Scientists have discovered three human skulls, all dated to be about 160,000 years old, in the Afar region of Ethiopia. BBC News Online explains why researchers regard the finds as so significant.
What precisely was found just outside the Herto village in the east of Ethiopia?
A US/Ethiopian team has been working on the site since 1997. The three skulls - from two adults and one child - represent the most complete crania so far recovered. The fossilised fragments of seven other individuals have also been unearthed but these, in some cases, are represented by just teeth. Hippo and buffalo remains were also found, as were more than 640 stone artefacts - tools made from volcanic rocks and glasses.
What do we know about the place where these individuals were living?
The evidence suggests these ancient Herto people lived near the shore of a shallow freshwater lake created when the Awash River temporarily dammed about 260,000 years ago. The lake contained abundant hippos, crocodiles and catfish, while buffalo roamed the land. Much of Europe was buried in ice at this time.
HUMAN FAMILY TREE
Scientists are trying to piece together the species relationships
Why have the skulls caused so much excitement?
They are the oldest yet found of Homo sapiens - modern looking people who would not appear unusual today if you met them. The previous oldest H. sapiens skulls were from South Africa (Klasies River Mouth, circa 100,000 years) and Israel (Qafzeh and Skhul, circa 90,000-130,000 years). Scientists are sure of the greater antiquity of the Herto specimens because they were pulled from sediments sandwiched between volcanic rocks that have been well dated using a radioactive argon technique.
But the skulls are slightly different from modern humans today?
The largest of the skulls, probably from an adult male in his late 20s to early 30s, is a bit larger than the extremes seen in modern Homo sapiens; the braincase is longer and the brow ridges are more pronounced. As a result, the researchers have given the fossils a subspecies name, Homo sapiens idaltu, to differentiate them from contemporary humans, Homo sapiens sapiens.
How do these skulls fit with what we know of other human-like creatures, or hominids?
The differences described above are hugely significant because they echo features seen in some older African hominid fossils, such as Homo heidelbergensis, whilst at the same time displaying a very modern look we would recognise today. In essence, the researchers argue, the Herto skulls fill a gap between the more archaic humans who went before and the very modern people who came after. The Herto people could be our direct and immediate ancestors.
And how does this fit with what we have learnt recently from genetic studies?
By looking at the genetic variation in all living populations today and in studying the errors that have arisen in our genome over time, molecular biologists have come to one conclusion: we diverged as a species less than 200,000 years ago in Africa. A recent study even narrowed the location down to Tanzania and Ethiopia. The Herto skulls therefore represent a confirmation of the genetic studies. They show the right features in the right place at the right time.
What can we now say about the origin of humans?
There are two major schools of thought. One says Homo sapiens migrated out of Africa recently to supplant all the other human-like species around the world, such as Europe's Neanderthals; the "Multiregional" school says modern humans arose in many areas of Europe, Asia and Africa from other hominids who had migrated out of Africa at a much earlier time.
Because the Herto fossils show anatomically modern features that pre-date most Neanderthals, it seems inconceivable that we could have descended from them, as some scientists have proposed.
The Herto skulls support the first school, the so-called "Out of Africa" hypothesis.