The end of Darwinism.
Nature 412, 131 - 132 (2001) © Macmillan Publishers Ltd.
Palaeontology: Return to the planet of the apes
HENRY GEE
Henry Gee is a senior editor at Nature.
e-mail: h.gee@nature.com
Fossil evidence of human evolutionary history is fragmentary and open to various interpretations. Fossil evidence of chimpanzee evolution is absent altogether.
Discoveries of fossil hominids are like buses: nothing for a while, then three come along at once. Earlier this year, Leakey et al.1 announced Kenyanthropus platyops, a 3.5-million-year-old creature with a disconcertingly modern-looking face, given its otherwise primitive cranium. At the same time, another team2, 3, also working in Kenya, described remains of a new species, Orrorin tugenensis, which at 6 million years old is possibly the earliest known hominid. These discoveries were discussed in accompanying News and Views articles4, 5.
In papers beginning on page 175 of this issue, Yohannes Haile-Selassie and colleagues now describe hominid specimens6 and palaeoenvironments7 from Ethiopia dated at between 5.2 and 5.8 million years old. The hominids are assigned to Ardipithecus ramidus kadabba, an archaic subspecies of A. ramidus, an early hominid previously discovered8 in 4.4-million-year-old sediments in Ethiopia. The designation of A. r. kadabba as a subspecies will be controversial. But all concerned agree that both Orrorin tugenensis and A. r. kadabba are primitive, and they are thought to lie in the family tree close to the point at which the ancestries of extant chimpanzees and humans diverged. Their phylogenetic position is thus pivotal. Definitive resolutions of the status of these creatures could reveal much about the nature, lifestyle and behaviour of the most recent common ancestor of humans and chimpanzees, and the course of human evolution in general. Whether this potential can be fulfilled is another question entirely.
Their respective discoverers claim that both A. r. kadabba and Orrorin were bipedal. When A. ramidus was first described8, bipedality was one of the few features that marked it as a hominid. But A. r. kadabba and Orrorin are more primitive still, raising the question of whether bipedality is a diagnostic hominid trait. In other words, bipedality, as an habitual form of locomotion, might have occurred in lineages of apes that are now extinct. This idea has found support, albeit controversially, in the claim that Oreopithecus bambolii, an ape that lived 7–9 million years ago on an isolated island that is now Tuscany, was bipedal to some extent — and yet this creature is thought to have become bipedal independently and was only distantly related to hominids9.
The idea that bipedality was once more widespread than its current humans-only distribution has several implications. First, one would be forced to consider that the ancestors of chimpanzees as well as of hominids were bipedal, and that the distinctive knuckle-walking habit of living chimpanzees is a secondary acquisition. This challenges the controversial idea10, 11 that the most recent common ancestor of chimpanzees and humans was capable of knuckle-walking.
Second, it is possible that some of these fossils might not be hominids at all (see Box 1 for a guide to the terminology). After all, most researchers agree that the most recent common ancestor of humans and chimpanzees lived around 5–6 million years ago (but see ref. 12), so some of the fossils currently described as hominid might be more akin to chimpanzees, or may represent an entirely extinct offshoot of the ancestry of hominids and chimpanzees — a cousin of the latest common ancestor, if you like.
Moreover, it remains the case that although hominid fossils are famously rare, the chimpanzee lineage has no fossil record whatsoever. One explanation has been that chimpanzees have always lived in forested environments, and that forest creatures are rarely preserved as fossils. Hominids only become (relatively) abundant as fossils after they moved from forests to more open habitats. However, this argument is turned on its head by strong evidence that Orrorin3 and Ardipithecus7 lived in woodland. The fossils of animals such as monkeys and small antelopes found alongside the hominids, as well as palaeobotanical and isotopic evidence, suggest that Ardipithecus lived in a relatively well-forested and high-altitude environment. Indeed, this creature may have been confined to such a habitat: as WoldeGabriel and colleagues7 show, searches for early hominids in geological settings indicative of the open-country habitats associated with later hominids were less rather than more likely to produce results. So it may be that hominids were woodland creatures until about 4.4 million years ago8, 13.
Given that chimpanzees today live in environments rather like those inhabited by Ardipithecus and Orrorin, could it be that at least one of these early hominids is actually more akin to chimpanzees? Questions have been raised about the status of Orrorin as a hominid5, 6. For their part, Senut and colleagues2 defend the hominid status of Orrorin and propose that Ardipithecus is an ancestor of chimpanzees. But they do not discuss the implications of this view for the history of chimpanzees, as distinct from that of hominids.
Sadly, I doubt that the status of these creatures can be resolved to general satisfaction. Some researchers have suggested14 that the dental and skeletal traits conventionally used as the basis for hominid systematics are unreliable guides for reconstructing evolutionary history, in that the phylogenies created using these traits differ from those based on molecular information from living primates. Given that bones and teeth are, for practical purposes, all there is to go on, uncertainty is likely to reign for some time, leaving the nature of the latest common ancestor — and the general course of early hominid evolution — as mysterious as ever.
Is the outlook completely gloomy? Perhaps not. The accumulating data on palaeoenvironments should at least improve our understanding of the lives and times of early hominids (and perhaps of early chimps), even though the evolutionary relationships remain murky.
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