Apes Among the Tangled Branches of Human Origins

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Science  29 Jan 2010:
Vol. 327, Issue 5965, pp. 532-534
DOI: 10.1126/science.1184703

The detailed description of Ardipithecus ramidus (1) more than lived up to the buzz of anticipation that preceded it in the paleoanthropological community. A. ramidus is a purported hominin (the group comprising humans and their extinct relatives after they diverged from our closest living relatives, the chimpanzees) from the Middle Awash region of Ethiopia. The focus of attention has been on how A. ramidus may relate to later fossil hominins and to living apes and humans (see the first figure), but to appreciate the place of A. ramidus in human origins, we must also view it from the perspective of the hominoids (apes) that lived in the Miocene, 23 to 5 million years ago (see the second figure).

A. ramidus is known from more than 100 specimens, including a remarkably preserved partial skeleton, that date back to 4.4 million years ago (2). Several other hominin contenders are known from the late Miocene (7 to 5 million years ago), including Ardipithecus kadabba, Orrorin tugenensis, and Sahelanthropus tchadensis, but our knowledge of their anatomy is much less complete. Not all paleoanthropologists (including this author) accept that A. ramidus is a hominin or agree with the evolutionary and paleobiological interpretations that have been proposed (2), but there is no doubt about its critical importance for understanding human origins (3, 4). The unveiling of A. ramidus has required a major rethinking of what the last common ancestor of humans and chimpanzees looked like and which initial evolutionary steps may have characterized the earliest hominins. A. ramidus also helps to close the gap between the last common ancestor of humans and chimpanzees (estimated at 7 to 5 million years ago) and the earliest undoubted hominin, Australopithecus anamensis (4.2 million years ago) (5).

A classification of the extant catarrhines (Old World monkeys and apes).

During the early Miocene (23 to 16 million years ago), the precursors of hominoids—the proconsuloids—were a remarkably diverse group of catarrhine primates (the group comprising Old World monkeys and apes, see the first figure) restricted to the tropical forests and woodlands of Africa and the Arabian Peninsula (6). Between 17 and 14 million years ago, environments in Africa became drier and increasingly more seasonal. Proconsuloid diversity declined, and cercopithecoids (Old World monkeys) and early hominoids, such as Kenyapithecus, Equatorius, and Nacholapithecus, became the dominant taxa (see the second figure). These hominoids and other catarrhines responded to increased seasonality by developing dietary adaptations for eating leaves or for processing hard food items, and by developing a range of specialized locomotor behaviors (68).

About 16 to 15 million years ago, apes expanded their geographic range out of Africa to colonize much of Eurasia. This influx of hominoids into Eurasia coincided with the middle Miocene climatic optimum, a phase of global warming that allowed tropical and subtropical mammals to extend their ranges northward. The earliest Eurasian apes, Griphopithecus and Kenyapithecus, are known from sites in Turkey and central Europe. Like their African contemporaries, they had thick-enameled molars and robust jaws, adaptations for exploiting a broad spectrum of seasonally available foods.

Between 13 and 9 million years ago, hominoid diversity in western and central Europe increased to include Pierolapithecus, Anoiapithecus, and at least four species of Dryopithecus (9, 10). Pierolapithecus and Anoiapithecus from Spain are probably stem hominids (great apes and humans, see the first figure). Dryopithecus has been inferred to be a stem hominid, an early member of the orangutan lineage, or a stem hominine (African great apes and humans, see the first figure), but the first of these options is the most plausible.

A diversity of hominoids also occurred in Asia during the middle and late Miocene, extending from Indo-Pakistan to Thailand. Of these, Ankarapithecus, Sivapithecus, Lufengpithecus, Khoratpithecus, and Gigantopithecus are all likely to be closely related to the extant orangutan (11).

Gradual cooling during the middle Miocene led to greater seasonality in western and central Europe and a shift from subtropical evergreen forests to predominantly deciduous broadleaved woodlands. This shift was accompanied by a dramatic turnover of the mammalian fauna at 9.6 million years ago, termed the Mid-Vallesian Crisis, when most hominoids became extinct (12). The highly specialized stem hominid Oreopithecus survived on European island refugia until 6 to 7 million years ago. In southeast Europe and southwest Asia, hominoids specialized for dry open woodlands, including Ouranopithecus and Udabnopithecus, survived well into the late Miocene (10 to 7 million years ago). Ouranopithecus probably offers the best evidence of an early hominine in Eurasia, which implies that African great apes extended their range from Africa into southeast Europe and southwest Asia about 10 million years ago.

About 7 to 8 million years ago, uplift of the Tibetan Plateau and increased intensity of the Asian monsoon, together with the global expansion of C4 grasses, led to a further decline in the diversity of Eurasian hominoids. By 5 million years ago, hominoids had become extinct throughout Eurasia, except for those surviving in the present-day range of Asian hominoids (orangutans and hylobatids, see the first figure), extending from southern China to Southeast Asia.

In Africa, the fossil record for hominoids between 13 and 7 million years ago is relatively sparse. This has led some authors to postulate that the hominines initially diverged in Eurasia before migrating back into Africa (13, 14). However, recent discoveries and a growing appreciation of later Miocene hominoid diversity in Africa make this an untenable scenario. The recently described 10-million-year-old Nakalipithecus from Kenya is closely related to Ouranopithecus but is older and has more primitive teeth, implying that these taxa shared a last common ancestor in Africa (15). It has been suggested that Samburupithecus (9.5 million years old) from Kenya and Chororapithecus (10 to 10.5 million years old) from Ethiopia are related to gorillas, but the evidence is slim, and they are probably stem hominines (16). Further recent fossil finds confirm that in the middle and late Miocene, Africa was populated by a multitude of hominoids, but the current material is too scanty to designate additional species (15, 17, 18).

Hominoid relationships.

Schematic representation of the inferred evolutionary relationships between Miocene apes, early hominins, and extant hominoids. Solid gray bars represent the known time range of each genus, thin dark lines are inferred relationships between the genera, and thin dashed lines with “?” denote uncertain relationships.

As paleontological exploration intensifies across Africa, our knowledge of hominoids in this critical time period will steadily grow. Rather than just a few relictual evolutionary strands surviving to the end of the Miocene and giving rise to modern hominine lineages, as was previously thought, ape diversity in Africa during the late Miocene looks very bushlike. The relationships between Ardipithecus and earlier hominids will remain enigmatic until the quality of the fossil evidence from the late Miocene of Africa improves, but this will eventually prove critical in resolving its affinities to later hominins. The important questions then become: Where did Ardipithecus and the other early hominin contenders come from? Are they truly members of the hominin lineage, or simply apes among the tangled branches that constitute the basal hominine bush?


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