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The Evolutionary History of the and Trevathan 1995:162). Rotation is Modern Birth Mechanism: necessary because the fetal head is largest in Looking at Skeletal and Cultural the sagittal dimension. Structurally, the maternal birth canal is comprised of Adaptations misaligned planes, including the pelvic inlet, midplane, and the pelvic outlet. The largest Sharon Buck dimension at the pelvic inlet is the transverse direction and the largest at the Introduction pelvic outlet is the sagittal direction The development of bipedalism (Rosenberg and Trevathan 1995:162). changed the course of hominin evolutionary Therefore, the fetus rotates so that the history. One of the most significant impacts sagittal dimension can line up with the it had, outside of locomotion, was on birth. largest diameter of each plane (Wittman and The advent of bipedalism, combined with Wall 2007:745). The fetus emerges in the later encephalization, has placed many occiput anterior position because of the evolutionary constraints on the birth process. rotation. This can cause problems which As a result of evolutionary pressures, a require assisted birth. While the occiput series of skeletal and cultural adaptations anterior position has problems, it is occurred making the modern birth process preferred because it is most favourable for unique. This paper reviews the literature and distributing the forces of uterine contraction fossil evidence and will outline the along the fetal spine, and it leads to the most differences between three forms of birth efficient cervical dilation. Another benefit of mechanisms: modern, ancient and non- this position is that it results in the most human primate. A look at the variability efficient fetal drive down the birth canal within these processes will lead to a more (Trevathan 1988:674). complete understanding of evolutionary The process of birth involves many history and a more critical analysis of the complicated movements of the fetus through fossil record. This review of research on the the birth canal. The biomedical community evolution of human birth mechanisms is typically describes an eight step process essential, not only to understand our history based on the gynecoid shape, known as the but also to apply these insights in a modern cardinal movements (Figure 1) (Trevathan context. Knowledge of human and non- 1988: 675). Although understanding the human primate birth mechanisms is useful cardinal movement can assist in under- for the biomedical community, primate standing the birth process, this process can conservationists and socio-cultural anthro- be different based on natural variability in pologists trying to understand birthing pelvic morphology and/or fetal presen- practices throughout different cultures. tation. As laid out by Trevathan (1988:676), the first step consists of the fetus engaging Modern birth mechanism the birth canal, typically in the oblique or The features of the modern birth transverse position; the fetus then descends mechanism have a long evolutionary history through the pelvic inlet and begins to enter a that correlates to bipedal locomotion and the position of flexion (Trevathan 1988:675). process of encephalization. Three main Flexion is assumed to allow the smallest features characterize the modern birth dimension of the cranium to descend first. mechanism: rotation, navigation of the three Internal rotation, the most recognized pelvic planes and the emergence of the fetus feature, occurs so that the head can line up in the occiput anterior position (Rosenberg

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TOTEM vol 19 2010-2011 TOTEM 83 with the largest dimension of the pelvis and and hyperandrogenism. Platypelloid-shaped to allow the shoulders to pass through the pelves are seen as the least desirable shape pelvic inlet. After the internal rotation, an for birth as it places the most constraints on extension movement occurs where the fetal fetal head size and shoulder width (Walrath head passes under the pubis. The head then 2003:10). While platypelloid pelves can returns to the original transverse position make birth more difficult, the shape has also after the shoulders pass the pelvic inlet, been termed “ultra human.” This term is which is known as restitution (Trevathan used because in modern women a platy- 1988:676). Restitution is followed by pelloid shape often correlates to attaining external rotation of the emerged cranium, erect posture early in life, and bipedal which reflects the internal rotation of the posture is considered both biologically and shoulders navigating the pelvic outlet. The culturally to be an important human trait final step of expulsion is marked by the (Abitbol 1996:242). Other factors that can emergence of the anterior shoulder followed affect pelvic shape include amount of by the posterior shoulder (Trevathan 1988: physical activity during childhood, genetic 676). factors, obstetric requirements, environ- The steps outlined above are merely mental factors, cultural factors, and hormone rough guidelines for understanding the birth imbalances in females as seen in some mechanism. The biomedical concept of a women with android-shaped pelves (Abitbol singular mechanism has many flaws. For 1996:243). It should be noted that these example, Walrath (2003:13) stated that shapes are broad categories and there can be according to a study where neonates were intermediate pelvic shapes (Figure 3). The not actively ‘delivered’ or manipulated by pelvic typology just discussed only the birth assistant, the posterior shoulder considers the amount of variability in a typically emerges first. Another line of minimal sense; this can affect the perception evidence comes from a study showing that of ancient pelvic shapes by trying to make in some cases the occiput anterior position them fit into one of these categories. does not occur. According to a study of Secondary altriciality is regarded as 40,000 births conducted by Walrath (2003: one of the most important factors of modern 15), there was a 20% occurrence of birth, as it has vast implications for the malpositions. Clearly, the variability evolution of encephalization. Secondary inherent in these cardinal birth movements altriciality is defined as the infant emerging needs to be studied in depth. in a state of helplessness due to having only Pelvic morphology is another factor a third of the brain size relative to mature that affects modern birth. There are four adults (Walker and Ruff 1993:227). Within main types described: gynecoid, anthropoid, the first year of infancy, their brains develop android, and platypelloid (Figure 2). to more than 2/3 of their adult counterparts Gynecoid is seen as the ideal pelvic shape as infants have a fetal rate of brain develop- for birth even though it occurs in less than ment throughout their first year (Walker and 50% of women (Walrath 2003:11). Anthro- Ruff 1993:227). This rate of brain growth is poid pelves, while differently structured than so substantial that anthropologists have gynecoid examples, similarly result in an referred to this period as exterogestation or efficient birth mechanism (Walrath 2003: external gestation (Rosenberg and Trevathan 10). Android is a typically male type pelvis; 1995:166). This adaptation essentially for this reason, when it is seen in females, it allows encephalization despite the con- is often associated with hormone imbalances

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straints of the bony pelvic canal because the primate birth mechanism and its similarities fetal rate of brain growth continues for the and differences. first year of life. Some anthropologists believe that secondary altriciality only Nonhuman primate birth mechanism developed in the genus Homo around 1.5 Nonhuman primate birth is mya, although this is hard to establish considerably different than the modern (Walker and Ruff 1993:233). This human birth mechanism. The main adaptation had a large impact on the social differences in the birth mechanism include structure and life history of our ancestors. the following: that the fetus engages the The modes of parental care would have birth canal anteriorly, that there is no consequently changed drastically due to rotation during descent and that the neonate raising such a helpless neonate (Rosenberg emerges in the occiput posterior position. and Trevathan 1995:166-67). As with many These differences are a result of the simple features of birth, the impact of this cylindrical shape of the quadrupedal pelvis evolutionary process has profound impli- (Walrath 2003:16). It is believed that birth in cations for our species. Having a basic nonhuman primates is considerably less knowledge of the modern birth process is complex than it is for humans because of necessary to understanding the non-human pelvic morphology. Walrath (2003:17), however, pointed out that nonhuman

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primates have rarely been observed giving evolution occurring after the origin of birth in the wild. The captive nonhuman bipedalism. This analysis of the nun-human primate populations unfortunately cannot primate birth mechanism allows for the answer this question as caesarean sections academic exploration of the ancient birth are typically undertaken. Contrary to this mechanism in order to recognize how the belief in ease of birth, there is some mechanism changed. evidence of difficult birth among nonhuman primates. These difficulties may arise from Ancient obstetric mechanism the general encephalization of primates as The fossil evidence that the ancient well as the trend of having closely matched birth mechanism is based on is very limited; dimensions between the pelvic canal and the therefore, the variability may not be fully fetal head (Walrath 2003:16). Recognizing understood or appreciated. Of the fossil the possibility of “human”-like difficulty in remains, anthropologists can really only use nonhuman primates is also important in A.L. 288-1 (Lucy) and Sts 14 to understand understanding the ease or difficulty of birth australopithecine pelvic canals (Tague and in the genera Australopithecus and early Lovejoy 1986:238). Many anthropologists Homo. There is the possibility that birth in see A.L. 288-1, an Australopithecus nonhuman primates as described in the afarensis, as the best specimen for recon- literature is oversimplified to make the structing the ancient birth canal because of modern birth mechanism seem more its remarkable preservation. The pelvis of complex (Walrath 2003:14). The level of A.L.288-1 consists of a complete sacrum variability seen in nonhuman primates needs and left innominate. The entire pelvis was to be analyzed in a more systematic manner reconstructed using mirror imaging (Tague in order for anthropologists to understand and Lovejoy 1986:238). According to Tague which modern birth features are the result of and Lovejoy (1986:237), A.L.288-1 is

TOTEM vol 19 2010-2011 TOTEM 86 indeed a female but differs in the sexually cranial capacity of the fetus. An dimorphic areas found in modern females. Australopithecus fetus would have engaged There are many features of A.L.288-1 that the birth canal as Homo did, transversely. are interpreted as male-like, such as the The australopithecine fetus would have angulation of the sacrum, the overall funnel continued descent along the transverse shape, the intermediate subpubic angle and diameter and there would have been no the straight ischiopubic ramus (Tague and internal rotation (Tague and Lovejoy 1986: Lovejoy 1986:251). The only modern 248). Another modern human-like feature female trait that A.L. 288-1 has is a long that A.L. 288-1 might have displayed was iliopectineal line, which, in fact, probably occiput anterior emergence patterns. Based provided no obstetric advantages (Tague and on pelvic morphology, this emergence Lovejoy 1986:243). Overall, the morph- pattern may have occurred in approximately ology of A.L.288-1’s pelvis in each plane 50% of australopithecine births. This emer- results in a hyperplatypelloid shape which gence pattern would have occurred due to constricts the bony birth canal (Tague and the external rotation which allows the Lovejoy 1986:244). The authors suggested shoulders to pass through the pelvic outlet. that the long and extremely curved This possibility of the neonate emerging in iliopectineal line greatly influenced the the occiput anterior position may have platypelloid shape seen in A.L. 288-1 resulted in the social imperative of assisted (Tague and Lovejoy 1986:244). birth (Rosenberg and Trevathan 2003: 84) The hyperplatypelloid shape is a The pelvis of Sts 14, an result of a compromise between bipedality, Australopithecus africanus, is not as well birth and support of viscera (Tague and preserved as that of A.L.288-1; it is Lovejoy 1986:250). The three skeletal represented by the first two sacral vertebrae, adaptations that result in a platypelloid a nearly complete right os coxae, and pelvis include: an elongated ilia which elements of the left os coxae which were repositions gluteal muscles, decreased fully reconstructed using extrapolation and distance between the sacroiliac and hip mirror imaging (Abitbol 1995:143). The joints, and a ventrally titled pelvis that aligns overall morphology of the Sts 14 pelvis was the sacroiliac and hip joints and ultimately one of platypelloidy, although the shape was results in lumbar lordosis (Tague and less extreme than that of A.L. 288-1 Lovejoy 1986:250). All of these adaptations, (Abitbol 1995:157). There were some which support viscera and posture, create a problems with the reconstruction because short and narrow pelvis. In order to rectify three of the sacral vertebrae were entirely this problem, the ilia are flared laterally and based on extrapolation. Moreover, the the sacrum is broadened. This combination results of the reconstruction showed that the of adaptations results in the platypelloid pelvis had a reduced posterior sagittal pelvis of A.L. 288-1 (Tague and Lovejoy diameter and a similar sacral angulation to 1986:250). A.L.288-1, which is deemed male-like The birth mechanism of A.L.288-1 (Abitbol 1995:156). The overall features of would have proceeded with some features of the pelvis in Sts 14 suggest that birth was modern birth but without any rotation most likely nonrotational (Abitbol 1995: (Rosenberg and Trevathan 2003:84). Tague 157). and Lovejoy (1986:248) argue that birth in Many significant features of modern A.L. 288-1 would have been easy despite birth, such as rotational birth and the infant the platypelloid shape, due to the small being born in a state of secondary

TOTEM vol 19 2010-2011 TOTEM 87 altriciality, probably developed during the mya, just prior to the emergence of early forms of the genus Homo when there H.erectus. was a leap in encephalization (Wittman and The Gona H. erectus pelvis was Wall 2007:740). Unfortunately, there is only discovered in the Afar region of Ethiopia one female H. erectus pelvis. Prior to the and has been dated to 0.9 to 1.4 mya discovery of this pelvis, the lack of fossil (Simpson et al. 2008:1089). There is some evidence led Walker and Ruff (1993) to deformation in the pelvis due to in situ postulate an ancient birth mechanism using fracture and the cementation of some KNM-WT 15000 (Nariokotome Boy), an fragments. Despite the reconstruction of the adolescent male Homo erectus, as the pelvis, some asymmetry was still present model. The pelvis of KNM-WT 15000 though not enough to dismiss the analysis. consists of parts of all three pelvic elements. Pelvic features of the Gona pelvis that are Reconstruction was undertaken using mirror important for birth include an antero- imaging as well as reference to KNM-ER posteriorly broadened birth canal, a greater 3228 (male H. erectus) and a modern male transverse diameter of both the pelvic mid- pelvis (Walker and Ruff 1993:222). The plane, and an outlet than is often seen in authors use the reconstruction to posit that modern females (Simpson et al. 2008: secondary altriciality had developed by 1.5 1090). These features suggest that the Gona mya. They worked under a series of H. erectus female was able to give birth to a assumptions: that KNM-WT 15000 was fetus with a cranial capacity up to 315 ml. male, that he would have grown in a manner This cranial capacity would have meant that similar to modern humans, and that the H. erectus could have experienced modern- amount of sexual dimorphism in H. erectus like prenatal brain growth as well as a is similar to modern humans (Walker and postnatal brain development rate inter- Ruff 1993: 231). They compare the lengths mediate between modern humans and of the curve between the articular surface chimpanzees (Simpson et al. 2008:1090). and the sciatic spine, to other early Homo This illustrates that by the early Pleistocene, specimens for which sex is known and to secondary altriciality was necessary to modern males and females. The results continue the process of encephalization and reinforced the fact that KNM-WT 15000 is that the human pelvis was still undergoing male and provided ratios for the degree of changes to accommodate both bipedalism sexual dimorphism (Walker and Ruff 1993: and encephalization. 231-32). Using the ratios obtained, Walker Known, identifiable fossils are and Ruff (1993:232) compared the expected incredibly important for understanding the transverse diameter of the female pelvis to ancient birth mechanism. However, the adult cranial capacity of H. erectus problems with the fossil record include its specimens. These two measurements helped limited nature, the integrity of the fossils, establish whether there were enough pelvic and the lack of soft tissue preservation. As constraints to necessitate a state of discussed previously, there are truly only secondary altriciality to reach adult cranial two fossils on which to base australop- capacity (Walker and Ruff 1993:233). The ithecine birth mechanisms. For early Homo, conclusion of the study indicated that an there are only Nariokotome Boy (which has adult cranial capacity of 900 cc or above problems in terms of accuracy of would require secondary altriciality (Walker reconstruction) and the recently discovered and Ruff 1993:233). Therefore, secondary Gona specimen. There are additional altriciality probably developed around 1.5 problems with the Nariokotome Boy

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analysis, specifically in terms of the “cultural universal”. Assisted birth is assumptions made by Walker and Ruff significant because of the difficulties (1993). In their study, Thompson and associated with the occiput anterior position. Nelson (2000) questioned a central assump- When modern females attempt to self- tion about the growth and development of deliver, they run the risk of bending the Nariokotome Boy suggesting that he would neonate against the natural curvature of the not have grown in a manner similar to spine. This pull force can damage the modern humans but rather in a manner neonate’s spinal cord, brachial plexus and similar to gorillas (486). This growth pattern other muscles and bones (Rosenberg and invalidates most of the obstetric data gained Trevathan 1995:163). Another difficulty is from Nariokotome Boy. There are also that a self-delivering modern mother cannot disagreements in the anthropological realm efficiently clear the airways of the neonate. regarding the usefulness of both A.L. 288-1 Additionally, if there are complications such and Sts 14. Many anthropologists argue that as the umbilical cord being tangled or A.L. 288-1 is not a good specimen for wrapped around the neonate, the mother reconstruction because of the amount of post often does not have enough leverage to mortem deformation. Some academics even remedy these problems (Rosenberg and postulate that A.L.288-1 may in fact be a Trevathan 1995:163). These complications male, which obviously invalidates any do not mean that modern women cannot reconstruction of the birth canal (Rosenberg self-deliver, but even a slight reduction in and Trevathan 1995:166). Finally, the lack infant and maternal mortality is important of soft tissue preservation hinders enough for this behaviour to be reconstruction efforts. The placenta deter- evolutionarily adaptive (Trevathan 1988: mines the physiology of the birth mechan- 679). Having examined assisted birth, a look ism, and despite all the calculations based on at the skeletal adaptations impacting the the bony birth canal, if the placenta was birthing process is warranted. significantly different than modern humans, the anthropological conception of the Skeletal adaptations ancient birth mechanism may be Since the advent of obligate fundamentally flawed (Walrath 2003:15). bipedalism, a whole set of skeletal Comprehending the three types of birth adaptations have had to occur for it to be an mechanism illustrated above permit an efficient means of locomotion. Some of the examination of the cultural and skeletal changes necessary included anterior move- adaptations as well the complications that ment of the foramen magnum, anterior can occur in the modern birth mechanism. displacement of the sacrum (which created cervical and lumbar curves), lengthening of Assisted birth the lower extremities, development of Seeking assistance during birth is a valgus knees, development of stable planter uniquely human behaviour. Many anthro- foot, loss of the divergent big toe, and the pologists believe that assisted birth is a alteration of the pelvis from a simple social evolutionary adaptation that occurred cylinder to misaligned planes (Wittman and to decrease the mortality associated with Wall 2007:740). The changes in the pelvis birth. The importance of this social have had great significance for birth behaviour is demonstrated through the term mechanisms. ‘obligate midwifery’ and Rosenberg and The changes in the female pelvis are Trevathan (1995:164) deemed it almost a incredibly important because there has to be

TOTEM vol 19 2010-2011 TOTEM 89 a compromise between the functions of wedged vertebrae while males only have locomotion, posture, support of viscera, and two (Whitcome et al. 2007:1076). The third birth (Tague and Lovejoy 1986:237). The wedged vertebra allows an increase in two largest requirements for successful birth lordosis without dangerous intervertebral throughout hominin evolution have been the rotation. This is extremely important accommodation of encephalization and because women compensate for fetal load by broad rigid shoulders. The changes in the extending the lower back by as much as 28o pelvis that result from birth requirements are (Whitcome et al. 2007:1075-76). This is not as significant as one might expect. This significant since, during pregnancy, the may be due to the malleability of the fetal mass of the abdomen increases up to 31% head and the fact that the fetus is born in a (Whitcome et al. 2007: 1075). If there was state of secondary altriciality (Wittman and no increased reinforcement in the lumbar Wall 2007:740). There are, however, a few region of females, there would be associated important pelvic changes resulting from problems including shearing forces along bipedalism that affect birth. One such the spine, increased risk of forward change is the broadening of the sacrum and displacement of lumbar vertebrae and its movement caudally. While this change greater lower back pain during pregnancy relates strictly to bipedalism and is not a (Whitcome et al. 2007:1075). The fact that sexually dimorphic trait, a broader sacrum females have three wedged vertebrae increases the transverse diameter of the birth reduces the shearing forces along the lumbar canal (Wittman and Wall 2007:742). The region up to 30% (Whitcome et al. 2007: ischial spines are more prominent for greater 1076). The lumbar region is clearly integral muscle attachment to stabilize viscera to the health of pregnant females so it during erect posture which constricts the undergoes strong selective pressure and a midplane of the pelvis (Wittman and Wall similar degree of sexual dimorphism in this 2007:743). area is seen as early as the australopithecines Another significant skeletal adapt- (Whitcome et al. 2007:1076). Despite this ation was lumbar lordosis, which relates to suite of impressive skeletal adaptations maternal health and the accommodation of allowing for bipedalism and successful birth, fetal load. The modern human skeleton there are still some problems that bipedal shows an adaptation to bipedalism in the locomotion introduced into the birthing lumbar region of the spine. This modifi- process. cation manifests itself as a derived concavity in the lumbar region (Whitcome et al. 2007: Complications 1075). This concavity developed because Complications in the birthing process there are a different number of vertebrae, occur as a result of the evolution of large different lengths of vertebrae and wedged brained offspring and a small, narrow birth vertebrae in bipeds when compared to canal. Two serious complications result quadrupeds. Lordosis is necessary to stab- from the constraints of the pelvic canal. ilize the upper body and to place the center Cephalopelvic disproportion is a compli- of mass over the hips (Whitcome et al. cation when the fetal head is too large to 2007:1075). The lumbar region is a very enter the birth canal. The fetus becomes sexually dimorphic region of the skeleton blocked and birth cannot progress because females need additional curvature to (Rosenberg and Trevathan 1995:161). The center their weight when carrying fetal load second serious complication is shoulder (Figure 4). Modern females have three dystocia which occurs when the fetal head

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enters the birth canal, but the shoulders the cesarean section has allowed for greater become lodged at some point during the survival rates (Weiner et al. 2008: 469). navigation of the pelvic planes (Trevathan Some anthropologists believe that 1988:677). These complications have difficulty during birth is merely a myth typically led to death for both the infant and perpetuated by the biomedical community the mother in the past, but the invention of (Walrath 2003:14). The notion that birth is a

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“scar of evolution” or that an obstetrical long lasting impacts. The consequent dilemma is a prevalent idea since birth now process of encephalization placed con- takes place in a hospital setting (Walrath flicting pressures on the birth process. The 2003:14). The idea of difficult birth also evolution of the ancient obstetric mechanism comes from the singular birth mechanism. to the modern obstetric mechanism is Evolutionary theory typically focuses on the fascinating, as conflicting pressures existed variability in humans; however, for this with regards to bipedalism and the particular topic, a monotypic birth continuation of our genus. The compromises mechanism is postulated. It is possible that made in the female body have resulted in a complications are merely misinterpretations unique human birth mechanism. Under- of variability (Walrath 2003:15). There are standing the suite of biological and cultural different birth mechanisms prevalent in adaptations in the most complete sense is other cultures. One such example of an essential for anthropologists to comprehend alternate birth mechanism is women from and critically analyze the fossil record. The other cultures who deliver in a squatting evolution of birth is critical because it has position rather than in a hospital bed the first and most direct impact on natural (Rosenberg and Trevathan 1995:163). selection as any serious complications may Walrath (2003) also points out that tolerance result in death for mother and fetus. Due to for birth variability is steadily decreasing. this immediate effect on the gene pool, it has This lack of tolerance is seen in length of shaped human evolution more than any birth; if females do not deliver within 14 other skeletal or cultural adaptation. hours, operative delivery is undertaken. This has decreased from 24 hours (Walrath 2003: 9). In addition, physicians often measure Acknowledgements the maternal pelvis to decide if vaginal labour will be allowed or if operative I would like to thank my family for their delivery should be undertaken (Walrath endless support in everything I strive for, as 2003:13). The “normal” birth mechanism is well as Samantha Gordon for being my contested through changing definitions of faithful editor. Finally, I would like to thank normal fetal presentation in the medical Professor Andrew Nelson for encouraging community and debate on which shoulder me to submit my paper to Totem in the first should be delivered first (Walrath 2003:13). place. Walrath (2003:18) states that in order to understand how birth progressed in the past, Works Cited a greater understanding of modern day biological and cultural variability must be Abitbol, Maurice M. 1995. Reconstruction undertaken first, as well as a closer of the STS 14 (Australopithecus examination of nonhuman primate birth africanus) Pelvis. American Journal mechanisms. of Physical Anthropology 96:143- 158. Conclusion The development of bipedalism had ———. 1996. The Shapes of the Female many effects on human evolution but one of Pelvis: Contributing Factors. Journal the more significant impacts was on the of Reproductive Medicine 41:242- birth process. An array of skeletal 250. adaptations took place which have had had

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Rosenberg, Karen and Wenda R. Trevathan. Weiner Stuart, Janet Monge and Alan Mann. 1995. Bipedalism and Human Birth: 2008. Bipedalism and Parturition: an The Obstetrical Dilemma Revisited. Evolutionary Imperative for Evolutionary Anthropology 4:161- Cesarean Delivery? Clinics in 168. Perinatology 35:466-478.

———. 2003. The Evolution of Human Whitcome, Katherine K., Liza J. Shapiro Birth. Scientific American 13:80-85. and Daniel E. Lieberman. 2007. Fetal Load and the Evolution of Simpson, Scott W., Jay Quade, Naomie Lumbar Lordosis in Bipedal Levin, Robert Butler, Guillaume Hominins. Nature 450:1075-1080. Dupont-Nivet, Melanie Everett, and Sileshi Semaw. 2008. A Female Wittman, Anna Blackburn and L. Lewis Homo erectus Pelvis from Gona, Wall. 2007. The Evolutionary Ethiopia. Science 322:1089-1092. Origins of Obstructed Labor: Bipedalism, Encephalization, and the Tague, Robert G. and C. Owen Lovejoy. Human Obstetric Dilemma. 1986. The Obstetric Pelvis of A.L. Obstetrical and Gynecological 288-1 (Lucy). Journal of Human Survey 62:739-748. Evolution 15:237-255.

Thompson, Jennifer L. and Andrew J. Nelson. 2000. The Place of Neandertals in the Evolution of Hominid Patterns of Growth and Development. Journal of Human Evolution 38:475-495.

Trevathan, Wenda R. 1988. Fetal Emergence Patterns in Evolutionary Perspective. American Anthropologist 90:674-681.

Walker, A. and Christopher B. Ruff. 1993. The Reconstruction of the Pelvis, in The Nariokotome Homo Erectus Skeleton. Edited by A. Walker and R. Leakey, 221-233. Cambridge, Massachusetts: Harvard University Press.

Walrath, Dana. 2003. Rethinking Pelvic Typologies and the Human Birth Mechanism. Current Anthropology 44:5-31.

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