Three fossilised skulls unearthed in Ethiopia are said by scientists to be among the most important discoveries ever made in the search for the origin of humans.
By Jonathan Amos
BBC News Online science staff
The crania of two adults and a child, all dated to be around 160,000 years old, were pulled out of sediments near a village called Herto in the Afar region in the east of the country.
Herto skull: Dated at between 160,000 and 154,000 years old
They are described as the oldest known fossils of modern humans, or Homo sapiens.
What excites scientists so much is that the specimens fit neatly with the genetic studies that have suggested this time and part of Africa for the emergence of mankind.
"All the genetics have pointed to a geologically recent origin for humans in Africa - and now we have the fossils," said Professor Tim White, one of the co-leaders on the research team that found the skulls.
"These specimens are critical because they bridge the gap between the earlier more archaic forms in Africa and the fully modern humans that we see 100,000 years ago," the University of California at Berkeley, US, paleoanthropologist told BBC News Online.
Out of Africa
The skulls are not an exact match to those of people living today; they are slightly larger, longer and have more pronounced brow ridges.
These minor but important differences have prompted the US/Ethiopian research team to assign the skulls to a new subspecies of humans called Homo sapiens idaltu (idaltu means "elder" in the local Afar language).
The Herto discoveries were hailed on Wednesday by those researchers who have championed the idea that all humans living today come from a population that emerged from Africa within the last 200,000 years.
The proponents of the so-called Out of Africa hypothesis think this late migration of humans supplanted all other human-like species alive around the world at the time - such as the Neanderthals in Europe.
If modern features already existed in Africa 160,000 years ago, they argued, we could not have descended from species like Neanderthals.
"These skulls are fantastic evidence in support of the Out of Africa idea," Professor Chris Stringer, from London's Natural History Museum, told BBC News Online.
"These people were living in the right place and at the right time to be possibly the ancestors of all of us."
The skulls were found in fragments, at a fossil-rich site first identified in 1997, in a dry and dusty valley.
Stone tools and the fossil skull of a butchered hippo were the first artefacts to be picked up. Buffalo fossils were later recovered indicating the ancient humans had a meat-rich diet.
The most complete of the adult skulls was seen protruding from the ancient sediment; it had been exposed by heavy rains and partially trampled by herds of cows.
The skull of the child - probably aged six or seven - had been shattered into more than 200 pieces and had to be painstakingly reconstructed.
All the skulls had cut marks indicating they had been de-fleshed in some kind of mortuary practice. The polishing on the skulls, however, suggests this was not simple cannibalism but more probably some kind of ritualistic behaviour.
This type of practice has been recorded in more modern societies, including some in New Guinea, in which the skulls of ancestors are preserved and worshipped.
The Herto skulls may therefore mark the earliest known example of conceptual thinking - the sophisticated behaviour that sets us apart from all other animals.
"This is very possibly the case," Professor White said.
The Ethiopian discoveries are reported in the journal Nature.