All living humans can trace their ancestry to Africans, who began settling the rest of the world sometime in the past 100,000 years. But precisely when the exodus took place has been a matter of sharp debate. Now, a study that provides a new date for an old skull offers fresh clues to when modern humans left Africa and where they went.
Geneticists and paleoanthropologists disagree about whether the ancestors of modern humans began to leave Africa as early as 100,000 years ago or as recently as 50,000 years ago. Although several recent genetic studies have supported a late exodus, fossil evidence has furnished few clues to the timing. Paleontologists have not found any fossils from the crucial period from the most likely source--sub-Saharan Africa.
It seems that they just had to take a closer look at a stunning skull that's been sitting on a South African museum shelf for more than 50 years. A team of international researchers led by paleoanthropologist Frederick Grine, of Stony Brook University in New York re-analyzed this skull, discovered in 1952 in a dry river bed near Hofmeyr, South Africa. Its age and identity had remained mysterious, because initial attempts to date it failed. Some researchers thought it could be less than 10,000 years old.
In this week's Science, the team reports that it used state-of-the-art optically stimulated luminescence and uranium series to pinpoint the skull's age to about 36,000 years. Although they could not use radiocarbon dating on the bone itself, the researchers dated the cementlike carbonate that coated the inside of the skull vault. Two different tests confirmed that the soil was deposited at one time, shortly after the fossil was buried.
Further analysis showed that this skull looks more like fossils of modern humans who lived in Europe and Asia about 36,000 years ago than fossils of Africans or Europeans from the past 10,000 years. That result suggests that the Hofmeyr skull is closely related to the modern humans who first swept into Europe and Asia, and that they all were the offspring of a source population in sub-Saharan Africa. The study also provides the first fossil evidence that the ancestral stock of modern humans left Africa recently. "If there was a very late migration out of Africa, you would expect Europeans at that time to look like Africans at that time," says Grine.
Two independent dating experts say the results look solid. "I do not see how the skull could possibly be older than the age of the carbonate," says Ann Wintle, an expert in luminescence dating methods at the University of Wales in Aberystwyth, U.K. Grine will soon compare the Hofmeyr skull with a soon-to-be-published contemporary skull from Romania to see if they are long-lost cousins.