Few things can grow in the dry, sandy soil of Ethiopia's Middle Awash region. For Tim White, however, the land has - once again - produced a rich crop of discoveries that can further explain the complicated origins of humankind.
Professor White, one of the world's most prominent anthropologists, yesterday unveiled the fruits of his latest research - three well- preserved skulls belonging to the earliest members of our own species, Homo sapiens.
The three individuals, two adults and one child, lived about 160,000 years ago, making their skulls about 60,000 years older than the previous oldest fossils of anatomically "modern" humans.
Professor White, who is based at the University of California, Berkeley, said that the three very probably belonged to the group of ancient humans from which everyone alive today is descended.
"With these new crania [skulls] we can now see what our direct ancestors looked like," said Professor White, whose study is published in the journal Nature.
The date of the fossils is important because it matches precisely the age at which Homo sapiens is said to have diverged from its ancestral line as calculated from the genetic analysis of human DNA.
"We've lacked intermediate fossils between pre-humans and modern humans, between 100,000 and 300,000 years ago, and that's where these fossils fit," Professor White said. "Now, the fossil record meshes with the molecular evidence."
For decades, Professor White and his team of scientists have scoured the Middle Awash in search of fossils and stone artefacts that could shed light on the evolution of early man.
Their strategy is to walk the fossil-rich sites at regular intervals, examining every stone and pebble in case a fragment of bone, tooth or stone tool has emerged from the ground as the soil erodes. One day in November 1997, the scientists were on such a survey of a dry, desolate valley bordering the Middle Awash river near the village of Herto when Professor White spotted some stone tools and the fossilised skull of a butchered hippopotamus.
Eleven days later he returned with a team to excavate the site more thoroughly. Out of the same patch of land, now further exposed by heavy rains, emerged the most complete of the adult skulls. All three fossils were sandwiched between two volcanic layers that could be accurately dated to about 160,000BC. The age of the bones and their remarkable state of preservation made the discovery unprecedented.
"It's a great find," said Professor Chris Stringer, head of human origins at the Natural History Museum in London.
"These could be the earliest ancestors of all of us. They are in the right time and the right place and it is powerful evidence that Homo sapiens originated in Africa."
Clark Howell, a team member, said: "This set of fossils is stupendous. This is truly a revolutionary scientific discovery."
A key anatomical feature of the skulls was their similarity to skulls of modern humans and the distinct separation from other species of hominids, such as Neanderthal man, Dr Howell said. "These well-dated and anatomically diagnostic Herto fossils are unmistakably non-Neanderthal. These fossils show that near-humans had evolved in Africa long before the European Neanderthals disappeared," Dr Howell said.
"They thereby demonstrate conclusively there was never a Neanderthal stage in human evolution," he added.
For decades, anthropologists have argued over whether Neanderthals were a direct ancestor of modern humans or whether they were a distant cousin whose lineage died out more than 30,000 years ago.
Berhane Asfaw of the Rift Valley Research Service in Addis Ababa, who is another member of Professor White's team, said the three Herto skulls finally answer the question. "These fossilised skulls from Herto show modern humans were living at around 160,000 years ago with full-fledged Homo sapiens features. The hypothesis is now tested [and] we can conclusively say Neanderthals had nothing to do with modern humans. They went extinct."
Although the three skulls show unequivocal similarities with the skulls of anatomically modern humans they also possess traits at the back of the skull they share with "Rhodesia man", found in what is now Zimbabwe, who lived about 500,000 years ago.
Because of that, researchers placed the fossils in a separate subspecies category called Homo sapiens idaltu - the word "idaltu" in the Afar language means "elder". The skull of the child suggests it died at the age of six or seven and delicate cut marks on the bone suggest it was subjected to mortuary rituals, such as defleshing, and the presence of apparently polished surfaces on the skull indicates it was handled repeatedly after death.
Professor White said the Herto people probably engaged in funeral rituals, perhaps the preservation and worship of the skulls of close relatives.
No other human bones were found at the site, suggesting the skulls were carried and "moved around", Professor White said. "They probably cut muscles and broke skull bases of some to extract the brain but why, whether as part of a cannibalistic ritual, we have no way of knowing.
"These were people using a sophisticated stone technology, using chipped hand axes and other stone tools," Professor White added.Reuse content