Review

Fossil apes and human evolution

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Science  07 May 2021:
Vol. 372, Issue 6542, eabb4363
DOI: 10.1126/science.abb4363

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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. Almécija 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

Structured Abstract

BACKGROUND

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.

ADVANCES

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 trap“—an 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.

OUTLOOK

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.

The evolutionary history of apes and humans is largely incomplete.

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.

Abstract

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.

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