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Michael Hopkin

Published online: NATURE 16 February 2005; | doi:10.1038/news050214-10

Radioactive dating finds that fossil skulls are 195,000 years old.

This skull, called Omo II, is from the earliest known member of our species. © Michael Day

Two Ethiopian fossils have been crowned as the oldest known members of our species. An estimated 195,000 years old, the pair were witness to the earliest days of Homo sapiens.

The discovery adds yet more weight to the argument that Africa, and Ethiopia in particular, was the birthplace of humans. The dating sits well with genetic analyses of modern populations, which suggest that H. sapiens first appeared in Africa around 200,000 years ago.

The fossils, called Omo I and Omo II, were found in 1967 at Kibish, near Ethiopia's Omo River, by the famed fossil-hunter Richard Leakey. Although Leakey realized that Omo I, at least, was a H. sapiens, the dating of mollusc shells found with the bones suggested that the specimens were only 130,000 years old.

"In 1967, dating techniques weren't what they are now," says John Fleagle of Stony Brook University, New York, who took part in the latest analysis, published in Nature¹. And besides, Leakey and his colleagues were more concerned with hunting for something millions of years older. "The fact of the matter is, they wanted early hominids; modern humans were like chump change," Fleagle says.

The finds confirm that East Africa was a key area in this story.  Chris Stringer Natural History Museum in London

As a result, nobody attempted to date the fossils' burial site more accurately, despite its significance in helping to settle the debate over humanity's African roots. "When modern human origins became a big issue in the early 1980s, Ethiopia was closed," Fleagle says.

Argon dating

And when the researchers, led by Ian McDougall of the Australian National University in Canberra, attempted to visit Kibish on their latest expedition, it was far from plain sailing. "The logistics are a nightmare. We spent days and weeks waiting just to get a boat to go there," recalls Fleagle.

© Nature

When they finally made it, McDougall's team collected samples of the rock where the Omo fossils were found. Using an improved dating method based on the rate of decay of radioactive argon, the researchers put the age of rock just below the fossils at 196,000 years.

The rock layers were formed in rapid bursts, corresponding to wet periods during which huge amounts of organic matter were dumped in the region by the overflowing River Nile, Fleagle says. This means that the fossils are likely to be only slightly younger than the rocks on which they were lying.

The age of the Omo fossils provides yet more support for the 'out of Africa' theory, which contends that humankind spent most of its life in Africa, before sweeping across the world during the past 40,000 years. "The finds confirm that east Africa was a key area in this story," says Chris Stringer, who studies human origins at the Natural History Museum in London.

Ethiopian hotspot

But it is still unclear whether Ethiopia can claim to be the sole crucible of humankind, or whether modern humans arose more widely and gradually across the continent. "Archaeological finds from southern Africa suggest that that region may have played an important part in the development of modern human behaviour, which is also part of what defines us as a species," Stringer says.

Nevertheless, the dating of the Omo fossils earmarks them as older than a set of ancient human skulls found in Herto, Ethiopia. These were unveiled in 2003 and hailed at the time as the oldest humans (see "Skulls reveal dawn of mankind"). The Herto hominids were christened as a new subspecies, H. sapiens idaltu, meaning 'elder'.

Such a move is unnecessary for the Omo specimens, Fleagle says. Omo I has always been viewed as thoroughly modern in appearance. And although Omo II, which consists of just a skull with no face, has more primitive features, Fleagle maintains that it is still best assigned to H. sapiens, particularly as both skeletons are now thought to be the same age.

"The only interpretation is that there was a lot of diversity at that time," Fleagle reflects. "There are no simple linear patterns, so I'd be reluctant to draw a line anywhere. And anyway, if you do that, how many subspecies are you going to end up with?"