LETTER

Pitfalls reconstructing the last common ancestor of chimpanzees and humans LETTER Sergio Almécijaa,b,1

The morphological nature of the last common ancestor evolutionary model from Pan into humans relies solely (LCA) of chimpanzees/bonobos and humans is a fasci- on the intermediate position (in PC1) of a “simulated” nating topic in human evolution. Available evidence Australopithecus afarensis adult, based on the “Dikika suggests that both lineages share a LCA that lived in baby” (∼3-y-old at death). However, one of these au- Africa ∼8–6 Myr. However, the hominoid fossil record of thors has previously shown that an actual A. afarensis this time period is inadequate, prompting the use of adult falls within the human and orangutan variation novel methodological approaches to reconstruct the (figure 2B in ref. 3). Furthermore, this simulated Dikika anatomy of this key ancestor. In a recent example, adult is—like humans—more similar to Nasalis than Young et al. (1) claim that “fossil hominin shoulders sup- to African apes (figure 2D and table S1 in ref. 1; in- port an African ape-like last common ancestor of hu- congruent with figure S5 in ref. 1). Fourth, Young et al. mans and chimpanzees.” Although I applaud these do not actually test alternative models of evolution authors for implementing novel comparative methods (see a formal test including convergence in ref. 4). In- to paleoanthropology, fundamental aspects of the ex- stead, they only compare the morphometric lengths perimental design were disregarded, leading to biased of two phylogenetic trees on morphospace. Fifth, all conclusions. Herein I provide constructive criticisms on branches in their trees were unfoundedly assumed to that study, also addressing recurrent pitfalls in the field, have equal length, forcing a punctuated model of evo- to help redirect future research on human paleobiology. lution (change being independent of time, contrary to First, the biological meaning of the shape data in their own conclusions), and disregarding the molecu- the Young et al. (1) analysis is unclear. Procrustes su- larly established branch lengths of most species in perimposition allows one to elegantly size-adjust data their sample (Fig. 1). Some of these pitfalls are also to analyze pure shape. However, Young et al. use noted by Melillo (5), who further highlighted functional allometric residuals as their input data without dem- aspects preserved in the scapula of “Kadanuumuu,” onstrating the existence of a common allometric trend an adult specimen of A. afarensis. in the sample, predicting a significant amount of scap- Perhaps the most relevant message of the study ular shape variation. This can fail to correctly identify was overlooked: Homo and Pan share ∼99% of their individuals of the same shape at different size, pro- genes, even given their remarkable phenotypic differ- ducing misleading results (2). Later, Young et al. (1) ences (resulting from gene regulation), demonstrating argue that scapular differences between Pan and Go- that phylogenetic proximity does not predict pheno- rilla are associated with body size, undermining the typic evolution. Thus, if indeed the LCA exhibited a assumption about their “allometrically adjusted” data. Pan-like scapula, given the mosaic nature of ape and Second, Young et al. favor a Pan-like LCA even though human evolution and that other anatomical regions their results show that Pan and Homo evolved in op- reveal alternative stories (4, 6), this study (1) highlights posite directions along the major axis of shape varia- even more the complexity of reconstructing the LCA’s tion (PC1; figure 2 E and F in ref. 1). Third, the linear mosaic anatomy.

aCenter for the Advanced Study of Human Paleobiology, Department of Anthropology, The George Washington University, Washington, DC 20052; and bInstitut Català de Paleontologia Miquel Crusafont, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Spain Author contributions: S.A. designed research and wrote the paper. The author declares no conflict of interest. 1Email: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1524165113 PNAS | February 23, 2016 | vol. 113 | no. 8 | E943–E944 Downloaded by guest on September 27, 2021 ?

Myr 0 Pan Homo Gorilla Pongo 5 Hylobates LCA: ‘Pan-cestor’ paradigm 10 LCA: complex mosaic Sivapithecus 15 Pierolapithecus 20

Fig. 1. Time-calibrated hominoid phylogeny highlighting the position of the LCA of Pan and Homo. The “Pan-cestor” paradigm (red), assumes that Pan represents a static living fossil undistinguishable from the LCA, constituting the “starting point” for human evolution (arrow). However, fossil hominoids (exemplified by Pierolapithecus and Sivapithecus) indicate that homoplasy and mosaicism were the rule, not the exception, during ape and human evolution. Under this model (green), the unique combination of plesiomorphic and autapomorphic features exhibited by each lineage (including Pan and Homo) and the evolutionary time (branch lengths) must be considered when modeling the tempo and mode of evolution and reconstructing ancestral morphologies.

1 Young NM, Capellini TD, Roach NT, Alemseged Z (2015) Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees. Proc Natl Acad Sci USA 112(38):11829–11834. 2 Jungers WL, Falsetti AB, Wall CE (1995) Shape, relative size, and size-adjustments in morphometrics. Yearb Phys Anthropol 38:137–161. 3 Green DJ, Alemseged Z (2012) Australopithecus afarensis scapular ontogeny, function, and the role of climbing in human evolution. Science 338(6106):514–517. 4 Almécija S, Smaers JB, Jungers WL (2015) The evolution of human and ape hand proportions. Nat Commun 6:7717. 5 Melillo SM (2015) An alternative interpretation of the Australopithecus scapula. Proc Natl Acad Sci USA 112(52):E7159. 6 Almécija S, et al. (2013) The femur of Orrorin tugenensis exhibits morphometric affinities with both Miocene apes and later hominins. Nat Commun 4:2888.

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