Why do gorillas have big fingers

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My blog All of Tumblr. RSS feed. We inspect human and ape hand-length proportions using phylogenetically informed morphometric analyses and test alternative models of evolution along the anthropoid tree of life, including fossils like the plesiomorphic ape Proconsul heseloni and the hominins Ardipithecus ramidus and Australopithecus sediba. Our results reveal high levels of hand disparity among modern hominoids, which are explained by different evolutionary processes: autapomorphic evolution in hylobatids extreme digital and thumb elongation , convergent adaptation between chimpanzees and orangutans digital elongation and comparatively little change in gorillas and hominins.

The human and australopith high thumb-to-digits ratio required little change since the LCA, and was acquired convergently with other highly dexterous anthropoids. The hand is one of the most distinctive traits of humankind and one of our main sources of interaction with the environment 1.

The human hand can be distinguished from that of apes by its long thumb relative to fingers 1 , 2 , 3 , 4 Fig. During the first half of the twentieth century, theories on human evolution were dominated by the view that humans split very early from the common stock of apes, and largely preserved generalized plesiomorphic hand proportions similar to other anthropoid primates 6 , 7 , 8.

To the contrary, extant apes were seen as extremely specialized animals adapted for below-branch suspension 6 , 7. However, since the molecular revolution in the —s which provided unequivocal evidence for humans and chimpanzees being sister taxa 9 a prevalent and influential evolutionary paradigm—said to be based on parsimony—has assumed that the last common ancestor LCA of chimpanzees and humans was similar to a modern chimpanzee for example, ref.

However, the current fossil evidence of early hominins 2 , 5 , 13 , 14 and fossil apes 15 , 16 , 17 , 18 challenges this paradigm. Collectively these fossils suggest instead that hand proportions approaching the modern human condition could in fact be largely plesiomorphic 2 , 4 , 13 , as was previously suggested before the advent of molecular phylogenetics.

If that were the case, this would have profound implications relevant to the locomotor adaptations of the chimpanzee-human LCA, as well as the relationship between human hand structure and the origins of systematized stone tool culture. Box represents the interquartile range, centerline is the median, whiskers represent non-outlier range and dots are outliers. The ranges of humans and modern apes are highlighted green and red-shaded areas, respectively. The values for Pr. To address this complex discussion, and to provide a deeper understanding on the evolution of the human and ape hand, in this study we perform a stepwise series of detailed morphometric and evolutionary analyses on the hand-length proportions of modern apes and humans, as compared with a large sample of extant anthropoid primates and key fossils preserving sufficiently complete associated hands.

This fossil sample is constituted by the early hominins Ardipithecus ramidus 4. First, we inspect thumb length relative to the lateral digits as revealed by ray four; that is, intrinsic hand proportions to show that humans are distinctive from apes for this important functional measure, but not from some other anthropoids.

Second, we analyse individual hand elements as proportions adjusted via overall body size that is, extrinsic hand proportions to test whether modern apes—and more especially African great apes—represent a homogeneous group from which humans depart.

Here we further show that modern hominoids constitute a highly heterogeneous group with differences that cannot be explained by phylogenetic proximity or size-related effects.

Third, we enlist phylogenetically informed comparative methods to map how the evolution of hand-length proportions has played out along the individual lineages of our comparative sample.

These methods employ statistical models that establish principles of how continuous trait change is likely to have unfolded over time, and we explore those principles to infer how the variation observed in comparative trait measurements is likely to have changed along the individual branches of a independently derived molecular-based phylogeny. Importantly from a statistical viewpoint, these methods allow the comparative data including the fossils to be analysed within an alternative-hypothesis-testing framework that assesses the statistical fit of alternative evolutionary scenarios.

In our case, we determine how hand-length proportions changed over time and quantify the relative likelihood support of alternative evolutionary hypothesis, thus providing a novel and rigorous analysis of human and ape hand evolution.

Our results reveal that the different hand morphologies exhibited by modern hominoids reflect different evolutionary processes: hylobatids display an autapomorphic hand due to extreme digital and thumb elongation; chimpanzees and orangutans exhibit convergent adaptation related to digital elongation to a lesser degree than hylobatids ; whereas the gorilla and hominin lineages experienced little change by comparison that is, their overall hand proportions are largely plesiomorphic within catarrhines.

These results support the view that the long thumb relative to fingers characterizing the human and australopith hand required little change since the chimpanzee-human LCA, and was acquired in convergence with other highly dexterous anthropoids such as capuchins and gelada baboons.

Hand proportions of humans are usually compared with those of apes using the thumb-to-digit ratio or IHPs , which is a good functional measure of thumb opposability and therefore a proxy for manual dexterity for example, refs 1 , 14 , Accordingly, we queried our anthropoid sample see details of our sample in Supplementary Table 1 to see whether our IHP measure as revealed by the thumb-to-fourth ray ratio; Fig.

Fossil hominins fall within the modern human range, but Ar. Most non-hominoid anthropoids, including the fossil ape Pr. Both Cebus and Theropithecus overlap in this index with humans, supporting the relationship between this ratio and enhanced manipulative skills see Supplementary Note 1.

Despite the aforementioned functional connections, IHPs provide limited information regarding what distinguishes humans from apes: is it a longer thumb, shorter digits or a combination of both? More specifically, which elements contribute most to the overall ray length? Major trends of EHP variation between the individuals in our anthropoid sample are summarized and inspected by means of principal components analysis of extant and fossil individuals Supplementary Table 3 , revealing high EHP heterogeneity in extant hominoids and in non-hominoid anthropoids; Fig.

In other words, there is a clear EHP structure that allows the characterization of the hominoid taxa. Differences among extant great ape genera are more apparent when the eigenanalysis is carried out exclusively on great ape individuals Supplementary Fig. EHPs of selected species are depicted to help understand extreme morphologies along the major axes of variation in shape space Fig.

A complex pattern is revealed: hylobatids, orangutans and chimpanzees in this order exhibit longer digits than humans, but gorillas do not. Thumb length follows a rather different trend: hylobatids have both the longest digits and the longest thumbs, whereas Theropithecus displays the shortest digits but not the shortest thumbs rather, eastern gorillas do. For Ar. Fossil hominins display a modern human pattern, but Ar.

The observed differences in EHP between hominoid taxa cannot be merely attributed to size-dependent effects that is, allometry; Supplementary Fig.

Species are arranged by maximum length of ray IV notice that the thumb does not follow the same trend. Previous observations on modern ape thoraces and limbs suggest that living apes show similar but not identical adaptations to accommodate similar functional demands related to specialized climbing and suspension especially Pan and Pongo , reinforcing the role of parallelism in ape evolution 3 , 21 , 22 , a phenomenon explained by common evolutionary developmental pathways in closely related taxa This method models adaptive evolutionary scenarios by fitting a multi-regime Ornstein Uhlenbeck OU stabilizing selection model 25 to the tip data.

Regimes are here understood as comprising a group of taxonomic units that are inferred to have similar phenotypes. Adaptive peaks can be understood as the optimal phenotypic values that characterize the different regimes.

The advantage of the surface method is that it locates regime shifts without a prior identification of regimes. The method hereby fits a series of stabilizing selection models and uses a data-driven stepwise algorithm to locate phenotypic shifts on the tree.

At each step, a new regime shift is added to the branch of the phylogeny that most improves model fit across all the variables inspected, and shifts are added until no further improvement is achieved. To verify true convergence, this method then evaluates whether the AICc score is further improved by allowing different species to shift towards shared adaptive regimes rather than requiring each one to occupy its own peak.

To verify this result, we compare the statistical fit of this evolutionary scenario with that of five other evolutionary hypotheses based on the respective relative AICc weights Supplementary Fig. To test the sensitivity of our results to a possible sampling bias due to the higher number of hominoid species in comparison with monkey clades in our sample, we repeat the analysis once more after excluding the most speciose and morphologically derived group of hominoids the hylobatid species , together with the fossil closest to the hominoid LCA in our sample that is, Pr.

Importantly in terms of human and ape evolution, irrespective of the difference in results between the full vs reduced hominoid sample, the similarities between the EHP of hominins and gorillas are reconstructed as representing the plesiomorphic condition for the African ape and human clade Fig.

Adaptive optima are based on the two major axes of extrinsic hand proportions EHP variation between extant and fossil species accounting for Branches are colour-coded according to different adaptive regimes revealing that Pan and Pongo -red edges- are convergent.

Clades are colour-coded circles as follows: brown, platyrrhines; dark green, cercopithecids; purple, hylobatids; light green, orangutans; red, gorillas; orange, chimpanzees; pink, fossil hominins; light blue, modern humans.

The nodes corresponding to the last common ancestor LCA of great apes-humans and chimpanzees-humans are highlighted. Phylomorphospace projection of the phylogeny presented in Fig. Owing to space constrictions, macaque species are not labelled. Outlines scaled to similar length of extant and fossil apes and Ar.

Furthermore, to visually track major evolutionary changes driving differences between apes and humans, we summarize the evolutionary history of hominoid hand length diversification as compared with platyrrhine and cercopithecid monkey out-groups by means of a phylomorphospace approach These are the steps that we followed: First, we reconstructed hypothetical ancestral morphologies that is, internal nodes in Fig.

Second, we mapped our time-calibrated phylogenetic tree Fig. The lengths and orientations of the branches of this phylomorphospace allows one to intuitively visualize the magnitude and directionality of inferred shape changes along each branch of the tree.

Owing to the possible impact of Ar. Although the position of Ar. In contrast, hominins and gorillas especially eastern gorillas have slightly reduced their digital lengths although both would still represent the same evolutionary regime, see Fig. In terms of thumb evolution, only a modest reduction in extant great apes and slight elongation in later hominins appears to have occurred.

It is worth noticing that, irrespective of which Ar. This supports the idea that chimpanzees exhibit derived hands, in this case convergent with Pongo Fig. This supports the idea of adaptive evolution that is, shape change associated with change in function 31 in hominoid finger length uncorrelated with phylogeny In other words, finger lengthening has been achieved homoplastically in different ape lineages probably in relation to increased suspensory behaviours , as also revealed by our multi-regime OU modelling Fig.

To inspect how the addition of more taxa with long fingers affects our evolutionary reconstructions of digital length, we revisit the phylomorphospace after excluding the thumb elements. Specifically, we incorporate the fossil ape Hispanopithecus laietanus 17 which does not preserve thumb elements; Fig.

However, its phylogenetic position is not resolved, being alternatively considered as a stem great ape, a stem pongine or even a stem hominine Fig. In the fourth ray morphospace Fig. When ancestral state reconstructions and phylogenetic mapping are inspected in this phylomorphospace, the overall evolutionary pattern reflecting homoplasy in modern and fossil ape digital elongation is also evident, irrespective of the BM estimate of Ar.

Specifically, these results also indicate independent digital elongation to different degrees in hylobatids, orangutans, chimpanzees, spider monkeys and Hi. Although chimpanzees and Hi. In contrast to chimpanzees and baboons that display long metacarpals relative to proximal phalanges as revealed by PC1 in Fig.

Overall, these results match the previously recognized mosaic nature of the Hi. More broadly, even though the living hominoid lineages represent the few remnants of a much more prolific group during the Miocene 22 , the evidence presented above indicate that hominoids constitute a highly diversified group in terms of hand proportions as identified in Fig. Its reconstructed hand is displayed in dorsal a and palmar b views, and together with its associated skeleton c.

This species represents the earliest specialized adaptations for below-branch suspension in the fossil ape record 33 , although its hand combining short metacarpals and long phalanges, dorsally oriented hamato-metacarpal and metacarpo-phalangeal joints, presents no modern analogues The phylogenetic position of Hispanopithecus is still highly debated: stem great ape d , stem pongine e or stem hominine e? The phylomorphospace approach was limited to the three long bones of ray IV to include the fossil ape Hispanopithecus laietanus and Ateles species.

The same analysis was iterated with the large a and small b body mass estimates of Ardipithecus ramidus finding no differences in the overall evolutionary pattern. Internal nodes that is, ancestral-state reconstructions and branch lengths are indicated for three different phylogenetic hypotheses: Hi.

Species names are indicated in a with the exception of macaques. Finally, we reconstruct the evolution of IHPs see Fig. On the basis of the previous results on EHP evolution Fig. Collectively, our results support several evolutionary scenarios with profound and far-reaching implications regarding ape and human origins see Supplementary Note 2 for an extended background in this matter : 1 extant apes are heterogeneous in terms of hand-length proportions as inspected by means of their EHP; Fig.

In other words, our results falsify the view that extant apes, and particularly African apes, constitute a homogeneous group with subtle deviations from a similar allometric pattern for example, ref. This previous idea, together with the phylogenetic proximity between Pan and Homo , has been commonly used as support for the hypothesis that hominins evolved from a Pan -like ancestor for example, ref.

For hand length proportions, our results indicate that Pan and Pongo are convergent Fig. Thus, in terms of evolution of digital elongation, we hypothesize that in some ape lineages natural selection acted on co variation in inter-limb lengths and hand proportions in the context of specialized adaptation for below-branch suspension.

This scenario matches previous evidence suggesting the extant ape lineages survived the late Miocene ape extinction event because they specialized, and were able to share habitats with the radiating and soon to be dominant cercopithecids 23 , This probably occurred with the advent of habitual bipedalism in hominins, and almost certainly preceded regular stone culture 4 , 5 , Our results provide a detailed picture on the evolution of the hand that is drawn from a multiple-regime model-fitting approach that infers the evolutionary scenario that indicates the optimal statistical fit for the observed differences in hand proportions between apes and humans, in terms of both the total amount and direction of shape changes.

These results are also most consistent with previous observations on pervasive homoplasy and complex evolution of the modern ape postcranium 3 , 21 , 35 , as well as with the available evidence from fossil apes and early hominins 1 , 2 , 22 , 29 , The IHPs were computed as the ratio between the long bones of the thumb metacarpal, proximal and distal phalanges and the long bones of the fourth ray but excluding the distal phalanx, which is not well represented in the fossil record that is, metacarpal, proximal and intermediate phalanges.

A total of modern anthropoids, including humans, all the species of great apes, hylobatids, as well as cercopithecid and platyrrhine monkeys Supplementary Table 1 were compared with available fossils Fig. As the emphasis of this work is on the evolution of the human hand, comparisons were made to our closest living relatives that is, the great apes at the species level.

Hylobatids were pooled at the family level and extant non-hominoid anthropoids at the genus level. Some of the monkey groups are represented by small samples for example, Theropithecus, Mandrillus due to the difficulty of finding associated distal phalanges pollical in this case in the museum collections most of them were apparently lost during the skinning and preparation process.

However, we still included these taxa because they provide relevant phylogenetic background to understanding the evolution of hand proportions in apes and humans.

The fossil sample included the associated hands of Ar. For Pr. IHP in Pr. EHPs were computed for an extant sample of anthropoid primates Supplementary Table 1 and the fossils described above by standardizing the length in mm of each of the six manual elements inspected in the IHP by cube root of the BM kg associated with each individual. Major trends in EHP variation between the individuals of our sample were examined by means of a principal components analysis carried out on the covariance matrix Fig.

EHPs were further examined for the fourth ray only if thumb bones are missing to include the late Miocene ape Hispanopithecus laietanus IPS ; Figs 5 and 6 , for which manual lengths were taken from the original fossil We relied on ratios to assess intrinsic and extrinsic hand proportions in our sample, and thus quantify the actual shape of each individual as a scale-free proportion. We favour ratios here over residuals because residuals derived from allometric regressions are not a property inherent to the individuals, but rather are sample-dependent However, to test whether differences between the hand length proportions in our ape sample could be attributed to size-related shape changes that is, allometry , we constructed separate bivaritate plots for the natural log-transformed lengths of the thumb and fourth ray relative BM Supplementary Fig.

Least square regressions were fitted to these data independently for the extant hominid genera and hylobatids, and grade shifts were inspected through Bonferroni post hoc comparisons between estimated marginal means Supplementary Table 5 after checking for homogeneity of slopes via analysis of covariance ANCOVA.

Known BMs kg were taken from museum records for the extant samples whenever available. Generic regressions are provided in Supplementary Table 6. We also derived our own BM estimates for fossils. The case of Ar. For the Proconsul heseloni individuals, BM estimates using different methods and regressions from various preserved anatomical regions were also available The time-scaled phylogeny used in this work is based on a consensus chronometric tree of extant anthropoid taxa downloaded from 10kTrees Website ver.

With the exception of Neanderthals for which molecular data is available , other fossil species were introduced post hoc. For these fossil species, as a criterion of standardization, ghost lineages of one million years were added to the published age of the fossil. There is not general consensus for placement of the late Miocene ape from Spain Hi.

Its phylogenetic position is debated between stem great ape 57 , stem pongine 33 or stem hominine