Homo Sapiens: Anatomically Modern Humans (AMH)

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Homo Sapiens: Anatomically Modern Humans (AMH) Feb 27, 2021 2:56:36 GMT

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Phalangeal curvature

Phalangeal curvature reduces diaphyseal strain associated with suspension (32, 33) and is therefore an important correlate of primate positional behavior (19, 30, 36, 37). To clarify how phalangeal curvature in the Ar. ramidus hand compares to our comparative sample, we quantified the curvature of the PP3 in a large sample of anthropoids and fossil taxa using the included angle method (Fig. 3). The overall pattern of phalangeal curvature in the extant and fossil sample is consistent with that of previous studies (19, 36, 37). The phalanges of Hispanopithecus, Danuvius, and Ar. ramidus are characterized by pronounced curvature, falling within the ranges of variation of the most suspensory anthropoids (Pan, Pongo, hylobatids, and atelines; Fig. 3). The phalanges of the remaining Miocene hominoid taxa (Pierolapithecus, Griphopithecus, Ekembo, Sivapithecus, and Oreopithecus) as well as Au. afarensis and Au. sediba fall within the overlapping ranges of multiple anthropoid taxa including Pan, hylobatids, cercopithecins, and colobines.

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.
Evolutionary modeling

To test alternative hypotheses about the relationship between hand morphology and locomotor behavior, we reconstructed the adaptive landscape of primate hand evolution using phylogenetic comparative methods [(38, 39); see also (5, 22, 40)]. These methods allow us to translate adaptive hypotheses into explicit evolutionary models tested against comparative data in a maximum likelihood framework. They also allow us to estimate evolutionary parameters such as the optimal hand morphology for a given selective regime (i.e., locomotor category).

Our evolutionary modeling analyses use the first three PCs from our PCA to reduce the dimensionality of the dataset. In our a priori adaptive hypotheses (fig. S9), we assigned Australopithecus and Homo to their own selective regime and Ar. ramidus to the same regime as Pan given the results of our morphometric analyses. The first hypothesis (H1) recognizes taxa that are mostly terrestrial (Gorilla beringei, Papio, Theropithecus, and Mandrillus), those that are semiterrestrial (Pan, G. gorilla, Semnopithecus entellus, Macaca mulatta, Macaca nemestrina, Macaca nigra, Cercocebus, and Chlorocebus aethiops), those that are mostly arboreal (all other non-hominin taxa), and those who frequently use their hands in nonlocomotor contexts including Homo and Australopithecus. The second hypothesis (H2) is identical to H1 but separates mostly arboreal quadrupedal taxa from those that are more suspensory (Pongo, hylobatids, Ateles, and Brachyteles). The third hypothesis (H3) includes regimes for digitigrade taxa (M. mulatta, M. nigra, Papio, Theropithecus, Mandrillus, and Cercocebus), knuckle-walking taxa (Pan and Gorilla), suspensory taxa (Pongo, hylobatids, Ateles, and Brachyteles), nonlocomotor taxa (Homo and Australopithecus), and palmigrade taxa (all remaining anthropoids). We tested an alternative hypothesis (H4) based on H3 in which we placed Pan and Ardipithecus in a regime with the suspensory taxa and Gorilla in its own terrestrial knuckle-walking regime (fig. S9). The assignment of cercopithecoid taxa into locomotor or hand posture categories was based on published literature (41). Next, we used SURFACE to identify selective regimes based on morphological similarity without identifying them a priori (39), which has been used previously to test hypotheses of human and primate evolution (5, 22, 40). Last, we tested the relative support of our results with two simpler alternative evolutionary hypotheses including evolution by genetic drift (i.e., Brownian motion; HBM) and adaptation with a single optimum [i.e., Ornstein-Uhlenbeck (OU); HOU1]. Support for HBM or HOU1 would suggest that our data do not reflect an adaptive signal related to locomotor behavior.

Our results show that hand morphology is best described by 11 selective regimes with all a priori adaptive hypotheses performing worse than the SURFACE model (Fig. 4 and tables S6 and S7). The selective regimes identified by SURFACE likely reflect a combination of locomotion and other factors (Fig. 4 and table S7). For example, the best supported a priori model fit using the “ouch” package in R included a convergent suspensory regime containing atelines and Asian hominoids, but SURFACE separated them into distinct regimes that likely reflect their distinct evolutionary histories (e.g., Ateles and Brachyteles pollical reduction) and body sizes. Likewise, we found support for terrestrial knuckle-walking (Pan and Gorilla) and digitigrady (M. mulatta, M. nigra, Papio, Theropithecus, Mandrillus, and Cercocebus) regimes that were subsequently separated by SURFACE. Note that SURFACE identified three regimes representing the spectrum of nonsuspensory quadrupedal hand postures and substrate preferences among cercopithecoids and platyrrhines ranging from arboreal palmigrady to terrestrial digitigrady, mostly reflecting variation in metacarpal and phalangeal lengths relative to the hand geometric mean. Colobus was placed in its own regime nearest the arboreal palmigrady optimum reflecting evolution in the direction of the Ateles and Brachyteles regime associated with pollical reduction. SURFACE placed Ar. ramidus into a regime with Pan paniscus and P. troglodytes, suggesting adaptation to an optimal hand shape related to shared aspects of positional behavior, body size, and evolutionary history. SURFACE detected an evolutionary shift in hand morphology between the Homo-Pan ancestral phenotypic optimum, including Ardipithecus and Pan, and all later hominins primarily related to decreased intrinsic phalangeal length and increased MC1 length.

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.

In addition to testing adaptive hypotheses, the OU model allows for the estimation of evolutionary parameters such as the phylogenetic half-life (t1/2; table S7), which is the average time it takes to evolve half the distance from the ancestral optimum to a new optimum given a regime shift. The phylogenetic half-life (t1/2) therefore quantifies the rate of adaptation. The estimate of t1/2 for PC1, which is the axis separating the ancestral hominin optimum from the primary hominin optimum, is 722 ka (thousand years) (table S7), which is relatively fast evolution compared to the total length of the tree [46.8 Ma (million years)]. As discussed above, PC1 is primarily associated with the intrinsic lengths of the metacarpals and phalanges. The estimates of t1/2 for PC2 and PC3 are 1.26 Ma and 5 ka, respectively. PC2 and PC3 are associated with metacarpal length, joint, and midshaft dimensions. The estimates of the phylogenetic half-life for each PC suggest that anthropoid metacarpal length and midshaft dimensions (i.e., MC1 relative to MC5) have evolved more slowly than phalangeal lengths and articular dimensions (e.g., the ulnar carpometacarpal and MCP joints). We used a Monte Carlo simulation method (42) to evaluate the statistical power to distinguish between alternative evolutionary models given our dataset and phylogeny (fig. S10). The simulation results show that all models can be distinguished from each other and support information criteria metrics [Akaike information criterion (AIC), small sample size corrected Akaike information criterion (AICc), and Schwarz information criterion (SIC)]. Previous studies have criticized the use of SURFACE for multivariate evolutionary modeling (43). We include the use of SURFACE for explicit comparison to recent similar studies (5, 22, 40) and as an additional perspective to our a priori models fit using the ouch package (38) in R. Overall, our use of Monte Carlo simulations provides additional support for the model fit by SURFACE.

We conducted a SURFACE analysis using the first three PC scores derived from our PCA on a modified dataset including H. laietanus (fig. S11 and table S8). SURFACE identified nine select regimes including a convergent regime containing H. laietanus, Pongo, Lagothrix, and Ateles. The locations of the evolutionary shifts estimated by SURFACE resemble our initial analysis using a larger dataset but with some differences. Ar. ramidus is placed in the same selective regime with P. troglodytes and P. paniscus, but expectedly, the analysis on the reduced dataset also places Au. afarensis in this regime since it excludes MC1 data. The Au. afarensis hand clusters with Au. sediba, H. naledi, and H. neanderthalensis in the PCA (fig. S5) but is distinguished from them along PC3 (fig. S11), which is associated with MCP and IP joint dimensions and MC4 length (table S2). Instead, SURFACE detects an evolutionary shift at the base of the Au. sediba and Homo clade. In addition, colobines, cercopithecins, and most papionins are placed in a single regime with evolutionary shifts toward a more terrestrial digitigrady regime in Papio and Theropithecus. Overall, the results of our SURFACE analysis on a modified dataset are consistent with our initial analyses because it recovers an evolutionary shift at the base of the great ape clade associated with suspensory adaptations.

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Homo Sapiens: Anatomically Modern Humans (AMH) Feb 27, 2021 5:55:16 GMT

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Ancestral estimations of human and great ape hand morphology

We estimated the ancestral hand morphology for humans and great apes using StableTraits software (44). StableTraits uses a stable model of continuous trait evolution to estimate ancestral values without assuming neutrality and gradualism, which accommodates a more biologically plausible set of evolutionary processes that produce evolutionary rate heterogeneity across the phylogeny. A Markov chain Monte Carlo (MCMC) algorithm was used to estimate the posterior distributions of ancestral states for each internal node of the phylogeny. We performed separate ancestral estimations on the first three PCs derived from our PCAs on our full (fig. S1) and modified datasets (fig. S5), which we visualized as phylomorphospace plots constructed using the “phytools” package (45) in R (Fig. 5). The hand morphology of the Homo-Pan LCA is estimated to be more similar to Pan than to any other extant taxon sampled here (Fig. 5, A and B). The 95% credible interval on the estimated hand morphology of the Homo-Pan LCA using our 26-variable dataset includes the mean value of G. gorilla along PC1 and PC3 and the mean value of P. paniscus along PC2. The 95% credible interval on the estimated hand morphology of the Homo-Pan LCA using our 17-variable dataset includes the mean value for P. troglodytes along the first three PCs.

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.

The results of our evolutionary modeling and ancestral estimation analyses provide complementary perspectives on the evolution of ape and human hands. For example, our initial SURFACE analysis estimated a large morphometric discontinuity between the estimated phenotypic optima for palmigrade monkeys and suspensory apes (Fig. 4). The 95% credible intervals for the ancestral hand morphology of great apes occupy this area of the morphospace between extant palmigrade monkeys and suspensory apes, consistent with ape evolution from an arboreal palmigrade ancestor (Fig. 5, C and D). Along the first two PCs, the estimated ancestral hand morphology of great apes includes H. laietanus, P. paniscus, Pongo, and Lagothrix. Overall, our ancestral estimations and evolutionary modeling analyses are consistent with the presence of suspensory adaptations in the hands of the LCA of great apes (18).
Relative size and scaling of metacarpal and phalangeal lengths

Last, we used phylogenetic generalized least squares (pGLS) to estimate the regression of log metacarpal and phalangeal lengths on log body mass using the “caper” package in R (Fig. 6 and table S9) (46). Ardipithecus, Pierolapithecus, Hispanopithecus, and Danuvius were excluded from the models, but plotted with the other data, given the uncertainties of their body mass estimates and the phylogenetic positions of the Miocene hominoids. Length of the MC1 relative to body mass shows substantial overlap between taxa and variation within locomotor modes. For example, H. sapiens, H. neanderthalensis, and H. naledi do not have relatively longer MC1s than Pan. Among the suspensory taxa, hylobatids have the longest MC1s and Ateles the shortest. Ardipithecus is similar to Homo and Pan in the relative length of the MC1 (Fig. 6A). Australopithecus and Homo have the relatively shortest MC5 lengths, which contributes to their high thumb-to-digit ratio (13, 15, 22). Terrestrial monkeys overlap with the more suspensory taxa (hylobatids, Ateles, and Pongo) and Pan in the relative length of the MC5. Ar. ramidus is intermediate between Pan and Gorilla in the relative length of the MC5 (Fig. 6B). The most suspensory taxa have relatively long phalanges of the third digit, whereas small-bodied arboreal quadrupeds, terrestrial monkeys, and other fossil hominins tend to have the relatively shortest phalanges (Fig. 6C). Nasalis has the largest body mass and the relatively longest phalanges of the arboreal cercopithecoids sampled here. Ar. ramidus is most closely aligned with P. paniscus and Danuvius, and intermediate between Hispanopithecus and Pierolapithecus, in the relative lengths of the third digit phalanges (Fig. 6D).

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.

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Homo Sapiens: Anatomically Modern Humans (AMH) Feb 27, 2021 20:13:27 GMT

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DISCUSSION

Our results show that, despite subtle differences from modern suspensory apes (e.g., decreased MC5 length), the Ar. ramidus hand (ARA-VP-6/500) displays morphometric affinities with great apes and shared a selective regime with modern chimpanzees and bonobos. First, morphometric analyses of the preserved metacarpals and phalanges of the ARA-VP-6/500 hand show that PCA separates extant anthropoid primates according to known differences in hand morphology that are related to behavioral differences (Fig. 1 and fig. S1), and Ardipithecus is most similar to extant great apes (Figs. 1 to 5). Building on this result, we found that both Ar. ramidus and Au. afarensis fall along a hypothetical evolutionary trajectory from a more chimpanzee-like, rather than monkey-like, LCA (Fig. 1). Second, we found that the relative length and curvature of the PP3 of Ar. ramidus, along with the shape of the MCP and proximal interphalangeal (PIP) joints, are most closely aligned with suspensory apes (Figs. 2 and 3). Last, we reconstructed the macroevolutionary adaptive landscape for the anthropoid hand and found that Ar. ramidus likely shared primitive suspensory adaptations with modern chimpanzees and bonobos, before an evolutionary shift detected at the base of the Australopithecus and Homo clade (Fig. 4). Hominin hand evolution from a more suspensory, ape-like ancestor rather than a more generalized, monkey-like ancestor is also supported by ancestral estimations (Fig. 5). Collectively, these results falsify the hypothesis that hominins evolved from an ancestor with a generalized hand that lacked suspensory adaptations (2–4, 10–12, 22).

The results of our evolutionary analyses contradict the original suggestion that the hand of Ar. ramidus is Old World monkey–like in intrinsic proportions (2, 3) or generalized relative to extant hominoids (22), which can be explained through both analytical and methodological differences. Our PC1 mostly reflects hand length and shares similarities to the one presented in a recent study including the Ar. ramidus hand (22). Critically, this multivariate axis alone cannot distinguish between African ape-like and monkey-like hypotheses for the hand of the LCA. The position of Ar. ramidus between Pan and Homo along PC1 (and within the “monkey” range of variation) is consistent with both hypotheses. Either Ar. ramidus had evolved away from an African ape–like LCA or, alternatively, retained an ancestral monkey-like hand. Our inclusion of more data (i.e., articular and midshaft dimensions) and increased taxonomic breadth results in better separation on subsequent PCs (i.e., variance is distributed among more PCs) and shows that a generalized, monkey-like hand for the Homo-Pan LCA is not supported by the fossil and comparative data.
Hand morphology and positional behavior

We interpret the shared aspects of nonpollical metacarpal and phalangeal morphology of Ar. ramidus, Pan, and, by estimation, the Homo-Pan LCA to reflect the use of below-branch, suspensory posture and locomotion as part of a varied positional repertoire including arboreality, vertical climbing, and, among the latter two, terrestrial quadrupedalism. The relatively long and curved phalanges of Ar. ramidus allowed for the application of a greater proportion of its digital flexor force to horizontal supports in below-branch suspension while reducing diaphyseal strain (32, 33). The proportion of the flexor force applied to the support contributes to the force of static friction (e.g., equivalent to the sum of the vertical components of FFD and FDP and the horizontal component of FFD′ in Fig. 2A multiplied by the coefficient of friction), which opposes motion between the digits and the support (33, 47).

The MCP and PIP joint morphology of Ar. ramidus is characteristic of a digital joint complex hypothesized to increase the magnitude of internal flexion moments produced by the extrinsic digital flexors, which is an important feature of below-branch positional behavior. Previous studies have noted the “flexion set” of metacarpal heads (9, 48) and phalangeal trochleae (49), as well as the relatively tall MCP and PIP joints of extant hominoids (20, 49), which would act to increase the moment arms of the long digital flexors, particularly in more flexed positions. In addition, the larger moment arms of hominoid digital flexors are associated with relatively large flexor musculature, implying increased ability to generate large moments at the MCP and PIP joints (50, 51). The characteristics of the forelimb musculature of Ar. ramidus are unknown, but the projections of its scaphoid tuberosity and hamate hamulus are consistent with a deepening of the carpal tunnel (2). Accordingly, the MCP and PIP joint morphology of Ar. ramidus is more closely aligned with Pan and Pongo than more generalized arboreal quadrupeds (Fig. 2B), implying a greater ability to generate forceful MCP joint flexion, which provides evidence for adaptation to suspensory locomotion in early hominins and the Homo-Pan LCA.

The relative length of the Ar. ramidus MC5 is intermediate between Pan and Gorilla but greater than that of all other hominins. Metacarpal length has mechanical consequences in the context of varied positional repertoires, hand postures, and substrate preferences. Nonpollical metacarpal length has been argued to be a correlate of suspensory positional behaviors (2, 21). The presence of shorter metacarpals relative to support diameters may cause increases in wrist flexion during suspension (31). However, metacarpal length increases do not positively contribute to performance in suspensory postures (e.g., uni- or bimanual arm hanging) and may only partly contribute to performance in forms of suspensory locomotion (e.g., brachiation and bridging) as a constituent of upper limb length. In brachiation, bridging, and vertical climbing, a longer upper limb allows individuals to travel greater distances per stride, which is hypothesized to reduce both metabolic energy expenditure and the probability of deficient grasps that could lead to deadly falls (33). The reconstructed upper limb length of Ar. ramidus suggests that it was not adapted to hylobatid-like, ricochetal brachiation or Pongo-like quadrumanous climbing and bridging (2). Instead, the hand of Ar. ramidus and the estimated morphology of the Homo-Pan LCA are more consistent with a Pan-like positional repertoire including orthograde vertical climbing and suspension. Therefore, it is premature to reject hypotheses of adaptation to below-branch suspension among fossil hominoids and early hominins on the basis of metacarpal length alone given its functional role in various primate positional repertoires, substrate preferences, and hand postures (33, 36, 41, 52).

A recent multivariate analysis of foot proportions suggested that the positional repertoire of Ar. ramidus and the Homo-Pan LCA also included terrestrial heel-strike plantigrady and vertical climbing (5). The presence of a suspensory adapted hand and a terrestrially adapted foot in Ar. ramidus and the Homo-Pan LCA is notable because this combination is only observed among extant knuckle-walking apes. The knuckle-walking hand posture is a compromise that enables large-bodied suspensory apes to spend a significant proportion of their time on the ground (51). Functionally, knuckle walking reduces external moments at the MCP joints despite the retention of elongated phalanges (36). The suspensory hand of Ar. ramidus, its terrestrial plantigrade foot posture (5), and the retention of knuckle-walking features in the wrists of early hominins (1), as well as other African ape–like regions of the skeleton (53–55), provide indirect support for the knuckle-walking hypothesis. Our evolutionary modeling analyses and ancestral estimations strongly support a more Pan-like, rather than monkey-like, hand morphology for the Homo-Pan LCA, which raises the critical question of which hand posture the LCA would have used if it was not a knuckle walker. We interpret the evolution of African ape knuckle walking to be contingent upon evolution from an ancestor adapted to climbing and suspension (51). Therefore, suspension, vertical climbing, and knuckle walking are naturally linked from both functional and evolutionary perspectives. Ultimately, the definitive resolution of the knuckle-walking hypothesis relies on the recovery of direct fossil evidence of chimpanzee and gorilla postcranial evolutionary history, but we interpret the preponderance of the available fossil and comparative evidence to support hypotheses of a large-bodied, semiterrestrial, knuckle-walking LCA with adaptations to climbing, suspension, and heel-strike plantigrady (1, 5, 40, 53–55, 81).
Hominin hand morphology, manipulative behavior, and bipedalism

We identified an evolutionary shift between Ar. ramidus and all later hominins that has been subsequently maintained for 3 Ma or more despite the gradual evolution of refinements to the hand and wrist throughout the Plio-Pleistocene (30, 34, 56–58). This evolutionary shift occurs within temporal proximity of the earliest known stone tools at Lomekwi 3, Kenya dated to 3.3 to 3.5 Ma (59) and stone tool cut marks on bones from Dikika, Ethiopia dated to >3.39 Ma (60). The evolutionary shift in hominin hand morphology implies a major difference between Ar. ramidus and later hominins in manipulative performance capabilities (34). The morphology and anatomy of chimpanzee and bonobo hands do not impede their use of organic tools in the wild, including the occasional use of unmodified stones by some chimpanzee populations, but they lack the intrinsic hand proportions, joint dimensions, and hypertrophied thenar and hypothenar musculature required to generate the forceful precision grips characteristic of hard hammer percussion (34, 56, 61–64). The hand of Ar. ramidus also lacks many of these important functional characteristics related to stone tool manufacture. The presence of tool-using behaviors among all extant great apes, combined with the ubiquitous use of more elaborate tools by chimpanzees, implies that the Homo-Pan LCA probably used tools (64, 65). Currently, there is no archeological evidence for hominin-like stone tool manufacture earlier than ca. 3.3 to 3.5 Ma, but we acknowledge that future discoveries may show otherwise. Nonetheless, with a synthesis of new evidence, we interpret the shared-derived aspects of Australopithecus and Homo hand morphology and the functional implications of their difference from Ar. ramidus to reflect adaptation to manipulative behaviors probably involving stone tool manufacture in some manner (Fig. 7).

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.

Compared to Ar. ramidus, the shorter nonpollical metacarpals and phalanges of Australopithecus and Homo allow for the formation of forceful precision grips when combined with an intrinsically elongated first ray (22, 25, 30, 34, 56, 61). The intrinsically longer MC1 of Australopithecus and Homo compared to Ar. ramidus, Pan, and Gorilla increases the moment arms of the thenar muscles (34, 62), which enables them to generate large-magnitude internal moments to oppose large-magnitude external moments generated by tool reaction forces hypothesized to be characteristic of hard hammer percussion (63). The increased dorsopalmar depth of the MC1 head relative to hand size in Australopithecus and Homo is consistent with the production of larger-magnitude internal moments at the pollical MCP joint (fig. S8A). Biomechanical modeling supports the hypothesis that human hand proportions enhance manual dexterity (66) and that the phenotypic optimum occupied by Australopithecus and Homo is consistent with enhanced manipulative performance (67).

At the same time, the Au. afarensis hand displays primitive retentions that may suggest some manipulative performance deficits relative to later hominins (56, 68, 69). The Au. afarensis MC5 (A.L. 333-14) has an ape-like palmar extension of its hamate facet, indicating articulation with the hook of the hamate, which is hypothesized to limit ulnar carpometacarpal flexion and supination useful for opposing the fifth digit with the pollex during manipulative behaviors (62, 69). Furthermore, the Au. afarensis MC1 (A.L. 333w-39) has a Pan-like origin of the first dorsal interosseous muscle (57), the trapezium (A.L. 333-80) has a strongly curved MC1 facet, and the MC3 (A.L. 333-16) lacks a styloid process (57, 58, 70). We acknowledge that the A.L. 333 hand sampled here is a composite of specimens potentially representing multiple individuals (25). A recent resampling analysis suggested that Au. afarensis might have had hand proportions slightly more Gorilla-like than previously thought (29). Our multivariate analyses show that the morphology of the Au. afarensis hand is most similar to Au. sediba and Homo but with some proximity to extant gorillas along PC1. Overall, the hand morphology of Au. afarensis is consistent with the ability to use human-like precision grips, but it might have limited its ability to make advanced stone tools (56).

A series of derived postcranial traits indicate that Ar. ramidus used an early form of facultative bipedality (3–5, 71–73). Primitive retentions in limb morphology, including a grasping hallux, signify the importance of arboreality in the positional repertoire of Ar. ramidus (3, 4) despite the presence of a terrestrial plantigrade quadrupedal ancestry shared by the hominine clade (5, 27). The loss of a grasping hallux that permitted the use of large-diameter arboreal supports occurs as early as ca. 3.7 Ma according to the morphology of the Laetoli footprints (74) and certainly by ca. 3.2 Ma as evidenced by the morphology of the Au. afarensis foot fossils from A.L. 333 (75). The ca. 3.4 Ma hominin partial foot from Burtele, Ethiopia (BRT-VP-2/73) was initially suggested to provide evidence for the retention of a more Ardipithecus-like grasping foot into the Pliocene (76). However, recent work has shown that the Burtele hominin foot had a hallucal proximal phalanx with a dorsally canted base; a mediolaterally widened, mildly dorsally domed first metatarsal head; low diaphyseal torsion; and a derived, asymmetric proximal articular surface of the first metatarsal, implying reduced hallucal grasping capabilities relative to Ar. ramidus and extant apes (77). The presence of these features show that the Burtele hominin had a more derived hallux relative to Ar. ramidus capable of increased metatarsophalangeal dorsiflexion in bipedalism. In contrast, the first metatarsal of Ar. ramidus from Aramis lacks these metatarsophalangeal and tarsometatarsal joint features characteristic of later hominins (73), including the Burtele hominin, and instead has an overall more Gorilla-like morphology (77).

The evolution of hominin hallucal adduction occurs alongside numerous other postcranial traits of the trunk, pelvis, and lower limb, indicative of a more derived form of obligate bipedalism in Au. afarensis compared to Ar. ramidus (3, 4). To our knowledge, these observations provide the first fossil evidence to support the hypothesis of a coevolutionary shift in hominin postcranial morphology related to obligate bipedalism and manipulative behaviors probably involving stone tools (Fig. 7) (23–25, 28). The evolutionary mechanisms underlying this coevolutionary shift are unclear, but our observations complement the hypothesis that hominin hand evolution could have occurred as a correlated response to selection on pedal phalanges in relation to bipedalism due to genetic and developmental integration (28). Therefore, both paleontological and neontological data support the hypothesis that hominin hands and feet evolved in a correlated fashion (28).

Overall, our analysis demonstrates that the hand morphology of Ar. ramidus is more closely aligned with chimpanzees and bonobos than generalized quadrupeds, which supports the hypothesis that hominins evolved from an ancestor with a positional repertoire including suspension, vertical climbing, and, possibly, knuckle walking (1, 5–7, 27, 55). The foot skeleton of Ar. ramidus also suggests that the positional repertoire of the LCA included terrestrial plantigrade quadrupedalism and vertical climbing (5), supporting previous studies based on primate comparative anatomy and biomechanics (26, 27). The poor temporal resolution and fragmentary nature of the early hominin fossil and archeological records limit our ability to disentangle causes and effects in a satisfying manner. However, the hand morphology of Ar. ramidus differs substantially from that of Au. afarensis and all later hominins, marking a significant transition in the paleobiology and evolutionary history of hominins related to obligate bipedality, enhanced manipulation, and stone tool use and manufacture (23, 24, 28). New fossil discoveries in critical times and places will continue to throw light on this aspect of human origins.

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Homo Sapiens: Anatomically Modern Humans (AMH) Mar 1, 2021 23:35:04 GMT

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New reconstructions of two famous ancient human ancestors have been produced by a team of academics.

The models of Lucy and the Taung child were made from silicone casts, with skin pigmentation and individual hairs painstakingly applied by hand.

Researchers sought to create the most lifelike reconstructions of these specimens ever produced by basing their constructs solely on data and leading theories.

http://instagram.com/p/CL15RsQpVAK

This, the international team of experts say, is long overdue as previous attempts to visualise our ancestors led to a multitude of purported appearances which are at odds with one another and have often been based on stereotypes and biases rooted in 'racist and misogynistic ideas and tribal concepts'.

Researchers sought to create the most lifelike reconstructions of Lucy and the Taung child (pictured) ever produced by basing their constructs solely on data and minimising artistic interpretation.

Experts say previous attempts to visualise our ancestors led to a multitude of purported appearances which are at odds with one another and have often been based on stereotypes (above) and biases rooted in 'racist and misogynistic ideas and tribal concepts'

Dr Ryan Campbell, co-author of the study from the University of Adelaide, says he was shocked when he compared a reconstruction of Lucy he saw at the Creation Museum in Petersburg, Kentucky with other versions on display around the world.

'I expected to find consistency in those reconstructions displayed in natural history museums, but the differences, even there, were so severe that I almost thought all previous practitioners had never encountered a single hominid reconstruction before commencing their own,' he says in a blog post.

The team set about collecting as much data as possible on both the specimens to determine their likely appearance.

'Our work shows that methods for achieving scientifically justified reconstructions are still not quite in our grasp, despite what many artists and institutions readily advertise,' says Gabriel Vinas, an artist from Arizona State University who was a co-author on the study and the person tasked with bringing the hominins to life.

He adds that previous attempts to show what ancient ancestors looked like have duped both the public and scientists with their stunning lifelike appearance and therefore avoided methodological scrutiny.

'We have assumed validity of a process before questioning artists as to how they validate the reliability of their methods, the applicability of their data, or even how they produce and publish data at all,' he says.

The project, published in Frontiers in Ecology and Evolution, saw the experts explain how they decided on Lucy and the Taung child's most likely appearance.

Last Edit: Mar 1, 2021 23:37:55 GMT by Admin

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Homo Sapiens: Anatomically Modern Humans (AMH) Mar 2, 2021 23:26:53 GMT

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Post by Admin on Mar 2, 2021 23:26:53 GMT

A priceless fossil was briefly brought to a UK research centre in complete secrecy two years ago, in an operation that had more than a touch of the spy novel about it.

The specimen was transported across South Africa with an armed guard, treated like an incognito VIP on an international flight, and then whisked slickly to the Diamond X-ray Light Source just south of Oxford.

It was at the British research facility that scientists were able to see some microscopic details in the ancient remains that could help unravel key clues to the origins of modern humans.

Details of the operation have been made public only now, as the first results from the X-ray investigations have been shared with the wider research community. doi.org/10.7554/eLife.64804

"It was immensely nerve-wracking," palaeoanthropologist Dominic Stratford recalls of the cloak-and-dagger mission, the first time any part of a prehistoric individual has been allowed out of South Africa.

Not only are the remains beyond value, after three million or more years embedded in sediments in the floor of a South African cave, they are immensely fragile.

What Prof Stratford had transported was the skull of "Little Foot", the most complete Australopithecine fossil ever recovered. And given the Australopithecines' position on the evolutionary road to modern humans, this makes Little Foot extra special.

Accompanying Prof Stratford and the skull was Ronald Clarke, the Witwatersrand University professor who led Little Foot's more-than-20-year excavation from the Sterkfontein Caves just outside Johannesburg.

Also in the party was Dr Amélie Beaudet, keen to use Diamond's powerful X-rays to peer inside the delicate object while doing no damage.

"With the X-rays, we found we could see tiny structures like the vasculature system, where there had been blood vessels inside Little Foot's bones, which normally would require physically slicing up a specimen," she told BBC News.

Prof Ian Tattersall, curator emeritus of human origins at the American Museum of Natural History in New York, is bowled over by the details revealed in the first scholarly paper to come out of the Diamond study.

"It is wonderful to have confirmation that micromorphology at this resolution can be recovered from a hominid this old," he said.

Dr Beaudet says they are also planning to measure a layer of material at the root of the teeth called cementum, which could indicate Little Foot's age when she died. It's thought the creature fell through an opening in the floor of the cave where she was found.

The sheer amount of data the team managed to collect at Diamond, seven terabytes (140 Blu-ray discs), has been a challenge. Generating it was, too.

The skull, not much smaller than a modern human's, was far larger than they had been accustomed to examining at the synchrotron's "I12" imaging beamline.

The South African researchers had first sent a plastercast of the specimen, explained Thomas Connolley, the principal scientist on the beamline.

This meant the team could rehearse how best to mount the real object safely. And it was too large to be imaged all at once, so that new techniques had to be developed to knit together a patchwork of stills to create a complete 3D rendering - all the while conscious of the skull's importance in human prehistory.

"Only two people were allowed to handle the fossil," he reported. "Prof Clarke and Dominic Stratford."

He added: "None of us were allowed to touch it - with very good reason! Little Foot is probably the oldest and best-preserved fossils of this type.

"This was a very special experiment," Dr Connolley admitted. "It was actually quite emotional to think that we were studying one of our very early ancestors - for everyone I think, certainly for me."

Dominic Stratford confesses to a little emotion, too.

"It was a fantastic moment when we were all 'hutched' together in the beamline control room - it's always difficult to imagine what's preserved, so when we finally started to see some images, it was completely remarkable," he recalled.

Dr Beaudet, now based at Cambridge University after several years on Prof Clarke's team at Witwatersrand, says it is the imprint of the brain on the inside of the skull that will be most interesting for what it reveals of the early development of human intelligence.

Already the team has identified the traces of blood vessels in the inner skull that are similar to ones found in modern humans.

"The main hypothesis is that in modern humans, these vessels are involved in thermoregulation - preventing our brain from becoming too hot. With Little Foot, the brain was the same size as a chimpanzee's. It was only later in evolution that the brain grew dramatically. But at some point, something had to change in the vascular system, too. So the fact that we can see these vessels in Little Foot is quite promising," she said.