A giant, long-vanished lake along the White Nile may have been a vital way station for early modern humans leaving Africa. Archaeologists say the 45,000-square-kilometer lake, which would be one of the ten largest lakes on Earth if it existed today, was in the right place at the right time for at least one of two key migrations. One exodus took people to what is now Israel before 100,000 years ago, and another peopled Eurasia 70,000 years
Geologists had seen traces of an ancient lake in the now-arid region south of Khartoum in Sudan, but it was too old for carbon dating so they didn't know when it dried up. So Martin Williams of the University of Adelaide in Australia teamed up with Tim Barrows of the University of Exeter, UK, to try other dating techniques. The two collected samples from former lake-shore deposits, and Barrows dated them to around 109,000 years ago.
Barrows and Williams traced the long-lost freshwater lake along some 650 kilometres of the White Nile, one of the two main tributaries to the Nile. At points, the lake seems to have stretched to almost 80 kilometres wide.
Its peak extent came in the last interglacial period, a warm interval before the last ice age. Barrows says it cannot have stayed at full size for long, because the lake-shore deposits he found took "thousands of years to form, but probably not tens of thousands". His dating techniques measure how long it has been since the deposits were exposed, so his date tells us when the lake first began to shrink.
The last interglacial period was a crucial time for early modern humans, who first appear in the fossil record nearly 200,000 years ago in Ethiopia. Bones from the Qafzeh and Es Skhul caves in Israel show that modern humans reached the eastern Mediterranean by about 110,000 to 100,000 years ago.
But mitochondrial DNA evidence shows that modern Eurasian populations left Africa much later, around 72,000 to 70,000 years ago, says Stephen Oppenheimer of the University of Oxford. He thinks the later migration crossed the mouth of the Red Sea, when the ice age had lowered sea levels by some 100 metres.
The lake was no more than about 12 metres deep, and like other shallow lakes in arid environments, its size would have varied seasonally. But that wouldn't have stopped people using it. "Even in arid times, these lake margins would have retained some stability," says Laura Basell of Queens University Belfast, UK.
Mitochondrial DNA studies suggest the human population in east Africa expanded about 100,000 years ago, when the lake should have been there. Other sites also show evidence of a wet spell around the date of the lake, says Basell, and a skull fragment indicates that modern humans were in Sudan 130,000 years ago.
So far there is no way to confirm that humans lived in the area, because there are very few archaeological sites known from the right period. Barrow did find some artifacts in the area, but none could be shown to be more than a few thousand years old. He says the current conflict in the region means it is too dangerous to return for further excavations. Nevertheless, anthropologists think it was probably inhabited. However, as climate changed and the monsoon rains that fed it dwindled, the lake shrank to nothing.
The lake's eventual disappearance was a disaster for people in the area, forcing them to move elsewhere, says archaeologist Ofer Bar-Yosef of Harvard University. The Nile valley was a good exit route, and migrants might have followed it north to the Mediterranean, and then round to its eastern end, the Levant. So the disappearance of the lake could explain the first migration out of Africa.
The lake may also have been important for the second migration, which populated Eurasia, but this is less clear. It depends on how long it took for the lake to disappear completely, which Barrow's findings do not tell us: his analysis only shows when the lake first began to shrink.If a shrunken version of the lake endured until 70,000 years ago or just before, it could have been a launch site for the second migration. These later migrants may have followed another route through the Ethiopian highlands to the shore of the Red Sea. By 75,000 to 65,000 years ago, the ice age was at its peak so sea levels were lower, making the strait of Bab-el-Mandeb at the southern end of the Red Sea passable.
Journal reference: Geology, DOI: 10.1130/G35238.1