The skulls were found within 650 feet (200 meters) of one another and had all eroded from the same geological layer of ancient river and beach deposits. No other skeletal bones were recovered and the team found no evidence for intentional burial.
Project geologists and paleontologists say that 160,000 years ago a shallow, freshwater lake teeming with crocodiles, catfish, and hippopotamus covered the region and supplied sustenance to the hominids that lived on the shore. Buffalo roamed the land.
A hippopotamus skull found amongst the remains has clear cut marks made by stone tools, though the team cannot tell whether the hominids actively hunted the animals or scavenged them.
The tools found amongst the remains document a transitional period in tool technology wherein earlier hand axe-dominated assemblages of the wood and bone Acheulean gave way to the flake-dominated assemblages of the Middle Stone Age, according to the team.
Each of the skulls also bears cut marks made by stone tools in what appear to be the result of some sort of post-death ritual.
"They were manipulating the skulls after the death of the individual as some sort of mortuary practice that involved not only de-fleshingor taking the flesh offbut also keeping the bones around for some purpose," said White.
There is no evidence to suggest cannibalism, but the closest analog to these skulls come from Papua New Guinea, where the remains of the dead are retained for ritual practices that involved cannibalism.
"Could you have the same sort of modification of bones without any ingestion of human flesh? Of course you can," said White. "It is a completely open question that we will need additional remains to answer."
Susan Antón, a paleoanthropologist at Rutgers University in New Brunswick, New Jersey, said that the cut marks do not appear cannibalistic in nature, but rather seem to represent some other sort of mortuary practice.
"This would signal the importance of culture in these ancient hominids, which should not come as a surprise to us," she said. "I expect they were much more like us than we have given them credit for being."
White and colleagues assigned the fossils to a new species of Homo sapiens after comparing them to other fossils as well as a worldwide sample of several thousand human skulls.
They found that the fossils from Herto are similar to, but do not duplicate, the anatomy of modern humans. Their faces are longer, the skulls more robust, and the brow ridges are larger than those of modern humans, for example.
"It was a very large individual with a complex set of characters that show it is anatomically similar to modern humans, but there are a few differences," said White. "To recognize the differences, we named the subspecies."
According to White, the differences link anatomically modern humans with more archaic forms of humans. This link also fits in with the hominids' tool technology which represents a transition from the Acheulean to the Middle Stone Age, he said.
Stringer says the characteristics White and colleagues use to justify assigning the Herto fossils to their own subspecies of Homo sapiens may not be so unusual among modern humans.
"Personally, I don't think the subspecies name is very useful," he said. "I think those features would be found in other parts of the world in the Pleistocene, for example in Australia."
Whether the fossils represent an immediate ancestor of modern humans or are indeed the first modern humans, Stringer says that the fossils are a fantastic find that places the origin of modern humans in Africa.
The next question is where in Africa did modern humans evolve?
"There might be just one small place where modern humans originated and spread out from there, or did different bits of Africa contribute to the overall modern human pattern?" said Stringer.
Further research and better dating of the African fossils may eventually reveal an answer.
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