A twist in our ancestry

Their geography changed,too. While earlier hominid fossils have been found only in Africa,newer ones also turn up at sites from Georgia all the way to Indonesia. These hominids are so much like modern humans that paleoanthropologists consider them the earliest members of our own genus,Homo.

But they didnt belong to our species,Homo sapiens. After all,their brains were still no more than two-thirds the size of our own,and they could only make crude stone tools. But if not Homo sapiens,then Homo what?

That turns out to be a remarkably hard question to answer.

Earlier this month,a team of scientists offered a rare glimpse of the diversity of early Homo fossils. In the journal Science,they compared five gorgeously preserved,1.8 million-year-old skulls from Dmanisi in the Republic of Georgia. One skull had a large jaw and a tiny brain less than half the size of a human one. But some of their brains were up to 25 per cent bigger than that specimens,while their jaws were much smaller.

They dont represent distinct species, said G Philip Rightmire of Harvard University,a co-author of the study. Theyre just one group.

Rightmire and his colleagues then compared the Dmanisi skulls to early Homo skulls found across the Old World. All of those far-flung fossils fell within the same range of variation.

Based on this analysis,the scientists declared that all those early Homo fossils belong to a single species,which they suggested should be called Homo erectus. If other researchers find evidence to support this view,Rightmire said,it would have a big impact on how we understand human evolution. Well have to go back to drawing boards and rethink the origins of Homo, he said.

If Homo erectus was indeed a single species,its range would have been tremendous compared with our closest ape relatives. Chimpanzees,for example,live only across a narrow band of sub-Saharan Africa. In fact,ape species tend to split into new ones.

Two million years ago,for example,a bend of the Congo River cut off the chimpanzees in the southern part of the species range. Those southern chimpanzees evolved into smaller,more slender apes that today are more peaceful than their warring cousins to the north. Their DNA reveals little sign that they have interbred with other chimpanzees over the past 2 million years despite living within a few miles of them. Biologists have given them their own species name: Pan paniscus,commonly known as bonobos.

If Homo erectus were like chimpanzees,it would be remarkable for them to hang together across rivers and deserts and mountain ranges as a single species. But Todd R Disotell,a biological anthropologist at New York University,suggests that early Homo might be more like another primate: baboons. Baboons live across much of Africa,as well as the Middle East. From place to place,they look so different.

Baboon expert Clifford J Jolly first proposed that our ancient relatives were joined together by a baboonlike web of connections. Distinct groups of hominids lived in distinct ranges. But in some places,they came into contact with other hominids,and their biology allowed them to interbreed.

While this explanation may turn out to be closer to the truth than the idea of a single species,it would leave us struggling to find the right name for our ancient relatives. Was each one a separate species,deserving of its own name? Pointing to baboons,Disotell said,We will never solve the species problem for fossils if we cant for living,breathing animals. NYT