Climate Change Allowed Humans to Migrate Out of Africa
Mati Milstein in Tel Aviv, Israel
National Geographic Adventure
|August 29, 2007|
Ancient cave formations found in Israel provide the first concrete evidence that climate changes allowed early humans to migrate out of Africa, researchers say.
A team of Israeli scientists studied stalactites and stalagmites, or speleothems, found in five caves deep in the Negev Desert in southern Israel (see Israel map).
The growth patterns of the formations, which only develop in the presence of rainwater, revealed a major cluster of unusually rainy periods beginning some 140,000 years ago, the scientists said.
The rainy spells match the period of the first modern human settlements in the Middle East, the team added.
"We found that the period of enhanced rainfall allowing the growth of speleothems occurred roughly 140,000 to 110,000 years ago, with its height being 130,000 to 125,000 years ago," said Anton Vaks, a doctoral student with the Geological Survey of Israel (GSI) and the Hebrew University of Jerusalem.
These dates correspond with modern human settlements found slightly farther north in Israel's Carmel region and near Nazareth. Archaeological evidence has dated those sites to about 100,000 to 130,000 years old.
The wet periods formed what were essentially climatic windows that allowed migration north through the Sahara and up into Asia via a "land bridge" on the Sinai Peninsula, Vaks explained.
"The desert began to shrink both from the south and also from the north," he said.
"The entire Sahara turned into something much, much smaller, and the desert barrier [out of Africa] was much less significant."
Nile Became "Highway"
The researchers analyzed the cave deposits using high-precision spectrometry to measure their periods of growth.
The wet seasons reflected in the formations likely helped ancient humans pass through the otherwise arid region, Vaks said.
"These monsoon rains strengthened the Nile's flow, forming a northbound 'highway,'" Vaks said.
"The climate along the shoreline of the Red Sea was also much less extreme during this period, and archaeologists have found evidence of migration along the coasts."
(See a map of ancient human migration.)
"It is reasonable that there is a connection between a wet period along the Sinai-Negev land bridge and the appearance of early modern man for the first time outside of Africa," he added.
Experts have been examining the influence of climate on human migration and evolution for years. But this is the first time researchers have turned up hard evidence, Vaks' team said.
(Read related story: "Climate Change May Have Helped Humans Out of Africa, Study Says" [June 12, 2006].)
"This is the first time there is both evidence and exact dating," said Hebrew University geographer and research team member Amos Frumkin.
"This evidence fits within a network of other information we have on the migration of modern humans from Africa to Asia."
Emory University anthropologist John Kingston, who was not involved in the study, agreed that the new find provides important physical clues to the history of early human migration.
"This is really significant in providing empirical evidence for ideas that existed already," he said. "To have empirical evidence like this is golden."
The connection between the rainy spells seen in the cave formations and the existing archaeological evidence in Carmel and Nazareth is also reasonable, Kingston added.
The use of speleothems to map climates is increasingly popular, he continued.
"What speleothems have that nothing else has is resolution," Kingston said.
"It's really a good terrestrial indicator. You can not only get the environmental information but link it to dates as well, and that's the key part here."
The research team also included Hebrew University's Alan Matthews and GSI's Ludwik Halicz. Their findings are published in the current issue of the journal Geology.
The comfortable corridor through the Sinai and Negev wilderness didn't last long, said GSI's Miryam Bar-Matthews, who took part in the research.
"Anton [Vaks'] work showed that those who moved northward from Africa could not return. Immediately afterward, the desert once again became a real harsh desert, so they couldn't move back," Bar-Matthews said.
Kingston agreed but added this was likely not the only period of comfortable passage through the northern Sahara.
"I would advocate that these corridors come and go," he said. "It's not like this was the last chance to get out of Africa or back in."
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