Research ArticleANTHROPOLOGY

Ardipithecus hand provides evidence that humans and chimpanzees evolved from an ancestor with suspensory adaptations

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Science Advances  24 Feb 2021:
Vol. 7, no. 9, eabf2474
DOI: 10.1126/sciadv.abf2474
  • Fig. 1 PCA on 26 logged geometric mean-standardized variables representing between-species variation in hand shape.

    Each point is a species mean, except among fossil hominins, and colors represent selective regimes identified by SURFACE. Homo and Australopithecus, gold; Ar. ramidus, P. troglodytes, and P. paniscus, purple; G. gorilla, Gorilla beringei beringei, and Gorilla beringei graueri, light green; Pongo pygmaeus and Pongo abelii, orange; Hylobates, Hoolock, Nomascus, and Symphalangus, blue; Macaca, Mandrillus, Cercocebus, Cebus, and Saimiri, dark red; Papio and Theropithecus, pink; Ateles and Brachyteles, light blue; Colobus, tan; remaining platyrrhines, cercopithecins, and colobines, gray. Note that most cercopithecoids and platyrrhines fall along a spectrum of generalized palmigrady to specialized digitigrady. The Ar. ramidus and Au. afarensis hands fall nearest to a hypothetical evolutionary trajectory from a suspensory, Pan-like ancestor instead of a more generalized, monkey-like ancestor. Purple and gray clouds represent hypothetical uncertainties surrounding suspensory Pan-like and generalized ancestral morphotypes, respectively.

  • Fig. 2 MCP and interphalangeal joint shape contributes to suspensory performance.

    (A) Free-body diagram depicting the third ray of a siamang positioned in a hook grip on a horizontal support following (32). The points of application, spatial orientations, and magnitudes of external and internal force vectors, as well as moment arm lengths, are hypothetical. Joint reaction forces are not depicted here for the sake of clarity. SRF, support reaction force (near proximal, middle, and distal phalanges); R, external moment arm; r, internal moment arm; FFD, muscle force vector associated with the flexor digitorum muscles (m. flexor digitorum superficialis and m. flexor digitorum profundus); FFDP, muscle force vector associated with the m. flexor digitorum profundus. Flexor forces applied to the support are the vertical components of FFD and FFDP and the horizontal component of FFD′. (B) Major axis of variance derived from a PCA on six logged geometric mean-standardized measurements (MC5 HML, MC5 HDP, PP3 TML, PP3 TDP, IP3 TML, and IP3 TDP). Positive values of PC1 are associated with decreased MC5 head breadth, increased PP3 TDP, and increased IP3 TDP.

  • Fig. 3 Proximal phalangeal curvature in anthropoid primates.

    The gray bar indicates the range of variation among the Miocene fossil hominoids included here. The curvature of the Ar. ramidus PP3 falls within the ranges of variation of P. troglodytes, Pongo, hylobatids, and atelines; between the highly suspensory Miocene hominoids Danuvius and Hispanopithecus; and above the ranges of variation of all other taxa.

  • Fig. 4 The evolution of anthropoid hand shape according to SURFACE.

    (A) Phylogenetic tree with branches painted according to selective regime. (B) Species means for PC1 and PC2 (small circles) and estimated phenotypic optima (large circles). (C) Species means for PC1 and PC3 (small circles) and estimated phenotypic optima (large circles). Ar. ramidus was placed in the same selective regime as P. troglodytes and P. paniscus. In contrast, all later hominins were placed in a selective regime with modern humans, which results in an evolutionary shift in hominin hand morphology from a Pan-like ancestor between ca. 4.4 and 3.2 Ma ago.

  • Fig. 5 Phylomorphospace plots and ancestral estimations.

    (A) PC1 and PC2 computed from PCA on 26 variables. (B) PC1 and PC3 computed from PCA on 26 variables. (C) PC1 and PC2 computed from PCA on 17 variables including H. laietanus. (D) PC1 and PC3 computed from PCA on 17 variables including H. laietanus. Purple ellipses, 95% credible intervals for the ancestral hand morphology of humans and chimpanzees. Orange ellipses, 95% credible intervals for the ancestral hand morphology of great apes. Points represent species means, except in the case of fossils, and their colors correspond to selective regimes identified by SURFACE for each dataset. OWM, Old World monkey; NWM, New World monkey.

  • Fig. 6 pGLS regression between manual ray segment lengths and body mass in anthropoid primates.

    (A) Relationship between MC1 length and body mass. (B) Relationship between MC5 length and body mass. The MC5 length of Hispanopithecus was estimated from its MC4 length using regression. (C) Relationship between PP3 length and body mass. (D) Relationship between IP3 length and body mass. Black symbols, palmigrady; gray symbols, digitigrady; red symbols, suspensory; blue symbols, knuckle walking; gold symbols, nonlocomotor (manipulative). The gray box surrounding Ar. ramidus represents the range of body mass estimates.

  • Fig. 7 The evolution of hominin hands and feet reflects an evolutionary shift toward enhanced manipulative capabilities and obligate bipedalism, respectively.

    Partial hands, partial feet, and stone tool exemplars are depicted here and supplemented by reference to more fragmentary specimens preserving functionally relevant anatomies. Gray bars, facultative bipedalism; black bars, obligate bipedalism; red bar, approximate timing of hypothesized hominin evolutionary shift. Asterisks indicate that the fossil was mirrored. BAR 349’00, curved juvenile manual proximal phalanx of Orrorin tugenensis; BAR 1901’01, pollical distal phalanx of O. tugenensis with extrinsic flexor insertion; OH 8, foot attributed to Homo habilis; KNM-WT 51260, MC3 with styloid process probably representing Homo erectus; U.W. 101 (foot 1), partial foot of H. naledi.

Supplementary Materials

  • Supplementary Materials

    Ardipithecus hand provides evidence that humans and chimpanzees evolved from an ancestor with suspensory adaptations

    Thomas C. Prang, Kristen Ramirez, Mark Grabowski, Scott A. Williams

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