Next time you are in a crowded bus, take a good look at your fellow passengers. With luck, they'll be a good sample of the enormous variety of contemporary humanity. Now imagine that all these passengers were buried and left to fossilize for a million years. How many distinct species of humanity would future palaeontologists record?
The chances are that they would recognize several distinct species. But we know this is the wrong answer. Biologists recognize that species reflect distinct populations that do not normally interbreed with with other populations to produce viable hybrids. Social pressures aside, there is no biological barrier that prevents any two living humans producing children. All modern humans, whatever their appearance, belong to a single biological species, whose individual members are highly variable. The breeding status of populations is impossible to read from fossils, of course, but the message is clear: when identifying extinct species, palaeontologists should be sensitive to the range of variation that exists in living species.
This issue lies at the heart of a report in the 2 October 1997 Nature from Dr Gen Suwa of the University of Tokyo and colleagues. The researchers describe the near-complete 1.4-m.y.-old skull of an extinct human relative, Australopithecus boisei, from Konso in Ethiopia. This skull is the first well-dated example of A. boisei in which the skull has been found in association with a jawbone. Although definitely a member of boisei, the skull displays some features associated with remains of other species, such as A. robustus from South Africa, and the poorly known A. aethiopicus, also from Ethiopia.
All these species were members of the so-called 'robust' australopithecines, a lineage which, between 2.5 and 3 Ma, split from the lineage that led to modern humans. Although they walked upright, like modern humans, they were highly specialized for a vegetarian existence. They became extinct more than a million years ago. The three species, East African boisei and aethiopicus, and South African robustus have been seen as distinct. But can that view be justified? The researchers point to the mixture of features apparent from the Konso skull, and advise caution.
The case of our own lineage shows how temptation can lead to confusion. Nobody disputes that all modern humans belong to one species, Homo sapiens. It is generally but not universally agreed that this species descended from H. erectus, which first appeared in Africa just over 2 Ma and subsequently spread across Eurasia. Again, it is generally but not universally agreed that H. erectus descended from indigenous African species such as H. habilis, around 2.5 Ma.
In recent years, this simple story has become blurred by a plethora of new names. The status of Homo habilis is most in question: some fossils assigned to it could come from an australopithecine. Others may be better placed with H. erectus, or, if one prefers, H. ergaster, seen by some as the immediate African precursor of the Eurasian H. erectus. The remainder could belong to genuine H. habilis, unless, of course, they fit in better with a similar-yet-different species called H. rudolfensis. Suddenly, what seemed to be an orderly evolutionary transition has become a sudden burst of speciation about 2.5 Ma, on which people have started to build scenarios of global climate change and so on‹yet such efforts are houses of cards unless the species issue is settled.
© Macmillan Magazines Ltd 1997 - NATURE NEWS SERVICE
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