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Becoming Bipeds

William Kimbel

Lowly Origin: Where, When, and Why Our Ancestors First Stood Up.
Jonathan Kingdon.
xx + 396 pp. Princeton University Press, 2003. $35.

With the exception of our large brain and attendant linguistic skills, no physical manifestation of humanity's divergence from an apelike ancestor has evoked greater ink flow than upright, two-legged walking. Because the fossil record of our lineage documents the primacy of bipedalism over brains (by at least 2 million years), modern explications of human origins have focused on the environmental challenges that early hominids overcame by standing upright on the ground and moving about on two legs. In most such tellings, the probability of making a successful shift to the ground from the trees varies inversely with the abruptness of the ecological transition across which it supposedly occurred. If the shift was precipitous—from tree-dwelling in a closed forest to ground-dwelling on an open savanna, for instance—then the plunge to terra firma must have been downright heroic and its success improbable.

In Lowly Origin, Jonathan Kingdon aims to dispel the timeworn idea that prehominids must have confronted such a drastic challenge. He was inspired by recently recovered African fossils indicating that from 6 to 4 million years ago bipedal hominids inhabited varied environments that included ample tree cover. Kingdon spins out a scenario in which the earliest hominids became terrestrial and bipedal in increments, long before moving into open country. In his view, the rise of hominid bipedalism was almost inevitable, and although it constituted a "symbolic moment" in human evolution, it was not a "sudden leap."

Kingdon, a senior research associate in the Institute of Biological Anthropology and Department of Zoology at the University of Oxford, is a largely self-trained expert on the biogeography of African mammals (he is the author and illustrator of the superb multivolume East African Mammals: An Atlas of Evolution in Africa). In Lowly Origin he has written an original and engaging account of human evolution as the evolution of "just one more African mammal." Readers accustomed to stories about human origins that invoke unusual, exceptional or unprecedented conditions and causes will find this book a refreshing change of pace.

According to Kingdon, the common ancestor of chimpanzees and humans was a Eurasian "Tree Ape" that immigrated into eastern and central Africa in the Late Miocene (about 10 million years ago). Some populations of these apes became part of an endemic mammalian fauna (surviving today only in part of southeastern Tanzania) adapted to life in coastal, open woodlands. In these littoral zones, in contrast to the closed inland forests inhabited by chimps and gorillas today, deciduous trees predominate in a seasonally and topographically varied range of relatively open habitats, and food resources are concentrated near the forest floor. During the latest Miocene, these "East Coast Ground Apes" (the ancestors of you and me), after being isolated from their inland-forest-loving cousins to the west (the ancestors of the chimpanzee) by expanding zones of aridity, evolved unique adaptations to committed terrestrial feeding. Kingdon imagines a kind of "squat-foraging": He believes that our vertically short pelvis, curved lumbar spine and forwardly placed articulation between the head and neck, which we usually associate with the acquisition of bipedalism, were initially mechanisms that helped to stabilize the upright trunk of these pre-bipeds while they fed on the ground.

Flowing from his intimate knowledge of African natural history, biogeography and evolution, Kingdon's scenario is vivid and compellingly detailed—all the more so because earliest human evolution in this telling is so unexceptional, the product of universal ecological factors and evolutionary forces operating on "just one more African mammal." Currently, though, his theory is not testable. Kingdon knows that the fossil evidence for these Miocene inhabitants of the East African coastal zone is nonexistent—although this is no defect in his argument in particular, as fossils documenting human and ape evolution between 12 and 6 million years ago are currently next-to-nonexistent anywhere in Africa. However, when confronted with the actual fossil record, Kingdon's storyline is less convincing. He downplays the importance of the 6- to 7-million-year-old Sahelanthropus tchadensis fossil from Chad, whose canine-tooth and skull-base characteristics appear to place it on our side of the chimp-human split (it is thus something of a geographic anomaly in his scheme), while promoting the contemporaneous Orrorin tugenensis from Kenya as the earliest known hominid species, for which a less convincing case has so far been made by its discoverers (in published descriptions, the Orrorin canines are distinctly apelike and the evidence for bipedalism is ambiguous).

By the time we reach a period for which there is a fairly dense fossil record (beginning about 4 million years ago), bipedal hominids were already spread out over much of Africa. Kingdon posits that long-distance inland migrations along wooded river courses during humid climatic phases spurred the transition from an upright squatting ground forager to striding bipeds—bipeds plural, because Kingdon believes, not unreasonably, that these river basins, isolated from one another by zones of aridity that hominids were not yet ready to master, were the main centers of hominid adaptive and taxonomic diversity in the Pliocene.

In the northeast, Australopithecus eventually spread widely to become the "first truly mobile, open country form." But Kingdon misinterprets these hominids, including A. afarensis (the species to which "Lucy" belongs), as clumsy bipeds with gorillalike life histories who retained too many apelike characteristics too late in time to figure in the ancestry of humans. (He thinks that all the known australopiths roaming eastern Africa in the Pliocene became overly specialized and went extinct in the early Pleistocene.) That role, in his view, is reserved for the southern species A. africanus, which Kingdon contends is fundamentally more humanlike than Lucy and other eastern hominids in its brain size (larger), degree of sexual dimorphism (smaller), and hand structure (more adept at manipulation, enhancing the efficiency of terrestrial foraging and predisposing it to tool manufacture)—none of which will sound very familiar to most paleoanthropologists or knowledgeable lay readers. As far as can be told, A. africanus and A. afarensis are basically similar in these attributes, and otherwise the evidence supporting a unique link between A. africanus and Homo is ambiguous at best. In fact the phylogenetic relationship of A. africanus to late Pliocene hominids remains one of the outstanding uncertainties in paleoanthropology.

Kingdon is not on intimate terms with these fossils; his ideas about them rely heavily on other sources, among which he seems unable to sort the credible from the implausible. For example, he asserts that some early Homo species were semi-aquatic (a latter-day nod toward Elaine Morgan's "Aquatic Ape") and that early Homo erectus (also known as H. ergaster) originated in the Atlas Mountains of northwestern Africa and then migrated into eastern Africa—ideas that are well outside the mainstream of paleoanthropological thinking.

Kingdon is at his best in drawing together the strands of ecology, geology and evolutionary biology in a sweeping view of changing ancient African landscapes. This is the best popular account of that subject I have read. Unfortunately, Kingdon populates the terrain with stand-ins for the real hominids, whose histories await a more discerning and persuasive rendering than that on offer here. Despite this central weakness, Lowly Origin deserves to be read and admired for its brilliant evocation of the African backdrop to our evolution.

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