Fossil apes and human evolution - Science Magazine
May 06, 2021 11 mins, 32 secs
It has often been suggested that the last common ancestor between humans and other apes, especially our closest relative, the chimpanzee, was ape- or chimp-like.

review this area and conclude that the morphology of fossil apes was varied and that it is likely that the last shared ape ancestor had its own set of traits, different from those of modern humans and modern apes, both of which have been undergoing separate suites of selection pressures.

Ever since the writings of Darwin and Huxley, humans’ place in nature relative to apes (nonhuman hominoids) and the geographic origins of the human lineage (hominins) have been heavily debated.

However, “bottom-up” perspectives from the fossil record suggest that modern hominoids represent a decimated and biased sample of a larger ancient radiation and present alternative possibilities for the morphology and geography of the Pan-Homo LCA.

Besides their fragmentary record, disagreements are due to the complexity of interpreting fossil morphologies that present mosaics of primitive and derived features, likely because of parallel evolution (i.e., homoplasy).

This has led some authors to exclude known Miocene apes from the modern hominoid radiation.

However, most researchers identify some fossil apes as either stem or crown members of the hominid clade [i.e., preceding the divergence between orangutans (pongines) and African great apes and humans (hominines), or as a part of the modern great ape radiation].

Some authors interpret dryopiths as stem hominines and support their back-to-Africa dispersal in the latest Miocene, subsequently evolving into modern African apes and hominins.

Increased habitat fragmentation during the late Miocene in Africa might explain the evolution of African ape knuckle walking and hominin bipedalism from an orthograde arboreal ancestor.

Early hominins likely originated in Africa from a Miocene LCA that does not match any living ape (e.g., it might not have been adapted specifically for suspension or knuckle walking).

Despite phylogenetic uncertainties, fossil apes remain essential to reconstruct the “starting point” from which humans and chimpanzees evolved.

Modern hominoids (that is, humans and apes) share multiple features (for example, an orthograde body plan facilitating upright positional behaviors).

However, the fossil record indicates that living hominoids constitute narrow representatives of an ancient radiation of more widely distributed, diverse species, none of which exhibit the entire suite of locomotor adaptations present in the extant relatives.

Current evidence suggests that hominins originated in Africa from Miocene ape ancestors unlike any living species.

In 1871, Darwin (1) speculated that humans originated in Africa based on the anatomical similarities with African apes (gorillas and chimpanzees) identified by Huxley (2).

After 150 years of continuous discoveries, essential information about human origins remains elusive owing to debates surrounding the interpretation of fossil apes (Figs. 1 and 2).

Red stars indicate regions with Miocene sediments (spanning ~23 to 5.3 Ma) where fossil apes have been uncovered.

(Some regions may contain more than one site; contiguous regions are indicated with different stars if they extend over more than one political zone.) It is possible that modern great ape habitats do not represent the ancestral environments where the great ape and human clade evolved.

Modern hominoids have higher ranges of joint mobility, such as the full elbow extension shown here, facilitated by a short ulnar olecranon process.

The orthograde body plan facilitates bipedal walking in modern humans and different combinations of arboreal climbing and below-branch suspension in apes.

Other fossil apes exhibit primitive “monkey-like” pronograde body plans with somewhat more modern ape-like forelimbs (e.g., Nacholapithecus).

All extant hominoids (apes and humans) are characterized by the lack of an external tail, high joint mobility (e.g., elbow, wrist, hip), and the possession of an “orthograde” (upright) body plan, as opposed to the more primitive, “pronograde” body plan of other anthropoids and most other mammals (Fig. 2).

The adjectives “lesser” and “great” refer to the smaller size of the former relative to great apes and human group, not to old evolutionary notions based on the Scala Naturae.

Given that some apes are more closely related to humans than to other apes, the word “ape” is a gradistic term used here informally to refer to all nonhominin hominoids.

Based on similarities between chimpanzees and gorillas, a prevalent evolutionary model argues that African apes represent “living fossils” and that knuckle-walking chimpanzees closely reflect the morphology and behavior of the Pan-Homo LCA—the “starting point” of human evolution (14, 15).

This working paradigm also postulates that modern African apes occupy the same habitats as their ancestors (16) (Fig. 1).

This assumption is based on a classical scenario that situates hominin origins in East Africa, owing to environmental changes after the rifting of East African Rift Valley during the Miocene (17).

However, the fossil record denotes a more complex picture: Miocene apes often display mosaic morphologies, and even those interpreted as crown hominoids do not exhibit all the features present in living apes (19) (Fig. 3).

A time-calibrated phylogenetic tree of living hominoids is depicted next to the spatiotemporal ranges of the fossil hominoids mentioned in the text.

The vertical green dashed line indicates that there is a continuity in the African fossil ape record.

Robust and lasting phylogenetic inferences of apes are difficult, in part, because of the fragmentary nature of the fossil record and probable high levels of homoplasy.

Another area of uncertainty relates to the position of many early and middle Miocene African apes relative to the crown hominoid node.

The discovery or recognition of more complete early Miocene fossil hylobatids would help resolve their position and, thus, what really defines the great ape and human family.

Second, the model relies on an outdated account of the fossil record (from the 1980s), when the earliest known hominin (Australopithecus afarensis) was recorded in East Africa, and no possible fossil gorillas and chimpanzees were known (17).

Subsequent fossil discoveries are incompatible with such a narrative: Australopithecus remains from Chad indicate that early hominins were living ~2500 km west of the East African Rift ~3.5 Ma (20).

Furthermore, if Sahelanthropus is a hominin, it would push back the human lineage presence in north-central Africa to ~7 Ma (21).

Moreover, continued fieldwork efforts in less explored areas have shown that hominoids lived across Afro-Arabia during the Miocene (22–25).

Discrepancies are caused by conflicting evolutionary signals among living and fossil hominoids, indicating rampant “homoplasy” (independent evolution causing “false homology”), and are further complicated by the highly incomplete and fragmentary nature of the hominoid fossil record.

This review argues that, despite the limitations, the information provided by fossil apes is essential to inform evolutionary scenarios of human origins.

Linnaeus included modern humans (Homo sapiens) within the order Primates, but it was not until 1863 that Huxley provided the first systematic review of differences and similarities between humans and apes (2).

Like extant great apes, humans display larger body size, larger relative brain size, a slower life-history profile, and more elaborate cognitive abilities than other primates (hylobatids included) (36).

Darwin also speculated that humans and modern African ape ancestors originated in Africa (1), based on the anatomical similarities identified by Huxley and his own observations that many living mammals are closely related to extinct species of the same region.

Most authors advocated an ancient divergence of humans from apes (51, 52) or favored a closer relationship to the great apes than to the lesser apes (53, 54).

A few proposed that humans were more closely related to one or both of the African apes (55, 56), although these views were not widely accepted (57).

By the 1960s, the Leakeys’ discoveries in Tanzania [e.g., Paranthropus boisei (58), Homo habilis (59)] reinforced the relevance of Africa in human evolution, which became firmly established as the “mother continent” with the A.

LCA models still centered on the available fossil apes (mostly represented by jaw fragments and isolated teeth) accumulated after decades of paleontological fieldwork in Africa and Eurasia.

The genus Sivapithecus was included in Dryopithecus, considered the ancestor of African apes, whereas Ramapithecus was considered ancestral to humans based on its short face (and inferred small canines) (63).

Leakey (64) and others agreed with Simons and Pilbeam that humans belong to their own family (Hominidae, or “hominids”), whereas great apes would belong to a distinct family (Pongidae, or “pongids”).

However, Leakey proposed reserving the genus Sivapithecus for the “Asian dryopithecines” and claimed that the human lineage could be traced back to, at least, the middle Miocene of Africa with Kenyapithecus wickeri (~14 Ma).

(65) and Goodman (66) found that some great apes—gorillas and chimpanzees—were more closely related to humans than to orangutans.

Sarich and Wilson developed an “immunological molecular clock” and concluded that African apes and humans share a common ancestor as recent as ~5 Ma (67).

For example, Washburn resurrected extant African apes as ancestral models in human evolution, proposing knuckle walking as the precursor of terrestrial bipedalism (68).

Ever since “the molecular revolution,” the perceived relevance of fossil apes in human evolution has been in jeopardy.

Besides their fragmentary nature, a persistent challenge is understanding the phylogenetic relationships among fossil apes, which exhibit mosaics of primitive and derived features with no modern analogs.

The conclusion that Sivapithecus is not a pongine relies on the assumption that suspensory adaptations and other orthograde-related features present in living hominoids were inherited from their LCA (18).

Sivapithecus and other fossil Asian great apes (e.g., Khoratpithecus, Ankarapithecus, Lufengpithecus) are generally considered pongines (Fig. 3) based on derived craniodental traits shared with Pongo (94, 96–98), although alternative views exist, particularly for Lufengpithecus (99).

By contrast, the phylogenetic positions of apes from the African early (e.g., Ekembo, Morotopithecus) and middle Miocene (Kenyapithecus, Nacholapithecus, Equatorius) remain very controversial.

This is likely the case for early Miocene African taxa.

In turn, Kenyapithecus and Nacholapithecus are commonly regarded as preceding the pongine-hominine split owing to the possession of some modern hominid craniodental synapomorphies combined with a more primitive postcranium than that of living great apes (94, 103).

This raises the question: Can some Miocene apes belong to the crown hominid clade despite lacking many of the features shared by extant great apes.

The large-bodied apes from the middle-to-late Miocene of Europe are at the center of discussions about great ape and human evolution (19, 28, 94, 104, 105).

The dryopith fossil record includes the oldest skeletons that consistently exhibit postcranial features of living hominoids (orthograde body plan and/or long and more curved digits).

One hundred fifty years after Darwin speculated that modern African ape and human ancestors originated in Africa, possible hominins have been found as far back as the latest Miocene of Africa (21, 33, 111): Sahelanthropus (~7 Ma), Orrorin (~6 Ma), and Ardipithecus kadabba (~5.8 to 5.2 Ma).

Kenyapiths indicate that putative stem hominids are first recorded in Eurasia and Africa before the earliest record of both European dryopiths and Asian pongines at ~12.5 Ma (94).

Given the suspensory specializations of late Miocene dryopiths (Hispanopithecus and Rudapithecus), if modern African apes originated from these forms, this scenario implies that the hominine ancestor could have been more reliant on suspension than living chimpanzees or gorillas.

The claim that hominines originated outside of Africa may be justified by cladistic analyses recovering dryopiths as stem hominines but may not be based on the lack of late Miocene great apes in Africa because fossils from this critical time period have been discovered (~13 to 7 Ma) (Fig. 3).

Under this view, bona fide extinct nonhominin hominines have yet to be found in largely unexplored regions of Africa, explaining the virtual lack of a gorilla and chimpanzee fossil record.

However, a critique to the original study concluded that the morphological affinities of Danuvius with modern great apes support a positional repertoire that includes orthogrady and suspension, but not bipedalism (124).

For instance, the inferred “long-back” morphology of Danuvius is characteristic of most quadrupedal monkeys and other Miocene apes (125), denoting the lack of trunk specialization seen in extant great apes.

Even if not directly related to hominins (or modern hominoids), the locomotor adaptations of Oreopithecus, and other Miocene apes, are worthy of further research to understand the selection pressures that led to the (independent) emergence of modern hominoid positional behaviors.

Miocene great apes can enlighten this question by helping to identify the polarity of evolutionary change preceding the Pan-Homo divergence (81, 82).

A holistic view indicates that the Pan-Homo LCA was a Miocene ape with extant great ape–like cognitive abilities, likely possessing a complex social structure and tool traditions (36, 38, 141).

The combined evidence of Miocene apes and early hominins indicate that the locomotor repertoire of the Pan-Homo LCA likely included a combination of positional behaviors not represented among living primates.

That hominins continuously evolved since the Pan-Homo LCA is universally accepted, but the possibility that all living hominoids (including chimpanzees) experienced their own evolutionary histories is sometimes disregarded.

Great apes might have initially thrived by evolving particular adaptations to more efficiently exploit their habitats, thereby occupying new adaptive peaks without abandoning the same area of the adaptive landscape broadly occupied by earlier stem hominoids.

The dietary, locomotor, and cognitive specializations of late Miocene great apes would have hindered their shift into new adaptive peaks suitable for the more open environments toward the latest Miocene (153).

This idea is difficult to reconcile with the premise that continuous-canopy forests covered the tropical belt of central and western Africa since the Miocene, unless gorillas and chimpanzees evolved in less densely forested habitats (30, 31, 114) and retreated to tropical forests when outcompeted by hominins and/or cercopithecoids.

Ironically, the same specializations that allowed great apes to survive despite major environmental challenges since the late Miocene might ultimately doom them to extinction.

Some essential changes (upright posture, enhanced cognition) are just the continuation of a trend started in Miocene hominoids (19, 36, 151).

While escaping from the great ape specialization trap, humans might have fallen into another evolutionary cul-de-sac, with current human activities and overpopulation leading the biosphere to a point beyond return (157).

However, current disagreements regarding ape and human evolution would be much more informed if, together with early hominins and living apes, Miocene apes were also included in the equation

This is the role of fossil apes in human evolution

A Review describes the unique and varied morphologies in fossil and modern apes, including humans

A Review describes the unique and varied morphologies in fossil and modern apes, including humans

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