx4.1 specifies pedicles, transverse processes and proximal ribs of the vertebral column. Development 2000;

127:2259-2267.

Lieberman DE, Pearson OM, Mowbray KM. Basicranial influence on overall cranial shape. J Hum

Evol. 2000;38:291-315.

Lovejoy CO. The natural history of human gait and posture. Part 1. Spine and pelvis. Gait Posture.

2005;21:95-112.

Manfreda E, Mitteroecker P, Bookstein FL, Schaefer K. The functional morphology of the first cervical vertebra in humans and nonhuman primates. Anat Rec (Part B: New Anat) . 2006;289B:184-

194.

Marroig G, Cheverud JM. A comparison of phenotypic variation and covariation patterns and the role of phylogeny, ecology, and ontogeny during cranial evolution of New World monkeys. Evolution.

2001; 55(12):2576-2600.

Marroig G, Shirai LT, Porto A, de Oliveira FB, de Conto V. The evolution of modularity in the mammalian skull II: evolutionary consequences. Evol Biol. 2009;36:136-148.

Melo D, Marroig G. Directional selection can drive the evolution of modularity in complex traits.

Proc Natl Acad Sci. 2015;112(2):470-475.

Meyer MR, Williams SA, Schmid PA, Churchill SE, Berger LR. The cervical spine of

Australopithecus sediba. J Hum Evol. 2017;104:32-49

Meyer MR. Ch. 5 The cervical vertebrae of KSD-VP-1/1. In: Haile-Selassie Y, Su DF, editors. The postcranial anatomy of Australopithecus afarensis: new insights from KSD-VP-1/1. Dordrecht:

Springer; 2016. p. 63-111.

Middleton ER. Ecogeographical influences on trunk modularity in recent humans. Doctoral dissertation, New York University; 2015.

Mikawa S, Morozumi T, Shimanuki SI, Hayashi T, Uenishi H, Domukai M, Okumura N, Awata T.

Fine mapping of a swine quantitative trait locus for number of vertebrae an analysis of an orphan nuclear receptor, germ cell nuclear factor (NR6A1). Genome Res 2007;17:586-593.

Mizoguchi Y. Structural covariation between the neurocranium and the cervical vertebrae: towards the solution of the brachycephalization problem. B Natl Acad Sci Tokyo D. 1995;21:11-35.

Nalley TK, Grider-Potter N. Functional morphology of the primate head and neck. Am J Phys

Anthropol. 2015;156(4):531-542.

Nalley TK. Positional behaviors and the neck: a comparative analysis of the cervical vertebrae of living primates and fossil hominoids. Doctoral dissertation, Arizona State University; 2013.

Naruse C, Shibata S, Tamura M, Kawaguchi T, Abe K, Sugihara K, Kato T, Nishiuchi T, Wakana S,

Ikawa M, Asano M. New insights into the role of Jmjd3 and Utx in axial skeletal formation in mice.

The FASEB Journal 2017; 31(6):2252-2266.

Negrín-Báez D, Navarro A, Afonso JM, Ginés R, Zamorano MJ. Detection of QTL associated with three skeletal deformities in gilthead seabream (Sparus aurata L.): Lordosis, vertebral fusion and jaw abnormality. Aquaculture 2015;448:123-127.

Nowak MG, Reichard UH. The torso-orthograde positional behavior of wild white-handed gibbons

(Hylobates lar). In Reichard UH, Hirai H, Barelli C, editors. Evolution of gibbons and siamangs. New

York: Springer; 2016. p. 205-227.

Oh SP, Yeo CY, Lee Y, Schrewe H, Whitman M, Li E. Activin type IIA and IIB receptors mediate

Gdf11 signaling in axial vertebral patterning. Genes & Dev 2002;16:2749-2754.

Olson EC, Miller RL. Morphological integration. Chicago: University of Chicago Press; 1958.

Pal GP, Routal RV, Saggu SK. The orientation of the articular facets of the zygapophyseal joints at the cervical and upper thoracic region. J Anat. 2001;198(4):431-441.

Pal GP, Routal RV. A study of weight transmission through the cervical and upper thoracic regions of the vertebral column in man. J Anat. 1986;148:245-261.

Pal GP, Routal RV. The role of the vertebral laminae in the stability of the cervical spine. J Anat.

1996;188:485-489.

Pal GP, Sherk HH. The vertical stability of the cervical spine. Spine. 1988;13(5):447-449.

Pavlicev M, Cheverud JM, Wagner GP. 2009. Measuring morphological integration using eigenvalue variance. Evol Biol 2009;36:157-170.

Peters H, Wilm B, Sakai N, Imai K, Maas R, Balling R. Pax1 and Pax9 synergistically regulate vertebral column development. Development 1999;126:5399-5408.

Porto A, de Oliveira FB, Shirai LT, de Conto V, Marroig G. The evolution of modularity in the mammalian skull I: morphological integration patterns and magnitudes. Evol Biol. 2009;36:118-135.

R Core Team. R: A language and environment for statistical computing. R Foundation for Statistical

Computing, Vienna, Austria. 2016. http://www.R-project.org/

Randau M, Cuff R, Hutchinson JR, Pierce SE, Goswami A. Regional differentiation of felid vertebral column evolution: a study of 3D shape trajectories. Org Divers Evol 2016;17(1):305-319.

Randau M, Goswami A. Morphological modularity in the vertebral column of Felidae (Mammalia,

Carnivora). BMC Evol Biol. 2017a;17:133.

Randau M, Goswami A. Unravelling intravertebral integration and modularity, disparity in Felidae

(Mammalia). Evol Dev. 2017b;19(2):85-95.

Reiter A. Die Frühentwicklung der menschlichen Wirbelsäule II. Die Entwicklung der

Occipitalsegmente und der Halswirbelsäule. Z Anat Entwicklungsgeschichte. 1944;113:66-104.

Reno PL, McCollum MA, Cohn MJ, Meindl RS, Hamrick M, Lovejoy CO. Patterns of correlation and covariation of anthropoid distal forelimb segments correspond to Hoxd expression territories. Mol

Dev Evol. 2008; 310B:240-258.

Rios L, Muñoz A, Cardoso H, Pastor F. Short communication: traits unique to genus Homo within primates at the cervical spine (C2-C7). Ann Anat. 2014;196(2-3):167-173.

Rolian C, Lieberman DE, Hallgrímsson B. The coevolution of human hands and feet. Evolution.

2010;64:1558-1568.

Rolian C, Willmore KE. Morphological integration at 50: patterns and processes of integration in biological anthropology. Evol Biol. 2009;36:1-4.

Rolian C. Genes, development, and evolvability in primate evolution. Evol Anthropol. 2014; 23:93-

104.

Rolian C. Integration and evolvability in primate hands and feet. Evol Biol. 2009;36:100-117.

Salesa MJ, Antón M, Turner A, Morales J. Aspects of the functional morphology in the cranial and cervical skeleton of the sabre-toothed cat Paramachairodus ogygia (Kaup, 1832) (Felidae,

Machairodontinae) from the Late Miocene of Spain: implications for the origins of the machairodont killing bite. Zool J Linn Soc. 2005;144(3):363-377.

Sanders WJ. Function, allometry, and evolution of the australopith lower precaudal spine. Doctoral dissertation, New York University; 1995.

Sarringhaus LA, MacLatchy LM, Mitani JC. Locomotor and postural development of wild chimpanzees. J Hum Evol. 2014;66:29-38.

Schlosser G. Modularity and the units of evolution. Theor Biosci. 2002;121(1):1-80.

Schultz AH. Vertebral column and thorax. Primatologia. 1961;4(5):1-64.

Schumacher A, Faust C, Magnuson T. Positional cloning of a global regulator of anterior-posterior patterning in mice. Nature 1995;383:250-253.

Shapiro LJ. Chapter Five: Functional morphology of the vertebral column in primates. In: Gebo DL, editor. Postcranial adaptation in nonhuman primates. DeKalb: Northern Illinois University Press;

1993. p. 121-149.

Shapiro LJ. Functional morphology of the primate spine with special reference ot the orthograde posture and bipedal locomotion. Doctoral dissertation, State University of New York, Stony Brook;

1991.

Singh N, Harvati K, Hublin JJ, Klingenberg CP. Morphological evolution through integration: a quantitative study of cranial integration in Homo, Pan, Gorilla and Pongo. J Hum Evol. 2012;62:155-

164.

Strait D, Ross CF. Kinematic data on primate head and neck posture: implications for the evolution of basicranial flexion and an evaluation of registration planes used in paleoanthropology. Am J Phys

Anthropol. 1999;108:205-222.

Urbanowicz M. Alteration of vertical dimension and its effect on head and neck posture. CRANIO.

1991;9(2):174-179.

Van der Geer AAE, Galis F. 2017. High incidence of cervical ribs indicates vulnerable condition in

Late Pleistocene woolly rhinoceroses. PeerJ 5:e3684.

Van der Lugt NM, Domen J, Linders K, van Roon M, Robanus-Maandag E, te Riele H, van der Valk

M, Deschamps J, Sofroniew M, van Lohuizen M. Posterior transformation, neurological abnormalities, and severe hematopoietic defects in mice with a targeted deletion of the bmi-1 proto- oncogene. Genes & Dev 1994;8:757-769.

Van Sittert SJ, Skinner JD, Mitchell G. From fetus to adult—an allometric analysis of the giraffe vertebral column. J Exp Zool (Mol Dev Evol). 2010; 314B:469-479.

Vidal PP, Graf W, Berthoz A. The orientation of the cervical vertebral column in unrestrained awake animals. I. Resting position. Exp Brain Res. 1986;61:549-559.

Villamil C. Locomotion and basicranial anatomy in primates and marsupials. J Hum Evol.

2017;111:163-178.

Wagner GP, Altenberg L. Perspective: complex adaptations and the evolution of evolvability.

Evolution. 1996;50(3):967-976.

Wagner GP, Booth G, Bagheri-Chaichian H. A population genetic theory of canalization. Evolution.

1997;51(2):329-347.

Wagner GP, Schwenk K. Evolutionarily stable configurations:functional integration and the evolution of phenotypic stability. Evol Biol. 2000;31:155-217.

Wagner GP. Homologues, natural kinds and the evolution of modularity. Amer Zool. 1996;36:36-43.

Walker A. Nuchal adaptations in Perodicticus potto. Primates. 1970;11:135-144.

Wallin J, Wilting J, Koseki H, Fritsch R, Christ B, Balling R. The role of Pax-1 in axial skeleton development. Development 1994;120:1109-1121.

Wellik DM. Hox patterning of the vertebrate axial skeleton. Dev Dynamics. 2007;236:2454-2463.

Winther RG. Varieties of modules: kinds, levels, origins, and behaviors. J Exp Zool Part A.

2001;291(2):116-129.

Yoo WG, An DH. The relationship between the active cervical range of motion and changes in head and neck posture after continuous VDT work. Industrial Health. 2009;47.2:183-188.

Yu BD, Hess JL, Horning SE, Brown GAJ, Korsmeyer SJ. 1995. Altered Hox expression and segmental identity in Mll-mutant mice. Nature 1995;378-505-508.

Zelditch ML, Carmichael AC. Ontogenetic variation in patterns of developmental and functional integration in skulls of Sigmodon fulviventer. Evolution. 1989;43(4):814-824.

Zelditch ML. Ontogenetic variation in patterns of phenotypic integration in the laboratory rat.

Evolution. 1987;42(1):28-41.

Zelditch ML. Ontogenetic variation in patterns of phenotypic integration in the laboratory rat.

Evolution. 1988;42(1):28-41.

Tables

Table 1. Measurements.

Name Description

Cranial base

1. Foramen magnum length Distance from the most anterior point on the edge of the foramen

magnum (basion) to its most posterior point (opisthion)

2. Minimum jugular Distance between the right and left most medial points on the edge of breadth the jugular canal

3. Maximum jugular Distance between the right and left most lateral points on the edge of breadth the jugular canal

4. Jugular length Distance from the most medial point and most lateral point on the edge

of the jugular canal

5. Basioccipital length Distance from basion to the midline point on the sphenooccipital suture

(sphenobasion)

6. Basioccipital breadth Maximum breadth at the sphenooccipital suture

7. Petrous length Length of the petrous portion of the temporal, measured from the most

medial point on the jugular to the most medial, anterior point on the

petrous process

8. Maximum breadth Distance between the right and left sides, measured at porion—the most

superior point on the opening of the external auditory meatus (EAM)

9. Bitympanic width Distance between the right and left EAM, measured at the most inferior

point of the opening

10. Petrous height Height of the temporal at the EAM, measured from porion to the most

inferior point of the opening

11. Medial breadth Distance from the most medial point on the jugular canal to

sphenobasion

12. Lateral breadth Distance from the most medial point on the jugular canal to most

inferior point on the opening of the EAM

13. Cranial base extension Distance from asterion to porion

Atlas (C1)

1. Ventral height * Craniocaudal height at the midline

2. Ventral thickness * Ventrodorsal breadth at midline [used only for comparisons of the

ventral portion of cervical vertebrae]

3. Dorsal height + Craniocaudal height at the midline

4. Dorsal thickness + Ventrodorsal breadth at midline

5. Maximum breadth Maximum breadth at the transverse processes

6. Cranial facet breadth + Maximum breadth measured at most lateral extension of the

cranial/superior articular facet

7. Caudal facet breadth Maximum breadth measured at most lateral extension of the

caudal/inferior articular facet

8. Interzygous height Height between the most cranial point on cranial articular facet and

most caudal point on caudal articular facet

9. Maximum depth Maximum depth of the vertebra, measured on midline from most

ventral edge of body to spine tip (not at an angle)

Axis (C2)

1. Dens height Height from midsagittal point just inferior to atlantoaxial facet to tip of

dens

2. Dens breadth Maximum transverse breadth

3. Dens depth Maximum ventrodorsal depth

4. Vertebral body height * Ventral height, measured from midsagittal point just caudal to

atlantoaxial facet

5. Vertebral body breadth * Maximum transverse width, measured at the caudal edge

6. Vertebral body depth * Maximum ventrodorsal depth, measured at the caudal edge

7. Transverse process Distance from most lateral point of transverse process to medial edge of length SAF

8. Cranial facet breadth + Maximum breadth measured at most lateral extension of the

cranial/superior articular facet

9. Caudal facet breadth + Maximum breadth measured at most lateral extension of the

caudal/inferior articular facet

10. Interzygous height Height between the most cranial point on cranial articular facet and

most caudal point on caudal articular facet

11. Maximum depth Maximum depth of the vertebra, measured on midline from most

ventral edge of body to spine tip (not at an angle)

12. Spine height + Craniocaudal height of the neural spine at its tallest point [used only for

comparisons of the ventral portion of cervical vertebrae]

13. Spine length + Ventrodorsal length of the neural spine, measured along caudal edge

[used only for comparisons of the ventral portion of cervical vertebrae]

General (C3-C7)

1. Vertebral body height * Craniocaudal height of the vertebral body, measured on the ventral side

2. Ventral body breadth * Maximum transverse width, measured at the caudal edge

3. Ventral body depth * Maximum ventrodorsal depth, measured at the caudal edge

4. Pedicle thickness + Mediolateral width of the pedicle

5. Lamina height + Craniocaudal height, measured at the midpoint between the most lateral

edge of the lamina and the spine

6. Cranial facet breadth + Maximum breadth measured at most lateral extension of the

cranial/superior articular facet

7. Caudal facet breadth Maximum breadth measured at most lateral extension of the

caudal/inferior articular facet

8. Interzygous height Height between the most cranial point on cranial articular facet and

most caudal point on caudal articular facet

9. Maximum depth Maximum depth of the vertebra, measured on midline from most

ventral edge of body to spine tip (not at an angle)

10. Spine length + Ventrodorsal length of the neural spine, measured along caudal edge

[used only for comparisons of the ventral portion of cervical vertebrae]

Unilateral measurements were taken on the right side, unless this was unavailable. For whole-element analyses, all measurements were used. * indicates measurements used for analyses of the ventral portion of the cervical vertebrae. + indicates measurements used for analyses of the dorsal portion of the cervical vertebrae.

Table 2. Mean integration results for whole vertebrae.

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

CB 0.726 0.810 0.894 107 0.743 0.826 0.910 104 0.764 0.837 0.910 85

C1 0.673 0.770 0.867 84 0.610 0.725 0.840 80 0.691 0.785 0.879 82

C2 0.413 0.504 0.594 77 0.305 0.411 0.516 86 0.365 0.458 0.551 81

C3 0.515 0.641 0.767 82 0.437 0.570 0.703 89 0.611 0.713 0.815 80

C4 0.510 0.648 0.787 83 0.563 0.684 0.805 85 0.605 0.716 0.827 83

C5 0.491 0.624 0.757 80 0.636 0.752 0.869 88 0.624 0.740 0.857 79

C6 0.398 0.529 0.659 89 0.517 0.629 0.741 90 0.595 0.708 0.822 82

C7 0.318 0.443 0.568 100 0.396 0.499 0.603 91 0.450 0.546 0.643 82

CB-C1 0.821 0.869 0.916 76 0.846 0.889 0.933 75 0.850 0.890 0.930 75

CB-C2 0.767 0.823 0.878 71 0.778 0.835 0.891 80 0.769 0.827 0.886 75

CB-C3 0.769 0.824 0.879 73 0.804 0.860 0.917 81 0.819 0.871 0.924 75

CB-C4 0.776 0.830 0.883 73 0.816 0.867 0.917 77 0.810 0.862 0.913 77

CB-C5 0.780 0.834 0.888 71 0.835 0.879 0.923 80 0.831 0.880 0.928 73

CB-C6 0.742 0.805 0.868 80 0.804 0.857 0.910 85 0.826 0.874 0.923 76

CB-C7 0.716 0.788 0.861 89 0.784 0.847 0.911 86 0.770 0.831 0.891 75

C1-C2 0.777 0.836 0.895 68 0.737 0.813 0.890 70 0.737 0.805 0.872 75

C1-C3 0.764 0.823 0.881 64 0.726 0.792 0.859 69 0.770 0.828 0.887 75

C1-C4 0.754 0.819 0.884 65 0.783 0.842 0.901 65 0.777 0.837 0.897 78

C1-C5 0.759 0.816 0.873 60 0.824 0.877 0.930 64 0.792 0.846 0.901 74

C1-C6 0.727 0.793 0.860 69 0.734 0.799 0.865 67 0.775 0.836 0.898 77

C1-C7 0.672 0.747 0.821 75 0.693 0.752 0.811 63 0.710 0.778 0.846 77

C2-C3 0.753 0.818 0.883 61 0.654 0.727 0.800 78 0.723 0.801 0.879 77

C2-C4 0.684 0.753 0.821 62 0.645 0.725 0.806 74 0.650 0.731 0.812 80

C2-C5 0.699 0.765 0.832 57 0.698 0.782 0.866 75 0.692 0.770 0.848 77

C2-C6 0.624 0.692 0.761 62 0.582 0.665 0.747 77 0.640 0.725 0.811 78

C2-C7 0.551 0.627 0.703 70 0.569 0.642 0.715 74 0.568 0.637 0.707 76

C3-C4 0.790 0.852 0.914 71 0.822 0.878 0.934 81 0.819 0.874 0.929 79

C3-C5 0.757 0.821 0.885 66 0.795 0.857 0.918 80 0.792 0.849 0.905 75

C3-C6 0.679 0.756 0.832 69 0.716 0.780 0.844 78 0.754 0.819 0.884 77

C3-C7 0.610 0.693 0.776 73 0.652 0.724 0.796 78 0.684 0.754 0.825 74

C4-C5 0.817 0.869 0.922 69 0.876 0.917 0.957 79 0.837 0.885 0.932 79

C4-C6 0.690 0.769 0.848 76 0.780 0.838 0.895 76 0.775 0.837 0.900 80

C4-C7 0.586 0.671 0.755 77 0.726 0.793 0.859 75 0.700 0.770 0.839 78

C5-C6 0.793 0.865 0.936 73 0.850 0.902 0.954 82 0.830 0.883 0.936 76

C5-C7 0.608 0.694 0.780 74 0.754 0.822 0.889 80 0.734 0.802 0.869 74

C6-C7 0.676 0.750 0.824 83 0.782 0.848 0.915 85 0.761 0.833 0.906 76

The minimum and maximum represent the minimum and maximum of the 99% confidence intervals, whereas mean is the actual calculated mean value. N stands for sample size.

Table 3. VE results for whole vertebrae.

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

CB 0.080 0.088 0.095 107 0.098 0.106 0.114 104 0.083 0.089 0.094 85

C1 0.117 0.144 0.170 84 0.069 0.085 0.101 80 0.092 0.104 0.116 82

C2 0.097 0.115 0.133 77 0.084 0.106 0.128 86 0.070 0.082 0.094 81

C3 0.122 0.143 0.163 82 0.115 0.142 0.169 89 0.092 0.104 0.115 80

C4 0.119 0.144 0.170 83 0.134 0.174 0.214 85 0.092 0.105 0.118 83

C5 0.093 0.115 0.136 80 0.133 0.167 0.201 88 0.076 0.086 0.097 79

C6 0.086 0.107 0.128 89 0.085 0.106 0.127 90 0.098 0.112 0.126 82

C7 0.086 0.102 0.118 100 0.091 0.116 0.142 91 0.085 0.099 0.113 82

CB-C1 0.060 0.065 0.070 76 0.057 0.061 0.065 75 0.068 0.072 0.077 75

CB-C2 0.062 0.068 0.074 71 0.055 0.059 0.062 80 0.066 0.070 0.075 75

CB-C3 0.062 0.066 0.071 73 0.071 0.077 0.083 81 0.063 0.067 0.072 75

CB-C4 0.059 0.065 0.071 73 0.068 0.074 0.080 77 0.062 0.066 0.069 77

CB-C5 0.057 0.062 0.067 71 0.074 0.081 0.089 80 0.060 0.064 0.067 73

CB-C6 0.053 0.057 0.061 80 0.060 0.064 0.068 85 0.062 0.065 0.068 76

CB-C7 0.052 0.057 0.061 89 0.068 0.074 0.080 86 0.060 0.063 0.067 75

C1-C2 0.111 0.123 0.135 68 0.080 0.091 0.102 70 0.076 0.083 0.089 75

C1-C3 0.105 0.119 0.134 64 0.071 0.082 0.093 69 0.068 0.072 0.076 75

C1-C4 0.095 0.109 0.123 65 0.082 0.096 0.109 65 0.067 0.071 0.076 78

C1-C5 0.070 0.080 0.090 60 0.092 0.101 0.109 64 0.064 0.069 0.073 74

C1-C6 0.077 0.089 0.101 69 0.066 0.075 0.083 67 0.075 0.081 0.087 77

C1-C7 0.073 0.085 0.096 75 0.068 0.077 0.085 63 0.075 0.081 0.087 77

C2-C3 0.118 0.130 0.142 61 0.098 0.111 0.123 78 0.079 0.084 0.088 77

C2-C4 0.102 0.113 0.124 62 0.096 0.110 0.124 74 0.078 0.083 0.089 80

C2-C5 0.107 0.118 0.130 57 0.095 0.105 0.116 75 0.071 0.076 0.082 77

C2-C6 0.097 0.108 0.118 62 0.079 0.088 0.097 77 0.070 0.076 0.082 78

C2-C7 0.095 0.104 0.113 70 0.074 0.086 0.097 74 0.072 0.079 0.086 76

C3-C4 0.143 0.156 0.169 71 0.133 0.152 0.171 81 0.112 0.119 0.126 79

C3-C5 0.133 0.148 0.164 66 0.130 0.147 0.165 80 0.096 0.103 0.109 75

C3-C6 0.115 0.128 0.142 69 0.101 0.112 0.123 78 0.086 0.093 0.101 77

C3-C7 0.093 0.105 0.116 73 0.112 0.127 0.143 78 0.083 0.090 0.096 74

C4-C5 0.119 0.130 0.141 69 0.143 0.163 0.183 79 0.106 0.113 0.120 79

C4-C6 0.109 0.121 0.132 76 0.103 0.115 0.128 76 0.098 0.105 0.112 80

C4-C7 0.085 0.094 0.104 77 0.117 0.138 0.158 75 0.085 0.093 0.101 78

C5-C6 0.098 0.107 0.117 73 0.129 0.140 0.152 82 0.100 0.108 0.116 76

C5-C7 0.089 0.100 0.111 74 0.133 0.153 0.173 80 0.087 0.096 0.105 74

C6-C7 0.099 0.110 0.121 83 0.106 0.119 0.132 85 0.109 0.119 0.130 76

The minimum and maximum represent the minimum and maximum of the 99% confidence intervals, whereas mean is the actual calculated mean value. N stands for sample size.

Table 4. Mean evolvability results for whole vertebrae.

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

CB 0.048 0.078 0.107 107 0.053 0.081 0.109 104 0.060 0.090 0.119 85

C1 0.086 0.166 0.247 84 0.084 0.187 0.291 80 0.078 0.161 0.244 82

C2 0.041 0.057 0.073 77 0.046 0.064 0.082 86 0.039 0.055 0.071 81

C3 0.061 0.094 0.127 82 0.063 0.102 0.142 89 0.059 0.100 0.140 80

C4 0.058 0.087 0.115 83 0.058 0.093 0.129 85 0.051 0.088 0.125 83

C5 0.063 0.092 0.120 80 0.054 0.088 0.122 88 0.048 0.093 0.139 79

C6 0.056 0.085 0.114 89 0.059 0.097 0.135 90 0.057 0.096 0.136 82

C7 0.041 0.058 0.075 100 0.055 0.084 0.112 91 0.051 0.074 0.098 82

CB-C1 0.072 0.108 0.143 76 0.069 0.111 0.152 75 0.075 0.113 0.150 75

CB-C2 0.052 0.070 0.088 71 0.054 0.069 0.084 80 0.055 0.076 0.096 75

CB-C3 0.063 0.085 0.107 73 0.062 0.087 0.112 81 0.068 0.093 0.118 75

CB-C4 0.056 0.073 0.089 73 0.060 0.085 0.109 77 0.063 0.087 0.111 77

CB-C5 0.059 0.078 0.098 71 0.059 0.080 0.101 80 0.064 0.091 0.119 73

CB-C6 0.055 0.076 0.097 80 0.061 0.085 0.109 85 0.069 0.095 0.120 76

CB-C7 0.052 0.069 0.086 89 0.061 0.081 0.101 86 0.062 0.084 0.107 75

C1-C2 0.063 0.101 0.139 68 0.064 0.117 0.171 70 0.060 0.101 0.142 75

C1-C3 0.086 0.133 0.180 64 0.083 0.137 0.190 69 0.083 0.127 0.171 75

C1-C4 0.077 0.129 0.181 65 0.076 0.140 0.204 65 0.072 0.120 0.167 78

C1-C5 0.080 0.124 0.169 60 0.076 0.130 0.184 64 0.077 0.123 0.169 74

C1-C6 0.078 0.128 0.179 69 0.082 0.141 0.201 67 0.077 0.128 0.179 77

C1-C7 0.061 0.107 0.153 75 0.066 0.096 0.126 63 0.070 0.115 0.160 77

C2-C3 0.055 0.078 0.101 61 0.053 0.077 0.100 78 0.053 0.075 0.098 77

C2-C4 0.049 0.067 0.086 62 0.050 0.072 0.093 74 0.049 0.070 0.092 80

C2-C5 0.054 0.074 0.094 57 0.049 0.070 0.092 75 0.048 0.072 0.096 77

C2-C6 0.047 0.067 0.086 62 0.052 0.076 0.101 77 0.049 0.073 0.098 78

C2-C7 0.043 0.059 0.074 70 0.048 0.067 0.085 74 0.047 0.064 0.081 76

C3-C4 0.056 0.087 0.117 71 0.058 0.096 0.134 81 0.055 0.093 0.130 79

C3-C5 0.064 0.094 0.124 66 0.061 0.094 0.127 80 0.058 0.097 0.135 75

C3-C6 0.058 0.087 0.117 69 0.067 0.105 0.142 78 0.065 0.096 0.126 77

C3-C7 0.052 0.075 0.098 73 0.064 0.096 0.127 78 0.060 0.087 0.115 74

C4-C5 0.059 0.086 0.113 69 0.053 0.089 0.125 79 0.054 0.090 0.126 79

C4-C6 0.054 0.081 0.108 76 0.056 0.091 0.126 76 0.057 0.089 0.121 80

C4-C7 0.046 0.065 0.084 77 0.057 0.086 0.114 75 0.056 0.082 0.108 78

C5-C6 0.063 0.088 0.112 73 0.055 0.092 0.129 82 0.057 0.093 0.128 76

C5-C7 0.052 0.074 0.095 74 0.056 0.089 0.121 80 0.055 0.086 0.117 74

C6-C7 0.046 0.068 0.091 83 0.057 0.089 0.121 85 0.054 0.084 0.115 76

Values are presented as e x 101 to facilitate viewing. The minimum and maximum represent the minimum and maximum of the 99% confidence intervals, whereas mean is the actual calculated mean value. N stands for sample size.

Table 5. Mean conditional evolvability results for whole vertebrae.

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

CB 0.068 0.139 0.211 107 0.057 0.136 0.215 104 0.065 0.139 0.213 85

C1 0.168 0.304 0.440 84 0.225 0.394 0.563 80 0.140 0.273 0.406 82

C2 0.199 0.267 0.334 77 0.274 0.357 0.440 86 0.207 0.285 0.364 81

C3 0.184 0.316 0.447 82 0.250 0.410 0.569 89 0.146 0.248 0.350 80

C4 0.163 0.287 0.411 83 0.148 0.267 0.385 85 0.125 0.221 0.317 83

C5 0.194 0.332 0.470 80 0.093 0.207 0.320 88 0.104 0.200 0.297 79

C6 0.249 0.373 0.497 89 0.199 0.329 0.458 90 0.136 0.256 0.377 82

C7 0.215 0.309 0.404 100 0.270 0.395 0.520 91 0.213 0.325 0.437 82

CB-C1 0.084 0.126 0.168 76 0.067 0.106 0.146 75 0.070 0.113 0.155 75

CB-C2 0.077 0.116 0.155 71 0.069 0.110 0.151 80 0.078 0.123 0.167 75

CB-C3 0.092 0.141 0.191 73 0.064 0.116 0.168 81 0.065 0.112 0.158 75

CB-C4 0.079 0.120 0.160 73 0.065 0.105 0.146 77 0.069 0.113 0.157 77

CB-C5 0.079 0.125 0.170 71 0.056 0.093 0.129 80 0.060 0.102 0.143 73

CB-C6 0.089 0.142 0.195 80 0.069 0.115 0.162 85 0.065 0.112 0.158 76

CB-C7 0.090 0.140 0.191 89 0.066 0.119 0.172 86 0.080 0.136 0.192 75

C1-C2 0.092 0.137 0.181 68 0.110 0.173 0.236 70 0.113 0.167 0.221 75

C1-C3 0.142 0.200 0.258 64 0.167 0.238 0.309 69 0.127 0.189 0.251 75

C1-C4 0.134 0.193 0.252 65 0.114 0.172 0.230 65 0.112 0.167 0.223 78

C1-C5 0.137 0.196 0.255 60 0.081 0.128 0.176 64 0.106 0.163 0.219 74

C1-C6 0.162 0.224 0.285 69 0.168 0.231 0.293 67 0.113 0.181 0.248 77

C1-C7 0.164 0.228 0.292 75 0.157 0.213 0.269 63 0.155 0.219 0.284 77

C2-C3 0.084 0.129 0.174 61 0.136 0.196 0.255 78 0.084 0.137 0.190 77

C2-C4 0.109 0.152 0.195 62 0.128 0.181 0.234 74 0.121 0.172 0.223 80

C2-C5 0.112 0.157 0.202 57 0.087 0.142 0.196 75 0.102 0.149 0.196 77

C2-C6 0.143 0.184 0.225 62 0.174 0.231 0.289 77 0.128 0.184 0.241 78

C2-C7 0.162 0.204 0.246 70 0.170 0.225 0.280 74 0.169 0.218 0.267 76

C3-C4 0.063 0.113 0.162 71 0.049 0.104 0.158 81 0.060 0.099 0.137 79

C3-C5 0.099 0.152 0.206 66 0.066 0.121 0.176 80 0.084 0.123 0.161 75

C3-C6 0.131 0.193 0.254 69 0.141 0.206 0.272 78 0.103 0.157 0.211 77

C3-C7 0.151 0.213 0.275 73 0.174 0.240 0.306 78 0.141 0.195 0.249 74

C4-C5 0.056 0.104 0.152 69 0.032 0.064 0.095 79 0.051 0.088 0.125 79

C4-C6 0.112 0.169 0.226 76 0.082 0.131 0.180 76 0.080 0.128 0.176 80

C4-C7 0.141 0.201 0.261 77 0.104 0.159 0.213 75 0.118 0.174 0.229 78

C5-C6 0.051 0.112 0.172 73 0.038 0.079 0.120 82 0.052 0.094 0.136 76

C5-C7 0.145 0.210 0.274 74 0.087 0.141 0.195 80 0.102 0.150 0.199 74

C6-C7 0.108 0.158 0.208 83 0.066 0.119 0.172 85 0.075 0.124 0.174 76

Values are presented as e x 102 to facilitate viewing. The minimum and maximum represent the minimum and maximum of the 99% confidence intervals, whereas mean is the actual calculated mean value. N stands for sample size.

Table 6. Mean integration for ventral (vertebral body) and dorsal (neural arch) portions of the cervical vertebrae.

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1 0.355 0.517 0.679 103 0.403 0.545 0.687 86 0.485 0.616 0.748 83

C2 0.363 0.480 0.598 83 0.264 0.372 0.479 92 0.388 0.493 0.599 83

C3 0.469 0.580 0.692 83 0.408 0.536 0.663 88 0.440 0.548 0.656 80

C4 0.414 0.527 0.641 84 0.377 0.481 0.585 83 0.418 0.538 0.657 83

C5 0.477 0.594 0.712 83 0.391 0.503 0.616 91 0.455 0.586 0.717 79

C6 0.433 0.551 0.669 93 0.355 0.474 0.594 93 0.443 0.567 0.692 82

C7 0.246 0.342 0.438 102 0.296 0.402 0.509 91 0.284 0.394 0.505 83

Ventral

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1-C2 0.139 0.210 0.282 94 0.129 0.219 0.310 82 0.125 0.225 0.325 82

C1-C3 0.108 0.179 0.251 93 0.137 0.224 0.312 82 0.121 0.218 0.315 81

C1-C4 0.130 0.198 0.266 89 0.140 0.228 0.316 77 0.113 0.208 0.303 82

C1-C5 0.171 0.247 0.324 82 0.169 0.262 0.356 78 0.135 0.242 0.349 81

C1-C6 0.164 0.250 0.336 93 0.142 0.223 0.304 78 0.101 0.182 0.263 85

C1-C7 0.212 0.301 0.389 98 0.131 0.227 0.323 75 0.101 0.180 0.260 84

C2-C3 0.308 0.429 0.550 86 0.213 0.310 0.406 96 0.230 0.330 0.429 81

C2-C4 0.223 0.312 0.402 85 0.173 0.264 0.355 91 0.152 0.231 0.309 82

C2-C5 0.169 0.251 0.333 76 0.150 0.238 0.326 92 0.165 0.244 0.322 80

C2-C6 0.105 0.184 0.263 86 0.112 0.193 0.275 91 0.131 0.204 0.277 82

C2-C7 0.103 0.186 0.270 90 0.111 0.187 0.264 90 0.123 0.193 0.264 82

C3-C4 0.392 0.503 0.615 93 0.458 0.577 0.695 93 0.356 0.450 0.545 80

C3-C5 0.322 0.435 0.549 85 0.396 0.508 0.620 94 0.314 0.404 0.494 79

C3-C6 0.217 0.318 0.418 92 0.277 0.396 0.514 93 0.199 0.274 0.349 80

C3-C7 0.239 0.337 0.435 95 0.189 0.286 0.383 92 0.215 0.309 0.403 80

C4-C5 0.382 0.487 0.593 87 0.564 0.678 0.792 95 0.356 0.441 0.527 81

C4-C6 0.244 0.341 0.439 96 0.351 0.467 0.583 93 0.261 0.344 0.426 82

C4-C7 0.242 0.338 0.435 95 0.231 0.337 0.443 91 0.217 0.307 0.397 81

C5-C6 0.358 0.471 0.584 88 0.416 0.542 0.668 95 0.316 0.421 0.527 81

C5-C7 0.224 0.329 0.434 86 0.266 0.381 0.496 93 0.261 0.355 0.449 80

C6-C7 0.426 0.545 0.664 100 0.318 0.450 0.582 97 0.322 0.427 0.531 84

Dorsal

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1-C2 0.656 0.766 0.875 87 0.501 0.626 0.750 78 0.661 0.755 0.849 78

C1-C3 0.602 0.716 0.830 81 0.600 0.701 0.802 72 0.580 0.683 0.786 78

C1-C4 0.597 0.699 0.801 80 0.559 0.661 0.763 67 0.610 0.710 0.809 80

C1-C5 0.559 0.668 0.777 87 0.560 0.666 0.773 72 0.646 0.742 0.837 77

C1-C6 0.527 0.652 0.776 92 0.546 0.657 0.769 74 0.628 0.731 0.835 78

C1-C7 0.440 0.569 0.699 92 0.485 0.587 0.689 70 0.523 0.648 0.773 81

C2-C3 0.777 0.859 0.940 73 0.609 0.725 0.842 82 0.731 0.837 0.944 81

C2-C4 0.707 0.793 0.878 74 0.482 0.584 0.687 79 0.623 0.729 0.834 81

C2-C5 0.670 0.762 0.854 80 0.524 0.630 0.735 83 0.652 0.742 0.833 79

C2-C6 0.597 0.698 0.799 85 0.474 0.578 0.683 84 0.619 0.721 0.823 79

C2-C7 0.451 0.554 0.657 82 0.456 0.562 0.669 79 0.507 0.613 0.720 80

C3-C4 0.628 0.737 0.847 79 0.588 0.701 0.814 82 0.642 0.753 0.863 81

C3-C5 0.634 0.748 0.862 84 0.554 0.672 0.790 83 0.642 0.743 0.843 79

C3-C6 0.604 0.710 0.816 84 0.572 0.680 0.787 81 0.596 0.708 0.821 79

C3-C7 0.520 0.622 0.724 84 0.527 0.632 0.737 78 0.507 0.618 0.730 79

C4-C5 0.667 0.792 0.916 86 0.640 0.746 0.852 83 0.719 0.809 0.899 81

C4-C6 0.595 0.725 0.855 87 0.513 0.628 0.744 81 0.681 0.783 0.885 81

C4-C7 0.462 0.587 0.712 86 0.525 0.624 0.722 77 0.538 0.659 0.780 81

C5-C6 0.659 0.790 0.920 93 0.677 0.788 0.898 89 0.737 0.834 0.932 78

C5-C7 0.511 0.634 0.757 92 0.576 0.684 0.792 85 0.591 0.712 0.834 78

C6-C7 0.526 0.649 0.772 97 0.575 0.697 0.820 90 0.607 0.729 0.850 78

The minimum and maximum represent the minimum and maximum of the 99% confidence intervals, whereas mean is the actual calculated mean value. N stands for sample size.

Table 7. VE results for whole vertebrae.

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1 0.036 0.051 0.065 103 0.023 0.039 0.055 86 0.078 0.097 0.116 83

C2 0.079 0.103 0.126 83 0.042 0.064 0.085 92 0.045 0.057 0.069 83

C3 0.086 0.104 0.121 83 0.078 0.102 0.127 88 0.035 0.043 0.052 80

C4 0.061 0.082 0.104 84 0.072 0.109 0.145 83 0.036 0.046 0.056 83

C5 0.047 0.067 0.087 83 0.081 0.115 0.150 91 0.030 0.038 0.047 79

C6 0.056 0.075 0.094 93 0.067 0.095 0.122 93 0.050 0.069 0.088 82

C7 0.057 0.077 0.097 102 0.076 0.105 0.134 91 0.039 0.052 0.065 83

Ventral

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1-C2 0.024 0.042 0.060 94 0.031 0.064 0.097 82 0.106 0.136 0.166 82

C1-C3 0.033 0.053 0.074 93 0.039 0.065 0.092 82 0.086 0.112 0.138 81

C1-C4 0.040 0.060 0.080 89 0.035 0.057 0.079 77 0.061 0.084 0.106 82

C1-C5 0.038 0.068 0.099 82 0.056 0.080 0.104 78 0.073 0.104 0.135 81

C1-C6 0.041 0.063 0.086 93 0.040 0.060 0.080 78 0.057 0.081 0.105 85

C1-C7 0.039 0.064 0.090 98 0.033 0.052 0.072 75 0.052 0.076 0.100 84

C2-C3 0.128 0.156 0.183 86 0.093 0.119 0.146 96 0.107 0.122 0.138 81

C2-C4 0.094 0.122 0.149 85 0.061 0.084 0.106 91 0.063 0.078 0.093 82

C2-C5 0.081 0.113 0.146 76 0.047 0.066 0.085 92 0.070 0.089 0.108 80

C2-C6 0.068 0.095 0.122 86 0.036 0.055 0.074 91 0.051 0.072 0.092 82

C2-C7 0.077 0.101 0.125 90 0.035 0.061 0.087 90 0.055 0.073 0.091 82

C3-C4 0.164 0.195 0.226 93 0.160 0.193 0.226 93 0.121 0.133 0.146 80

C3-C5 0.164 0.196 0.229 85 0.141 0.164 0.187 94 0.109 0.124 0.140 79

C3-C6 0.108 0.140 0.172 92 0.089 0.116 0.143 93 0.074 0.089 0.104 80

C3-C7 0.111 0.140 0.169 95 0.080 0.106 0.132 92 0.080 0.095 0.110 80

C4-C5 0.156 0.191 0.225 87 0.182 0.207 0.232 95 0.114 0.127 0.139 81

C4-C6 0.107 0.135 0.164 96 0.109 0.143 0.178 93 0.084 0.104 0.124 82

C4-C7 0.093 0.125 0.156 95 0.074 0.110 0.146 91 0.071 0.083 0.095 81

C5-C6 0.163 0.195 0.226 88 0.126 0.159 0.193 95 0.093 0.115 0.136 81

C5-C7 0.120 0.157 0.194 86 0.081 0.108 0.134 93 0.078 0.099 0.120 80

C6-C7 0.179 0.209 0.238 100 0.098 0.120 0.142 97 0.107 0.123 0.140 84

Dorsal

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1-C2 0.135 0.182 0.229 87 0.036 0.050 0.064 78 0.062 0.074 0.086 78

C1-C3 0.085 0.108 0.131 81 0.048 0.064 0.081 72 0.031 0.039 0.048 78

C1-C4 0.060 0.095 0.130 80 0.035 0.052 0.069 67 0.046 0.060 0.073 80

C1-C5 0.031 0.044 0.057 87 0.048 0.071 0.093 72 0.038 0.050 0.061 77

C1-C6 0.052 0.071 0.090 92 0.034 0.048 0.062 74 0.060 0.079 0.097 78

C1-C7 0.054 0.071 0.088 92 0.036 0.055 0.074 70 0.035 0.045 0.055 81

C2-C3 0.129 0.155 0.181 73 0.114 0.139 0.164 82 0.069 0.080 0.090 81

C2-C4 0.144 0.179 0.214 74 0.068 0.089 0.111 79 0.067 0.079 0.091 81

C2-C5 0.079 0.097 0.114 80 0.092 0.112 0.131 83 0.056 0.067 0.078 79

C2-C6 0.104 0.137 0.170 85 0.068 0.093 0.117 84 0.062 0.075 0.088 79

C2-C7 0.098 0.119 0.141 82 0.081 0.104 0.127 79 0.051 0.062 0.074 80

C3-C4 0.120 0.146 0.172 79 0.131 0.161 0.191 82 0.095 0.108 0.122 81

C3-C5 0.096 0.113 0.130 84 0.125 0.151 0.176 83 0.065 0.074 0.084 79

C3-C6 0.098 0.119 0.140 84 0.107 0.132 0.157 81 0.054 0.062 0.070 79

C3-C7 0.077 0.091 0.105 84 0.085 0.115 0.144 78 0.043 0.053 0.062 79

C4-C5 0.099 0.118 0.137 86 0.140 0.173 0.206 83 0.102 0.112 0.122 81

C4-C6 0.086 0.105 0.125 87 0.099 0.131 0.163 81 0.087 0.099 0.110 81

C4-C7 0.064 0.077 0.090 86 0.104 0.143 0.182 77 0.062 0.073 0.084 81

C5-C6 0.096 0.109 0.122 93 0.161 0.198 0.235 89 0.101 0.111 0.121 78

C5-C7 0.064 0.076 0.088 92 0.147 0.180 0.213 85 0.065 0.076 0.088 78

C6-C7 0.102 0.121 0.140 97 0.144 0.182 0.220 90 0.091 0.107 0.123 78

The minimum and maximum represent the minimum and maximum of the 99% confidence intervals, whereas mean is the actual calculated mean value. N stands for sample size.

Table 8. Mean evolvability for ventral (vertebral body) and dorsal (neural arch) portions of the cervical vertebrae.

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1 0.145 0.279 0.414 103 0.167 0.306 0.445 86 0.158 0.268 0.378 83

C2 0.061 0.088 0.115 83 0.076 0.108 0.141 92 0.069 0.102 0.135 83

C3 0.109 0.175 0.242 83 0.096 0.177 0.257 88 0.098 0.146 0.193 80

C4 0.097 0.164 0.231 84 0.082 0.123 0.164 83 0.088 0.132 0.175 83

C5 0.111 0.192 0.273 83 0.080 0.119 0.158 91 0.079 0.134 0.188 79

C6 0.092 0.158 0.224 93 0.081 0.128 0.175 93 0.086 0.133 0.180 82

C7 0.055 0.079 0.104 102 0.064 0.097 0.130 91 0.076 0.099 0.122 83

Ventral

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1-C2 0.076 0.100 0.124 94 0.077 0.111 0.145 82 0.069 0.103 0.137 82

C1-C3 0.094 0.118 0.142 93 0.076 0.108 0.141 82 0.071 0.104 0.137 81

C1-C4 0.088 0.114 0.139 89 0.073 0.104 0.135 77 0.068 0.095 0.122 82

C1-C5 0.084 0.108 0.132 82 0.073 0.103 0.133 78 0.066 0.096 0.126 81

C1-C6 0.076 0.102 0.128 93 0.082 0.110 0.139 78 0.079 0.109 0.139 85

C1-C7 0.067 0.094 0.120 98 0.078 0.109 0.140 75 0.077 0.106 0.134 84

C2-C3 0.054 0.077 0.101 86 0.052 0.072 0.091 96 0.052 0.075 0.097 81

C2-C4 0.055 0.075 0.095 85 0.056 0.074 0.092 91 0.055 0.072 0.089 82

C2-C5 0.050 0.067 0.084 76 0.064 0.080 0.096 92 0.051 0.070 0.088 80

C2-C6 0.049 0.066 0.082 86 0.063 0.077 0.090 91 0.063 0.080 0.097 82

C2-C7 0.044 0.059 0.074 90 0.060 0.074 0.089 90 0.060 0.079 0.097 82

C3-C4 0.053 0.085 0.117 93 0.049 0.075 0.101 93 0.051 0.073 0.096 80

C3-C5 0.055 0.081 0.107 85 0.055 0.081 0.107 94 0.050 0.071 0.092 79

C3-C6 0.052 0.075 0.098 92 0.059 0.080 0.101 93 0.062 0.082 0.101 80

C3-C7 0.049 0.070 0.091 95 0.056 0.076 0.097 92 0.060 0.081 0.101 80

C4-C5 0.052 0.080 0.109 87 0.049 0.079 0.109 95 0.046 0.066 0.086 81

C4-C6 0.051 0.075 0.100 96 0.056 0.080 0.104 93 0.054 0.072 0.091 82

C4-C7 0.046 0.066 0.087 95 0.055 0.076 0.097 91 0.056 0.074 0.092 81

C5-C6 0.043 0.069 0.094 88 0.059 0.087 0.116 95 0.053 0.074 0.094 81

C5-C7 0.042 0.061 0.081 86 0.061 0.083 0.105 93 0.054 0.072 0.091 80

C6-C7 0.034 0.057 0.080 100 0.057 0.080 0.102 97 0.062 0.085 0.108 84

Dorsal

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1-C2 0.120 0.230 0.340 87 0.128 0.236 0.344 78 0.133 0.226 0.319 78

C1-C3 0.190 0.303 0.417 81 0.210 0.331 0.451 72 0.165 0.265 0.365 78

C1-C4 0.190 0.329 0.468 80 0.157 0.276 0.394 67 0.162 0.260 0.358 80

C1-C5 0.200 0.309 0.417 87 0.143 0.258 0.373 72 0.157 0.253 0.349 77

C1-C6 0.181 0.298 0.415 92 0.152 0.272 0.392 74 0.157 0.243 0.329 78

C1-C7 0.108 0.193 0.278 92 0.125 0.196 0.266 70 0.109 0.202 0.294 81

C2-C3 0.114 0.180 0.246 73 0.120 0.195 0.270 82 0.106 0.158 0.210 81

C2-C4 0.116 0.193 0.270 74 0.101 0.137 0.173 79 0.096 0.142 0.188 81

C2-C5 0.124 0.205 0.286 80 0.095 0.133 0.171 83 0.096 0.153 0.210 79

C2-C6 0.112 0.169 0.226 85 0.094 0.144 0.193 84 0.100 0.147 0.194 79

C2-C7 0.073 0.103 0.134 82 0.086 0.120 0.154 79 0.079 0.113 0.148 80

C3-C4 0.142 0.225 0.308 79 0.115 0.195 0.275 82 0.109 0.184 0.259 81

C3-C5 0.171 0.265 0.360 84 0.122 0.200 0.277 83 0.120 0.197 0.274 79

C3-C6 0.162 0.241 0.319 84 0.138 0.222 0.307 81 0.130 0.185 0.239 79

C3-C7 0.109 0.174 0.238 84 0.118 0.193 0.269 78 0.106 0.151 0.196 79

C4-C5 0.145 0.248 0.352 86 0.089 0.146 0.204 83 0.102 0.179 0.255 81

C4-C6 0.156 0.238 0.320 87 0.096 0.159 0.222 81 0.116 0.180 0.245 81

C4-C7 0.098 0.167 0.236 86 0.089 0.136 0.183 77 0.098 0.149 0.200 81

C5-C6 0.159 0.256 0.352 93 0.089 0.151 0.214 89 0.110 0.180 0.250 78

C5-C7 0.113 0.175 0.237 92 0.082 0.130 0.178 85 0.088 0.150 0.212 78

C6-C7 0.089 0.156 0.223 97 0.076 0.137 0.198 90 0.089 0.141 0.192 78

Table 9. Mean conditional evolvability for ventral (vertebral body) and dorsal (neural arch) portions of the cervical vertebrae.

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1 0.056 0.123 0.190 103 0.065 0.125 0.186 86 0.045 0.097 0.149 83

C2 0.028 0.044 0.060 83 0.046 0.067 0.087 92 0.031 0.050 0.068 83

C3 0.041 0.069 0.096 83 0.046 0.071 0.097 88 0.037 0.064 0.090 80

C4 0.043 0.070 0.098 84 0.040 0.060 0.081 83 0.035 0.059 0.082 83

C5 0.043 0.069 0.095 83 0.037 0.057 0.077 91 0.029 0.050 0.072 79

C6 0.039 0.064 0.090 93 0.040 0.063 0.085 93 0.031 0.056 0.080 82

C7 0.035 0.050 0.065 102 0.038 0.055 0.072 91 0.041 0.060 0.078 83

Ventral

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1-C2 0.058 0.078 0.099 94 0.062 0.082 0.103 82 0.058 0.075 0.093 82

C1-C3 0.074 0.097 0.120 93 0.061 0.081 0.101 82 0.058 0.077 0.096 81

C1-C4 0.069 0.091 0.113 89 0.060 0.077 0.094 77 0.057 0.072 0.087 82

C1-C5 0.060 0.081 0.102 82 0.054 0.074 0.093 78 0.052 0.068 0.085 81

C1-C6 0.054 0.076 0.099 93 0.064 0.084 0.103 78 0.068 0.086 0.104 85

C1-C7 0.045 0.065 0.085 98 0.062 0.081 0.099 75 0.067 0.084 0.101 84

C2-C3 0.028 0.044 0.060 86 0.035 0.049 0.062 96 0.035 0.048 0.062 81

C2-C4 0.037 0.051 0.066 85 0.040 0.054 0.069 91 0.043 0.055 0.066 82

C2-C5 0.038 0.049 0.060 76 0.046 0.061 0.077 92 0.040 0.051 0.063 80

C2-C6 0.042 0.052 0.063 86 0.048 0.062 0.077 91 0.050 0.063 0.076 82

C2-C7 0.037 0.046 0.056 90 0.048 0.060 0.073 90 0.050 0.062 0.075 82

C3-C4 0.024 0.041 0.058 93 0.016 0.032 0.047 93 0.025 0.040 0.054 80

C3-C5 0.029 0.044 0.058 85 0.023 0.039 0.055 94 0.029 0.041 0.053 79

C3-C6 0.035 0.049 0.064 92 0.032 0.049 0.066 93 0.045 0.058 0.072 80

C3-C7 0.032 0.045 0.057 95 0.040 0.054 0.068 92 0.040 0.055 0.069 80

C4-C5 0.023 0.039 0.055 87 0.009 0.025 0.042 95 0.023 0.036 0.050 81

C4-C6 0.033 0.048 0.063 96 0.026 0.043 0.059 93 0.033 0.047 0.060 82

C4-C7 0.029 0.043 0.056 95 0.034 0.050 0.067 91 0.036 0.051 0.066 81

C5-C6 0.021 0.034 0.048 88 0.021 0.040 0.060 95 0.028 0.042 0.056 81

C5-C7 0.028 0.039 0.051 86 0.034 0.051 0.069 93 0.032 0.046 0.060 80

C6-C7 0.013 0.025 0.036 100 0.026 0.045 0.063 97 0.032 0.049 0.065 84

Dorsal

Homo Pan Hylobates

Min Mean Max N Min Mean Max N Min Mean Max N

C1-C2 0.019 0.047 0.074 87 0.043 0.076 0.109 78 0.024 0.049 0.073 78

C1-C3 0.036 0.083 0.130 81 0.050 0.089 0.129 72 0.040 0.081 0.122 78

C1-C4 0.041 0.094 0.147 80 0.048 0.081 0.115 67 0.036 0.072 0.108 80

C1-C5 0.048 0.101 0.154 87 0.041 0.073 0.105 72 0.029 0.062 0.094 77

C1-C6 0.048 0.099 0.151 92 0.046 0.081 0.115 74 0.029 0.062 0.095 78

C1-C7 0.045 0.075 0.104 92 0.048 0.073 0.099 70 0.035 0.063 0.091 81

C2-C3 0.007 0.025 0.042 73 0.024 0.050 0.076 82 0.005 0.026 0.046 81

C2-C4 0.016 0.037 0.058 74 0.034 0.058 0.081 79 0.018 0.038 0.057 81

C2-C5 0.019 0.047 0.075 80 0.027 0.049 0.071 83 0.017 0.038 0.058 79

C2-C6 0.026 0.050 0.074 85 0.035 0.058 0.081 84 0.019 0.040 0.061 79

C2-C7 0.028 0.045 0.062 82 0.032 0.052 0.071 79 0.025 0.043 0.062 80

C3-C4 0.027 0.055 0.084 79 0.027 0.052 0.077 82 0.017 0.042 0.066 81

C3-C5 0.027 0.065 0.103 84 0.031 0.061 0.090 83 0.022 0.047 0.071 79

C3-C6 0.033 0.068 0.103 84 0.037 0.066 0.095 81 0.025 0.053 0.081 79

C3-C7 0.036 0.062 0.087 84 0.037 0.066 0.095 78 0.032 0.057 0.081 79

C4-C5 0.014 0.049 0.084 86 0.015 0.033 0.051 83 0.011 0.031 0.052 81

C4-C6 0.026 0.065 0.103 87 0.033 0.054 0.075 81 0.015 0.038 0.061 81

C4-C7 0.040 0.064 0.088 86 0.029 0.048 0.068 77 0.025 0.049 0.073 81

C5-C6 0.014 0.052 0.090 93 0.012 0.029 0.046 89 0.008 0.027 0.047 78

C5-C7 0.034 0.061 0.089 92 0.022 0.038 0.055 85 0.019 0.040 0.061 78

C6-C7 0.026 0.050 0.073 97 0.020 0.036 0.052 90 0.016 0.036 0.057 78

Figure Legend

Figure 1. Hox expression patterns (A) in the cranial base and cervical vertebrae, following Kessel and

Gruss (1991) and Buchholtz et al. (2012), and hypothesized cervical vertebral modules based on Hox expression (B) and functional considerations (C), following Graf et al. (1995b). Dashed lines indicate relationships that were hypothesized by Randau and Goswami (2017a) but not tested in that study.

Figure 2. Measurements collected on the (A) cranial base and on the (B-C) typical cervical vertebrae.

In A), an inferior view of the human basicranium, only bitympanic breadth is not shown. In B), a cranial view of a human C6 vertebra, and C), a lateral view of a human C3 vertebra, only caudal facet breadth is not shown as it is not visible in either view. Numbers correspond to those in Table 1 for the cranial base and typical cervical vertebrae. Note that some measurements were taken at a different orientation than shown here (e.g. vertebral body breadth was taken from the caudal edge). Specialized measurements of the atlas (C1) and axis (C2) are not shown.

Figure 3. Heat maps showing integration among the cranial base (CB) and cervical vertebrae (C1 through C7). The number within each cell represents the mean value for the integration statistic for each pair of elements. The cranial base is strongly integrated with all cervical vertebrae, C2 and C7 are weakly integrated with other cervical elements, and C3 through C5 form a strong module that tapers off in C6 and C7.

Figure 4. Heat maps showing VE among the cranial base (CB) and cervical vertebrae (C1 through

C7). The number within each cell represents the VE value for each pair of elements. C3 through C5 are strongly integrated with each other. Hylobates tends to have significantly lower integration than

Homo and Pan in C2 through C3, and Pan has significantly higher integration in CB-C5, CB-C7, C1-

C5, C4-C5, C4-C7, C5-C6, and C5-C7 than either Homo or Hylobates.

Figure 5. Heat maps showing evolvability among the cranial base (CB) and cervical vertebrae (C1 through C7). The number within each cell represents the mean value for the evolvability statistic for each pair of elements, represented as e x 101 to facilitate viewing. C1 displays the highest evolvability by itself and in its relationship to other vertebrae, and C2 displays the lowest, both by itself and in relationship to other vertebrae.

Figure 6. Heat maps showing conditional evolvability among the cranial base (CB) and cervical vertebrae (C1 through C7). The number within each cell represents the mean value for the conditional evolvability statistic for each pair of elements, represented as e x 102 to facilitate viewing. Individual vertebrae have the highest conditional evolvability, and it is higher in C1, C2, C3, C6, and C7 than in

CB, C4, or C5. However, confidence intervals are large and these differences are not significant.

Figure 7. Heat maps showing integration among the ventral and dorsal portions of the cervical vertebrae (C1 through C7). For the individual species, heat maps show patterns within the ventral or dorsal aspects of the vertebral column. For the averages, heat maps show the comparison of ventral versus dorsal patterns. The number within each cell represents the mean value for the integration statistic for each pair of elements. The ventral and dorsal portions within C2 and within C7 are particularly weakly integrated. A strong C3 through C5 module is present in the ventral portion of the vertebrae, but may be shifted into a C4 through C6 module in the dorsal portion. Integration is higher among the dorsal elements than among the ventral elements throughout.

Figure 8. Heat maps showing integration among the ventral and dorsal portions of the cervical vertebrae (C1 through C7). For the individual species, heat maps show patterns within the ventral or dorsal aspects of the vertebral column. For the averages, heat maps show the comparison of ventral versus dorsal patterns. The number within each cell represents the mean value for the VE statistic for each pair of elements. There is high VE between C3-C4, C3-C5, and C4-C5 of the ventral portion in

all three taxa, but this may be shifted into a C4 through C6 module in the dorsal portion. VE is generally higher among the ventral portions of the vertebrae than in the dorsal portions.

Figure 9. Heat maps showing evolvability among the ventral and dorsal portions of the cervical vertebrae (C1 through C7). For the individual species, heat maps show patterns within the ventral or dorsal aspects of the vertebral column. For the averages, heat maps show the comparison of ventral

versus dorsal patterns. The number within each cell represents the mean value for the integration statistic for each pair of elements. C1 consistently shows the highest evolvability, and evolvability is higher among the dorsal elements than among the ventral elements throughout.

Figure 10. Heat maps showing conditional evolvability among the ventral and dorsal portions of the cervical vertebrae (C1 through C7). For the individual species, heat maps show patterns within the ventral or dorsal aspects of the vertebral column. For the averages, heat maps show the comparison of ventral versus dorsal patterns. The number within each cell represents the mean value for the conditional evolvability statistic for each pair of elements, represented as e x 101 to facilitate viewing.

Conditional evolvability is highest in the cranial base and its relationships across taxa, and is similar in the dorsal and ventral portions of the cervical vertebrae.