THE evolutionary grandmother of Homo sapiens may actually turn out to be our aunt. That is the possibility raised by new fossil evidence from a research team led by Meave and Louise Leakey of the Koobi Fora Research Project in Nairobi, Kenya.
Until recently, many palaeontologists thought that our own species, H. sapiens, evolved from the small-brained but otherwise relatively modern ancestor a few hundred thousand years ago. H. erectus, in turn, had evolved from the earlier , one of the earliest known members of the genus Homo, about 1.9 million years ago. Some experts also include an intermediate species, H. ergaster.
Now, however, the Leakeys鈥� team have described a new fossil they unearthed in 2000 in Ileret, in the far north of Kenya. It is a fragment of an upper jaw, or maxilla, that dates from 1.44 million years ago. Based on the shape and size of the teeth, the researchers conclude that the fragment belongs to H. habilis ().
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If correct, the fossil is a remarkable find: the next most recent H. habilis fossil dates from about 1.65 million years ago. If H. habilis really survived until 1.44 million years ago, then it lived alongside H. erectus for nearly half a million years 鈥� much longer than was thought. 鈥淲e鈥檝e more or less doubled the overlap,鈥� says Fred Spoor, a palaeontologist at University College London and one of the leaders of the project.
This long period of coexistence would make it less likely that H. habilis gave rise to H. erectus, and would certainly rule out the simplest scenario 鈥� that H. habilis gradually evolved into H. erectus. It also strengthens the idea that the two species occupied different ecological niches, with H. erectus perhaps eating more meat than H. habilis and the latter focusing on tough plant matter, says Spoor.
Several other experts on human origins aren鈥檛 buying the claim, though, at least at first glance. 鈥淚鈥檇 be charitable and say it鈥檒l be controversial,鈥� says Tim White, a palaeontologist at the University of California, Berkeley. 鈥淧ersonally, I think it鈥檒l be blown out of the water. After all, people argue about whole skulls, so you can imagine what they鈥檒l do with a worn-out, scrappy maxilla.鈥�
鈥淧eople argue about whole skulls, so you can imagine what they鈥檒l do with a worn-out, scrappy jawbone鈥�
White has not seen the actual fossil yet but says, judging from the photographs in the paper, that the teeth appear badly worn and distorted. This casts doubt on the tooth measurements that led the Leakey team to declare the fossil was from H. habilis. White notes that if instead the jawbone is merely an unusual H. erectus 鈥� or some other, unknown hominid 鈥� the Leakeys鈥� extended time line of coexistence duly vanishes.
The overlap is shaky at the older end, too: the first known H. erectus fossil, a skull fragment from 1.9 million years ago, is difficult to date accurately. The period of coexistence central to this new research 鈥� between 1.9 and 1.4 million years 鈥� 鈥渟uddenly evaporates, because you鈥檝e got no anchor point on the old end of erectus and the young end of habilis,鈥� says White.
Both sides agree on one thing, at least: the new fossil suggests that more than one hominid species lived in Africa about a million and a half years ago. 鈥淭he more this stuff comes out, the more you think to yourself, wow, there鈥檚 a lot of different hominids,鈥� says Jeffrey Schwartz, an expert in human origins at the University of Pittsburgh in Pennsylvania.
