A distinctive ancestor

There has been much focus on the evolution of primates and especially where and how humans diverged in this process. 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. Almcija et al. 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.

Science, this issue p. eabb4363

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. Humans diverged from apes [specifically, the chimpanzee lineage (Pan)] at some point between ~9.3 million and ~6.5 million years ago (Ma), and habitual bipedalism evolved early in hominins (accompanied by enhanced manipulation and, later on, cognition). To understand the selective pressures surrounding hominin origins, it is necessary to reconstruct the morphology, behavior, and environment of the Pan-Homo last common ancestor (LCA). Top-down approaches have relied on living apes (especially chimpanzees) to reconstruct hominin origins. 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. Reconciling these two views remains at the core of the human origins problem.

There is no consensus on the phylogenetic positions of the diverse and widely distributed Miocene apes. 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]. European Miocene apes have prominently figured in discussions about the geographic origin of hominines. Kenyapith apes dispersed from Africa into Eurasia ~16 to 14 Ma, and some of them likely gave rise to the European dryopith apes and the Asian pongines before 12.5 Ma. 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. Others interpret dryopiths as broadly ancestral to hominids or an evolutionary dead end.

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. Bipedalism might have allowed humans to escape the great ape specialization trapan adaptive feedback loop between diet, specialized arboreal locomotion, cognition, and life history. However, understanding the different selection pressures that underlie knuckle walking and bipedalism is hindered by locomotor uncertainties about the Pan-Homo LCA and its Miocene forebears. In turn, the functional interpretation of Miocene ape mosaic morphologies is challenging because it depends on the relevance of primitive features. Furthermore, adaptive complexes can be co-opted to perform new functions during evolution. For instance, features that are functionally related to quadrupedalism or orthogrady can be misinterpreted as bipedal adaptations. Miocene apes show that the orthograde body plan, which predates below-branch suspension, is likely an adaptation for vertical climbing that was subsequently co-opted for other orthograde behaviors, including habitual bipedalism.

Future research efforts on hominin origins should focus on (i) fieldwork in unexplored areas where Miocene apes have yet to be found, (ii) methodological advances in morphology-based phylogenetics and paleoproteomics to retrieve molecular data beyond ancient DNA limits, and (iii) modeling driven by experimental data that integrates morphological and biomechanical information, to test locomotor inferences for extinct taxa. It is also imperative to stop assigning a starring role to each new fossil discovery to fit evolutionary scenarios that are not based on testable hypotheses.

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.

Whereas the phylogenetic relationships among living species can be retrieved using genetic data, the position of most extinct species remains contentious. Surprisingly, complete-enough fossils that can be attributed to the gorilla and chimpanzee lineages remain to be discovered. Assuming different positions of available fossil apes (or ignoring them owing to uncertainty) markedly affects reconstructions of key ancestral nodes, such as that of the chimpanzee-human LCA.

Humans diverged from apes (chimpanzees, specifically) toward the end of the Miocene ~9.3 million to 6.5 million years ago. Understanding the origins of the human lineage (hominins) requires reconstructing the morphology, behavior, and environment of the chimpanzee-human last common ancestor. 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. Hence, some modern ape similarities might have evolved in parallel in response to similar selection pressures. 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). However, Darwin urged caution until more fossils became availablethe European Dryopithecus was the only recognized fossil ape at the time (3). 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).

Extant apes live in (or nearby) densely forested areas around the equator in Africa and Southeast Asia. Except for the recently recognized tapanuli orangutan (which may represent a subspecies of the Sumatran orangutan), each of the three extant great ape genera presently has two geographically separated species. The Congo River (highlighted in dark blue) acts as the current barrier between common chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). 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. Paleontologically, the vast majority of Africa, west of the Rift Valley, remains highly unexplored. Extant ape ranges were taken from the International Union for Conservation of Nature (IUCN Red List). Background image sources: Esri, DigitalGlobe, GeoEye, i-cubed, USDA FSA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS user community.

(A) Macaque (above) and chimpanzee (below) in typical postures, showing general differences between pronograde and orthograde body plan characteristics. In comparison to a pronograde monkey, the modern hominoid orthograde body plan is characterized by the lack of an external tail (the coccyx being its vestigial remnant), a ribcage that is mediolaterally broad and dorsoventrally shallow, dorsally placed scapulae that are cranially elevated and oriented, a shorter lower back, and long iliac blades. Modern hominoids have higher ranges of joint mobility, such as the full elbow extension shown here, facilitated by a short ulnar olecranon process. The inset further shows differences in lumbar vertebral anatomy, including more dorsally situated and oriented transverse processes in orthograde hominoids. (B) Representatives of each extant hominoid lineage (left column) show different postural variations associated with an orthograde body plan. The orthograde body plan facilitates bipedal walking in modern humans and different combinations of arboreal climbing and below-branch suspension in apes. Knuckle walking in highly terrestrial African apes is seen as a compromise positional behavior superimposed onto an orthograde ape with long forelimbs relative to the hindlimbs. Associated skeletons of fossil hominoids (right column) show that an orthograde body can be disassociated from specific adaptions for suspension (e.g., Pierolapithecus exhibits shorter and less curved digits than Hispanopithecus). Other fossil apes exhibit primitive monkey-like pronograde body plans with somewhat more modern ape-like forelimbs (e.g., Nacholapithecus). Approximate age in millions of years ago is given to representative fossils of each extinct genus: Ardipithecus (ARA-VP-6/500), Nacholapithecus (KNM-BG35250), Pierolapithecus (IPS21350), Hispanopithecus (IPS18800), and Oreopithecus (IGF 11778). Silhouettes of extant and fossil skeletons are shown at about the same scale.

Genomic data indicate that humans and chimpanzees are sister lineages (hominins and panins, respectively; Box 1) that diverged from a last common ancestor (LCA) toward the end of the Miocene, at some point between ~9.3 million and ~6.5 million years ago (Ma) (4, 5). 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). These body plans are associated with two different types of positional (postural and locomotor) behaviors: pronograde behaviors, taking place on nearly horizontal supports with the trunk held roughly horizontally; and orthograde (or antipronograde) behaviors, with the torso positioned vertically (6, 7). Extant ape features also include enhanced joint mobility, long forelimbs relative to hindlimbs, and (except gorillas) long hands with high-to-very-high finger curvature (810). The orthograde body plan is generally interpreted as a suspensory adaptation (11, 12), or as an adaptation for vertical climbing subsequently co-opted for suspension (13).

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. Finally, the taxonomic convention used (the most common), does not reflect that panins and hominins are monophyletic [although some do; e.g., (169)].

Order Primates

Suborder Strepsirrhini (non-tarsier prosimians: lemurs, galagos and lorises)

Suborder Haplorrhini (tarsiers and simians)

Infraorder Tarsiiformes (tarsiers)

Infraorder Simiiformes (or Anthropoidea: simians or anthropoids)

Parvorder Platyrrhini (New World monkeys)

Parvorder Catarrhini (Old World simians)

Superfamily Cercopithecoidea (Old World monkeys)

Superfamily Hominoidea (apes and humans)

Family Hylobatidae (lesser apes: gibbons and siamangs)

Family Hominidae (great apes and humans)

Subfamily Ponginae (the orangutan lineage)

Genus Pongo (orangutans)

Subfamily Homininae (the African ape and human lineage)

Tribe Gorillini (the gorilla lineage)

Genus Gorilla (gorillas)

Tribe Panini (the chimpanzee lineage)

Genus Pan (common chimpanzees and bonobos)

Tribe Hominini (the human lineage)

Genus Homo (humans)

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 LCAthe 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). For some, a chimpanzee-like Pan-Homo LCA could also imply that all extant ape locomotor adaptations were inherited from a modern ape-like ancestor (18). 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. Fossil taxa are color coded based on possible phylogenetic hypotheses. The vertical green dashed line indicates that there is a continuity in the African fossil ape record. However, currently, it is sparse between ~14 and 10 Ma. 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. Many Miocene ape taxa are represented only by fragmentary dentognathic fossils, and the utility of mandibles and molars for inferring phylogeny in apes has been questioned. 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. Splitting times are based on the molecular clock estimates of Springer et al. (168) (hominoids and hominids) and Moorjani et al. (4), which are more updated for hominines and Pan-Homo. Silhouettes are not to scale. Shaded boxes represent geographic distributions (green is Africa, gold is Europe, and purple is Asia).

The Pan-like LCA model builds on the East Side Story of hominin origins (17), a seriously challenged scenario. First, it is grounded in the living-ape geographic distribution, which may not match that at the time of the Pan-Homo split (Fig. 1). 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 (2225). In addition, remains of putative hominines have been found in East Africa (26, 27), perhaps even in Europe (28, 29). Finally, paleoenvironmental reconstructions for late Miocene apes and hominins suggest that the Pan-Homo LCA inhabited woodlands, not tropical rainforests (3033).

Current debates about the transition from an ape into a bipedal hominin are centered on the morphological and locomotor reconstruction of the Pan-Homo LCA, as well as its paleobiogeography. 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.

Since Linnaeus established modern taxonomy in 1758 (34) and until the 1960s, morphological similarity was the main basis for classifying organisms. 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). Imagining himself as a scientific Saturnian, Huxley stated that, The structural differences between Man and the Man-like apes certainly justify our regarding him as constituting a family apart from them; though, inasmuch as he differs less from them than they do from other families of the same order, there can be no justification for placing him in a distinct order [(2), p. 104]. Huxleys work was motivated by widespread claims (e.g., Cuvier, Owen) that humans uniqueness warranted their placement in a separate order. Darwin concurred with Huxley that humans should be classified in their own family within primates (1).

We now know that most human features are primitive traits inherited from primate (e.g., trichromatic stereoscopic vision, manual grasping) or earlier (e.g., five digits) ancestors (35). Even humans distinctively large brains and delayed maturation are framed within a primate trend of increased encephalization and slower life history compared with other mammals (35, 36). Some differences in brain size may partly reflect a neocortex enlargement related to enhanced visual and grasping abilities (37). 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). However, modern humans are extreme outliers in terms of delayed maturation, encephalization, advanced cognition, and manual dexterity, ultimately leading to symbolic language and technology (38).

Anatomically, only two adaptive complexes represent synapomorphies present in all hominins: the loss of the canine honing complex and features related to habitual bipedalism (33, 39). Most anthropoids possess large and sexually dimorphic canines coupled with body size differences between males and females, reflecting levels of agonistic behavior and sociosexual structure (40). The fossil record indicates that there was a reduction in canine height, leading to the loss of the honing complex in early hominins (41). Habitual bipedalism is reflected in several traits across the body (e.g., foramen magnum position and orientation; pelvic, lower-back, and lower-limb morphology), present (or inferred) in the earliest hominins (21, 33, 42).

Darwin linked the origin of bipedalism with an adaptive complex related to freeing the hands from locomotion to use and make tools (replacing large canines), leading to a reciprocal feedback loop involving brain size, cognition, culture, and, eventually, civilization (1). Multiple variants in the order of these events have been advocated, with the freeing of the hands alternatively linked to tools (43), food acquisition and carrying (15), or provisioning within a monogamous social structure (44), to name a few. There is general agreement that canine reduction (including social structure changes), enhanced manipulative capabilities, and bipedalism were interrelated during human evolution. However, determining the order of events and their causality requires reconstructing the ape-human LCA from which hominins originated. 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. However, given the limited ape fossil record at that time, he concluded that it was useless to speculate on this subject [(1), p. 199]. Using the French Dryopithecus to calibrate his clock, Darwin concluded that humans likely diverged as early as the Eocene and warned against the error of supposing that the early progenitor of the whole Simian stock, including man, was identical with, or even closely resembled, any existing ape or monkey [(1), p. 199]. These ideas inaugurated a century of discussions about humans place in nature.

Until the 1950s, the geographic origin of hominins was disputed between Africa, Asia, and Europe. After the publication of Darwins On the Origin of Species (45), Haeckel predicted that the missing link (dubbed Pithecanthropus, the ape-man) would be found in Asia (46). This idea led to Dubois 1891 discovery of Homo erectus in Indonesia (47). In 1925, Dart published the discovery of Australopithecus africanus, the man-ape from South Africa (48). However, the scientific community still focused on Europe because of the Piltdown fossils, until they were exposed as a hoax (49). Asia remained a mother continent contender owing to the man-like ape Ramapithecus, discovered in the Indian Siwaliks (50).

During this time, the relationships of humans to other primates were highly contentious. 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). These alternative phylogenetic hypotheses heavily affected reconstructions of the LCA. Some (e.g., Schultz, Straus) advocated for a generalized ape ancestor (52), whereas others relied on extant hominoid models. Notably, Keith developed a scenario in which a hylobatian brachiating stage preceded an African ape-like creature: a knuckle-walking troglodytian phase immediately preceding bipedalism (11). Focused on Keiths hylobatian stage, Morton proposed that the vertically suspended posture of a small-bodied hylobatid-like ancestor caused the erect posture of human bipedalism (12). Gregory, another prominent brachiationist, supported similar views (53). Morton argued that knuckle walking did not represent an intermediate stage preceding bipedalism but rather a reversion toward quadrupedalism in large-bodied apes specialized for brachiation. At that time, brachiation was used for any locomotion in which the body was suspended by the hands. Now, it refers to the pendulum-like arm-swinging locomotion of hylobatids (6).

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. afarensis discoveries during the 1970s (60, 61). 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. In 1965, Simons and Pilbeam (62) revised and organized available Miocene apes in three genera: Dryopithecus, Gigantopithecus, and Ramapithecus. 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). He also agreed that Ramapithecus was an Asian early human ancestor. 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).

Two major revolutions in the study of evolutionary relationships started in the 1960s. First, a series of studies jump-started the field of molecular anthropology: Blood protein comparisons by Zuckerkandl et al. (65) and Goodman (66) found that some great apesgorillas and chimpanzeeswere 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). These results led to decades-long debates regarding the African apehuman split. For example, Washburn resurrected extant African apes as ancestral models in human evolution, proposing knuckle walking as the precursor of terrestrial bipedalism (68). By contrast, paleontologists argued that the molecular clock was inaccurate because of the much older age of the purported human ancestors Kenyapithecus and Ramapithecus (69). Second, Hennigian cladistics (phylogenetic systematics), which only recognizes synapomorphies (shared derived features) as informative for reconstructing phylogeny (70), became slowly implemented in anthropology by the mid-1970s (71).

In the 1970s and 1980s, the relationships among gorillas, chimpanzees, and humans were still disputed. Chromosomal comparisons (72), DNA hybridization (73), and hemoglobin sequencing (74) supported a closer relationship between chimpanzees and humans, whereas morphology-based cladistics recovered gorilla-chimpanzee as monophyletic (75). In the late 1980s, the first single-locus DNA sequencing studies (76), followed in the 1990s with multiple loci analyses, finally resolved the trichotomy (77). Current genomic evidence indicates that humans are more closely related to chimpanzees (5), having diverged at some time between ~9.3 and ~6.5 Ma (4). Ever since the molecular revolution, the perceived relevance of fossil apes in human evolution has been in jeopardy.

Extant African apes have been considered ancestral models since Keiths troglodytian stage in the 1920s (11), and especially since the 1960s, with updated hypotheses inspired by the molecular revolution (68, 78) and field discoveries on chimpanzee behavior by Goodall (79). Leakey played a central role in promoting Goodalls pioneering research (subsequently fostering Fosseys research in gorillas and Galdikass research in orangutans). Now, a prominent paradigm proposes that chimpanzees represent living fossils that closely depict the Pan-Homo LCA (14, 16). This model combines molecular data with the anachronistic view that Gorilla and Pan are morphologically similar (75). Under these assumptions, knuckle walking, once used to defend African ape monophyly (80), is used to argue that African apes are morphologically conservative and only display size-related differences (14). This model contends that gorillas are allometrically enlarged chimps and that chimpanzees [or bonobos (78)] constitute a suitable model for the Pan-Homo LCA, perhaps even the hominine or hominid LCAs (14). This narrative also incorporates the paleobiogeographic assumption that African apes likely occupy the same habitats as their ancestors: Without new selection pressures, there was no need for evolution.

If hominins originated from a chimpanzee-like LCA, human bipedalism must have evolved from knuckle walking (15), a functional compromise enabling terrestrial travel while retaining climbing adaptations (80). Under this view, bipedal hominins originated from an ancestor that was already terrestrial while traveling. These conclusions are logical from a top-down perspective, based on the evidence provided by extant hominoids and early hominins. However, a fully informed theory of hominin origins must also apply a bottom-up approach (81, 82), from the perspective of extinct apes preceding the Pan-Homo split. It is also essential to clarify whether chimpanzees represent a good ancestral model for the Pan-Homo LCA. Unfortunately, the view from the bottom is blurry.

With more than 50 hominoid genera and a broad geographic distribution (Fig. 1), the Miocene has been dubbed the real planet of the apes (83). 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 Asian Miocene ape Sivapithecus best exemplifies this complexity. Discoveries during the 1970s and 1980s, including a facial skeleton (84), clarified that Ramapithecus is a junior synonym of Sivapithecus, which is likely related to orangutans (85). However, two Sivapithecus humeri show a primitive (pronograde-related) morphology, calling into question the close phylogenetic link with Pongo that had been inferred from facial similarities (86).

The root of this Sivapithecus dilemma (18) is identifying where phylogenetic signal is best captured in hominoids: the postcranium or the cranium? The former implies that a Pongo-like face evolved independently twice; the latter entails that some postcranial similarities among living apes evolved more than once. Both hypotheses highlight the phylogenetic noise that homoplasy introduces in phylogenetic inference. Indeed, several studies have found that homoplasy similarly affects both anatomical areas (87). 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). However, this is contradicted by differences among living apes [e.g., forelimb and hand anatomy, degree of limb elongation, hip abduction capability (8, 9, 19, 80, 8891)]. These studies concluded that apparent similarities could represent independently evolved biomechanical solutions to similar locomotor selection pressures. For instance, hand length similarities among living apes result from different combinations of metacarpal and/or phalangeal elongation in each extant genus (9).

Parallel evolutionhomoplasy among closely related taxa due to shared genetic and developmental pathwayscould explain some postcranial similarities related to suspensory behaviors among extant apes (80). Compared with convergences among distantly related taxa, parallelisms are more subtle and difficult to detect and they readily evolve when similar selection pressures appear. Within extant primates, suspensory adaptions evolved independently in atelines and between hylobatids and great apes (8, 80, 88, 91, 92). When the hominoid fossil record is added, independent evolution of suspensory adaptations has been inferred, too, for orangutans, chimpanzees, and some extinct lineages (9, 89, 93, 94). Knuckle walking has also been proposed to have different origins in gorillas and chimpanzees (80, 93, 95). As for suspension, the preexistence of an orthograde body plan, vertical climbing, and general arboreal heritage could have facilitated the independent evolution of knuckle walking to circumvent similar biomechanical demands during terrestrial quadrupedalism while preserving a powerful grasping hand suitable for arboreal locomotion (9).

The possibility of parallelisms indicates that ancestral nodes in the hominoid evolutionary tree, including the Pan-Homo LCA, cannot be readily inferred without incorporating fossils. In addition, fossils from known evolutionary lineages are commonly used to calibrate molecular clocks despite being subject to considerable uncertainty (4). Even worse, relatively complete fossil apes undisputedly assigned to early members of the gorilla and chimpanzee lineages remain to be found.

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, 9698), 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. Like Sivapithecus, they exhibit only some modern hominoid features superimposed onto a primitive-looking pronograde (monkey-like) body plan (Fig. 2). Some authors interpret this mosaicism as indicating that most Miocene apes do not belong within the crown hominoid radiation and, thus, are irrelevant to reconstructions of the Pan-Homo LCA (14). This is likely the case for early Miocene African taxa. However, the vertebrae of Morotopithecus [~20 Ma (100) or ~17 Ma (101)] display orthogrady-related features absent from other stem hominoids, indicating either a closer relationship with crown hominoids or an independent evolution of orthogrady (102). 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). Named after Dryopithecus (3), they are generally distinguished as a subfamily (Dryopithecinae) (94) or tribe (Dryopithecini) (28). However, it is unclear if they constitute a monophyletic group or a paraphyletic assemblage of stem and crown hominoids (94). Thus, we refer to them informally as dryopiths. These apes are dentally conservative, but each genus exhibits different cranial and postcranial morphology. 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). Dryopithecus (~12 to 11 Ma) is known from craniodental remains and isolated postcranials that are too scarce to reconstruct its overall anatomy (106). By contrast, Pierolapithecus (~12 Ma) is represented by a cranium with an associated partial skeleton (19). Cranially a great ape, its rib, clavicle, lumbar, and wrist morphologies are unambiguous evidence of an orthograde body plan. Yet, unlike chimpanzees and orangutans (but similar to gorillas), Pierolapithecus lacks specialized below-branch suspensory adaptations [see discussion in (10)]. The recently described Danuvius (~11.6 Ma, Germany), and the slightly younger (~10 to 9 Ma) Hispanopithecus (Spain) (105) and Rudapithecus (Hungary) (28) represent the oldest record of specialized below-branch suspensory adaptations (e.g., long and strongly curved phalanges; Fig. 2). Danuvius has also been argued to show adaptations to habitual bipedalism (but see below).

The different mosaic morphology exhibited by each dryopith genus is a major challenge for deciphering their phylogenetic relationships (Fig. 3). Current competing phylogenetic hypotheses consider dryopiths as stem hominoids (107, 108), stem hominids (94, 96, 109), or crown hominids closer to either pongines (105), hominines (28), or even hominins (29, 110). However, recent phylogenetic analyses of apes recovered dryopiths as stem hominids (97, 109), perhaps except Ouranopithecus (~9 to 8 Ma) and Graecopithecus (~7 Ma) (97). Ouranopithecus has been interpreted by some as a stem hominine, or even as a crown member more closely related to the gorilla or human lineages (110). Graecopithecus has also been advocated as a hominin (29), although the fragmentary available material hinders evaluation of this hypothesis. Such contrasting views about dryopiths stem from their incomplete and fragmentary fossil record coupled with pervasive homoplasy. However, because these factors are equal for all researchers, their different conclusions must also relate to analytical differences (e.g., taxonomy, sampling, polymorphic and continuous trait treatment). The root of the conflict is the remarkable differences in subjective definition and scoring of complex morphologies (e.g., incipient supraorbital torus).

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). However, others question the feasibility of identifying the earliest hominins among the diverse Miocene apes (96, 112). Puzzlingly, despite some claims based on scarce remains (113115), ancient representatives of the gorilla and chimpanzee lineages remain elusive. Some apes from the African late MioceneChororapithecus (26), Nakalipithecus (27), and Samburupithecus (116)have been interpreted as hominines, but the available fragmentary remains preclude a conclusive assessment. Furthermore, Samburupithecus is likely a late-occurring stem hominoid (97, 117).

During the middle Miocene (~16.5 to 14 Ma), apes are first found out of Africa. These are the genera Kenyapithecus (Turkey) and Griphopithecus (Turkey and central Europe). We informally refer to them as the kenyapiths because there is no consensus on their relationships (28, 94, 118). 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). Paleobiogeographical and paleontological data suggest that kenyapiths dispersed from Africa into Eurasia as one of the multiple catarrhine intercontinental dispersal events occurred during the Miocene (e.g., hylobatids, pliopithecoids) (83, 94). Although some competing evolutionary scenarios agree that kenyapiths gave rise to dryopiths in Europe, the phylogenetic and geographic origin of hominines remains contentious (28, 94).

If dryopiths are stem hominids, they could either be close to the crown group or constitute an evolutionary dead end, an independent experiment not directly related to either pongines or hominines. Alternatively, dryopiths might be crown hominids more closely related to one of these groups. If dryopiths are hominines, this implies that the latter could have originated in Europe and subsequently dispersed back to Africa during the late Miocene (28, 29, 83). This would coincide with vegetation structure changes caused by a trend of increased cooling and seasonality (32) that ultimately drove European apes to extinction [or back to Africa (28)]. In this scenario, hominines and pongines would be vicariant groups that originally evolved in Europe and Asia, respectively, from early kenyapith ancestors. 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). Both molecular and paleontological evidence (e.g., Sivapithecus) situate the pongine-hominine divergence within the middle Miocene. Hence, the debate cannot be settled without more conclusively resolving the phylogenetic relationships of middle Miocene dryopiths.

An alternative scenario proposes a vicariant divergence for hominines and pongines from kenyapith ancestors but favors the origin of hominines in Africa (94, 119). It argues for a second vicariant event between European dryopiths and Asian pongines soon after the kenyapith dispersal into Eurasia. Cladistically, dryopiths would be pongines but would share none of the currently recognized pongine autapomorphies, evolved after the second vicariant event. This scenario is difficult to test, but it would be consistent with the apparent absence of clear pongine synapomorphies in Lufengpithecus (99) and the more derived nasoalveolar morphology of Nacholapithecus (103) compared with some dryopiths (106). However, it would imply even higher levels of homoplasy, including the independent acquisition of an orthograde body plan in Africa and Eurasia from pronograde kenyapith ancestors.

A third possibility is that none of the taxa discussed above are closely related to the African ape and human clade (107). 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. According to Pilbeam, paleoanthropologists could be like the drunk looking for his keys under the lamppost where it was light rather than where he had dropped them, working with what we had rather than asking whether or not that was adequate [(108), pp. 155156]. Africa is a huge continent, and most paleontological discoveries are concentrated in a small portion of it. The greatest challenge is finding hominoid-bearing Mio-Pliocene sites outside East and South Africa, even though we know they exist (2022). Besides insufficient sampling effort, this is hindered by numerous impediments to fieldwork in most of Africa, including geopolitical conflicts, restricted land use development, lack of suitable outcrops (due to extensive vegetation cover), and taphonomic factors [tropical forests do not favor fossil preservation (120)].

The decades-long feud regarding arboreality and bipedalism in A. afarensis exemplifies the complexity of inferring function from anatomy. Totalist functional morphologists rely on a species total morphological pattern (121) to infer its locomotor repertoire. Totalists see a bipedal early hominin with some ape-like retentions (e.g., curved fingers) pointing to continued use of the trees and consider that certain not-yet-human-like features (e.g., hip) indicate a different type of bipedalism (122). Instead, directionalistsfor whom functional inferences are only possible for derived traits evolved for a specific functionfocus exclusively on bipedal adaptations (123). Totalist and directionalist interpretations of the fossil record differ in the adaptive significance attributed to primitive features, which result in different behavioral reconstructions. Two other related factors further complicate locomotor inferences in extinct species: First, different positional behaviors have similar mechanical demands [e.g., bipedalism, quadrupedalism and some types of climbing (39)]. Second, preexisting morphofunctional complexes originally selected to fulfill a particular function (adaptations) can be subsequently co-opted for a new role (exaptations).

The mosaic nature of hominoid morphological evolution makes the functional reconstruction of fossil apes especially challenging, as recently exemplified by Danuvius (104): It was described as possessing long and curved fingers, a long and flexible vertebral column, hip and knee joints indicative of extended postures, and an ankle configuration aligning the foot perpendicular to the long axis of the tibia. Such a combination of features was functionally interpreted as indicating below-branch suspension combined with above-branch bipedalism. 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). Part of the problem with the original interpretation is that it infers a derived locomotor behaviorbipedalismfrom primitive features that are also functionally related to quadrupedalism. 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. The Danuvius femoral head joint, being (primitively) posterosuperiorly expanded (126), is consistent with flexed quadrupedal hip postures that are not used during human-like bipedalism. In addition, the distal tibia configuration of Danuvius is shared with Ekembo and cercopithecoids (104), thus being likely plesiomorphic and not unique to bipeds. When the primitive and derived features of Danuvius are considered, a totalist would argue that it combined high degrees of plesiomorphic quadrupedal locomotion with novel (suspensory) behaviors, whereas a directionalist would downplay the primitive features in favor of the newly derived adaptive traits (i.e., suspension).

The late Miocene Oreopithecus (~7 Ma, Italy) is another example of conflicting phylogenetic and functional signals. Phylogenetic interpretations of Oreopithecus include cercopithecoid, stem hominoid, and hominid (even hominin) status (127). However, current phylogenetic analyses suggest that Oreopithecus could represent a late-occurring stem hominoid (97, 128), with postcranial adaptations to alternative types of orthogrady, such as forelimb-dominated behaviors (129) and terrestrial bipedalism (130). 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.

To distinguish true locomotor adaptations from exaptations, current research efforts focus on plastic ecophenotypic traits, potentially denoting how fossil hominoids were actually moving. Bone is a living tissue, and growth is expected to occur in predictable ways that reflect loading patterns throughout life (131). Thus, cross-sectional and trabecular bone properties and their links to behavior are widely investigated (132, 133). Yet, experimental studies indicate that internal bone morphology does not necessarily match stereotypical loading patterns (134). Ample evidence suggests that irregular loading, even in low magnitude, can be more osteogenically potent than stereotypical loading (135). This may bias interpretations of individual fossils with a species-atypical loading pattern during life (e.g., because of an injury). Bone (re)modeling also does not consistently occur in response to changes in loading pattern: It can occur in ways that detract from, rather than enhance, function (136) and may manifest differentially across the skeleton (137). Incongruence also exists between actual bone performance and expectations based on aspects of internal morphology (138). Finally, there is a strong genetic component to the responsiveness of bone (re)modeling to loading (136), which is largely unknown for most species. The confidence with which internal bone structures can be used to retrodict behavior in fossil species remains a work in progress.

Competing hypotheses about the locomotor behavior immediately preceding hominin bipedalism include terrestrial knuckle walking (15), palmigrade quadrupedalism (93), and different types of arboreal (orthograde) behaviors such as climbing and suspension (7), vertical climbing (139), or arboreal bipedalism and suspension (104, 140). Miocene great apes can enlighten this question by helping to identify the polarity of evolutionary change preceding the Pan-Homo divergence (81, 82). For instance, if Pierolapithecus is interpreted as an orthograde ape without specific suspensory adaptations but retaining quadrupedal adaptations [see alternatives in (10)], then the orthograde body plan and ulnocarpal contact loss could be interpreted as an adaptation to vertical climbing, subsequently co-opted for suspension (19). Similarly, habitual bipedalism might have directly evolved from other orthograde behaviors without an intermediate stage of advanced suspension or specialized knuckle walking. Hence, Pierolapithecus complements previous hypotheses that biomechanical aspects of the lower limb during quadrupedalism and vertical climbing could be functionally preadaptive for bipedalism (39, 139).

A holistic view indicates that the Pan-Homo LCA was a Miocene ape with extant great apelike cognitive abilities, likely possessing a complex social structure and tool traditions (36, 38, 141). This ape would exhibit some degree of body size and canine sexual dimorphism (with large honing male canines) (15), indicating a polygynous sociosexual system (40). Based on Miocene apes and earliest hominins, it is also likely that the Pan-Homo LCA was orthograde and proficient at vertical climbing [see alternative interpretation based on Ardipithecus (33, 93)], but not necessarily adapted specifically for below-branch suspension or knuckle walking (9, 33). Chimpanzees seem to retain the Pan-Homo LCA plesiomorphic condition in some regards [e.g., brain and body size (38), vertebral counts (125), foot morphology (142)]. However, in others [e.g., interlimb (93), hand (9), pelvis (143) length proportions; femur morphology (89)], early hominins are more similar to generalized Miocene apes. These results further reinforce the idea that functional aspects of other locomotor types were co-opted for bipedalism during hominin origins.

The East Side Story scenario links the divergence of chimpanzees and humans to the rifting of East Africa, which would have triggered a vicariant speciation event from the ancestral Pan-Homo LCA (17). Chimpanzees would have remained frozen in time in their ancestral tropical forest environment, whereas humans would be the descendants of the group left behind on the east side of the Rift. Major climate and landscape changes would have then forced the earliest hominins to adapt to more open (grassland savanna) environments by acquiring bipedalismand the rest is history. Several decades after the proposal of this scenario, where do we stand?

The landscape of East Africa has dramatically changed during the past 10 million years because of tectonic events leading to specific climatic conditions and associated changes in vegetation structure, from mixed tropical forest to more heterogeneous and arid environments than elsewhere in tropical Africa (144, 145). The trend of progressive aridification did not culminate in the predominance of savanna environments until ~2.0 Maroughly coinciding with hominin brain size increase and the appearance of H. erectusand was punctuated by alternating episodes of extreme humidity and aridity, resulting in a fluctuating extension of forests through time (144, 145). Despite ongoing discussions about early hominin paleoenvironments (woodland with forest patches versus wooded savanna) (146), evidence from Miocene apes (30, 31) supports that the Pan-Homo LCA inhabited some kind of woodland. Therefore, it has been suggested that the Pan-Homo LCA was probably more omnivorous than chimpanzees (ripe fruit specialists) and likely fed both in trees and on the ground (33), in agreement with isotopic analyses for Ardipithecus ramidus (41).

Bipedalism would have emerged because of the selection pressures created by the progressive fragmentation of forested habitats and the need for terrestrial travel from one feeding patch to the next. Data on extant ape positional behaviors (Fig. 4) suggest that hominin terrestrial bipedalism originated as a posture rather than a means of travel on the ground (147) or in trees (140). Rose (39) proposed a long process of increasing commitment to bipedality in the transition to more complex open habitats throughout the Plio-Pleistocene, and Potts (148) argued that key stages in hominin evolution may relate to adaptive responses to cope with highly variable environments. The fossil and archaeological records provide a new twist to the order of evolutionary events in early hominin evolution. The remains of Orrorin and Ar. ramidus indicate that habitual terrestrial bipedalism, enhanced precision grasping, and loss of canine honing evolved at the dawn of the human lineage well before brain enlargement (9, 33, 89, 93). It was not until later in time [maybe starting with Australopithecus (149) and continuing with Homo], that some preexisting hand attributes were co-opted for purposive and systematic stone toolmaking in more encephalized hominins with more advanced cognitive abilities (38, 150).

Although one particular behavior can dominate the locomotor repertoire of a given species, the full positional repertoire (postural and locomotor behaviors) of living primates is diverse, complex, and not fully understood. For example, some locomotor behaviors are not totally comparable (e.g., monkey quadrupedalism versus African ape knuckle walking). Furthermore, comprehensive data are not yet available for some extant hominoids (e.g., Gorilla). Bipedalism did not appear de novo in hominins; it existed as a posture or locomotion within a broader Miocene ape positional repertoire. 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. Over time, bipedal behaviors became the predominant activity within the repertoire of early hominins (and knuckle walking in the chimpanzee lineage). Locomotor behaviors (plus bipedal standing) in each taxon represent percentages of total positional behavior repertoire. (The full repertoire is not shown; hence, these do not add to 100%.) Data were taken from (92). Quadrupedalism includes Hunts categories quadrupedal walk and quadrupedal run, suspension includes suspensory, brachiate, clamber, and transfer. The locomotor repertoire compositions of the LCA and modern humans (Homo) are conjectural, for illustrative purposes.

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. Potts (151) suggested that the greater cognitive abilities of great apes originated to continue exploiting fruit supplies from densely forested environments in front of strong environmental variability. Coupled with locomotor adaptations (e.g., vertical climbing, suspension) enabling an efficient navigation through the canopy, this cognitive trap would consist of an adaptive feedback loop between diet, locomotion, cognition, and life history. Although hominids originated approximately during the Mid-Miocene Climatic Optimum (~17 to 15 Ma), their subsequent radiation from ~14 Ma onward paralleled a trend of climatic deterioration during the rest of the Miocene (152). 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. Nevertheless, this evolutionary strategy would become unsustainable once a particular paleoenvironmental threshold was surpassed. This could explain the fate of European dryopiths, which survived for some time under suboptimal conditions (despite the progressive trend of cooling and increased seasonality) until they vanished (94).

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). The Miocene planet of the apes gave way to the time of the more generalist Old World monkeys, enabling their survival in a wider variety of seasonal habitats (30, 92, 154). The same specialization trap can explain the delayed retreat of pongines (and hylobatids) to southeastern Asia throughout the Plio-Pleistocene. The highly specialized orangutans remain extant, but not for long because their habitat continues to shrink. African apes could have partially overcome the specialization trap by evolving (perhaps in parallel) semiterrestrial adaptationsknuckle walking. Gorillas also expanded their dietary range (more folivorous) and enlarged their body size. Contrary to the view that gorillas are enlarged chimpanzees, morphometric analyses indicate that gorillas underwent their own evolutionary history, resulting in different ontogenetic trajectories (155, 156) and postcranial differences that cannot be explained by size-scaling effects (9, 143). Why, when, and how many times knuckle walking evolved is more difficult to explain than the origin of hominin bipedalism. Habitat fragmentation coupled with a higher reliance on arboreal feeding might be invoked (i.e., knuckle walking serves both terrestrial and arboreal locomotion). 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.

Hominins might have escaped the great-ape specialization trap by evolving novel and more radical adaptations: bipedalism (another specialized orthograde locomotion), concomitant freeing of the hands, and subsequent enhanced manual dexterity, brain configuration, sociosexual behavior, and culturally mediated technology. Human evolution also reflects the progressive adaptation (biological first, cultural later) to ever-changing environments (39, 148). 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). Will humans escape their own specialization trap?

Fossils uniquely inform deep-time evolutionary studies, which is essential to plan for the future (158). However, we must be aware of the many existing limitations and the gaps in our knowledge. For example, we need more fossils because we are likely missing vastly more than what we have. More fieldwork is necessary to find fossil apes close to the gorilla or chimpanzee lineages, and it is essential to extend such efforts to unexplored or undersampled areas (Fig. 1). It is also essential to continue developing tools of phylogenetic inference. Bayesian approaches are promising, but uncertainty remains about their applicability to morphological data (159). Improvements in the treatment of continuous characters and recent methodological advances for analyzing three-dimensional geometric morphometric data within a cladistic framework (in combination with traditional characters) are promising for reconstructing fossil hominoid phylogeny (160). The oldest (recently retrieved) ancient DNA is ~1 Ma (161). Paleoproteomics could be a complementary solution because it has enabled sampling further back in time up to ~2 Ma, recently confirming the pongine status of Gigantopithecus (162). Future technological advances in paleoproteomics could potentially help to answer key questions by retrieving paleoproteomes from Miocene apes.

Locomotor reconstructions of the Pan-Homo LCA and other fossil hominoids are seriously hampered by the lack of current analogs. Washburn spotted the fundamental limitation: It is not possible to bring the past into the laboratory. No one can see a walking Australopithecus [(163), p. 67]. Such inferences rely on morphofunctional assumptions of bone, joint, or muscle function, but experimentally derived biomechanical data are required to test these assumptions and provide reliable inferences from fossils. Technological advances now facilitate noninvasive kinematic data collection from animals in their natural environments (164). In turn, experimental and morphological information should be integrated to better predict the locomotion of fossil hominoids. Forward dynamic simulations offer a powerful pathway for predicting de novo movements in fossil species while iterating possible effects of morphology and soft tissue (165).

Humans are storytellers: Theories of human evolution often resemble anthropogenic narratives that borrow the structure of a heros journey to explain essential aspects such as the origins of erect posture, the freeing of the hands, or brain enlargement (166). Intriguingly, such narratives have not drastically changed since Darwin (166). We must be aware of confirmation biases and ad hoc interpretations by researchers aiming to confer their new fossil the starring role within a preexisting narrative. Evolutionary scenarios are appealing because they provide plausible explanations based on current knowledge, but unless grounded in testable hypotheses, they are no more than just-so stories (167).

Many uncertainties persist about fossil apes, and the day in which the paleobiology of extinct species can be undisputedly reconstructed is still far away. 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 approach will allow us to better discern primitive and derived traits, the common from the specific, or the unique. This is the role of fossil apes in human evolution.

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