Humankind may have survived after leaving Africa thanks to seasonal droughts — not because they created a time of scarcity, but because they produced a time of plenty.

University of Texas at Austin anthropologist John Kappelman presented this counterintuitive idea October 19 in a talk titled “Blue Highways,” which followed his fossil digs along the Blue Nile tributaries in Ethiopia. Early humans are thought to have taken one of two routes out of Africa: along the Red Sea, or along the Nile Valley and out across Eurasia.

But “there’s been very little testing on the ground, recovering fossils and sites that actually permit us to evaluate either one of those two hypothetical migration events,” Kappelman said.

Most fossils found to date come from the rift valley on the eastern side of the continent, where dry, flat, exposed land makes for good fossil hunting. In the late 1990s, Kappelman started exploring the tributaries on the western side of the Nile, where no one had looked for fossils before. The last record of western exploration there was from British naturalist Sir Samuel Baker in the 1860s.

“This area that was a blank slate for Africa is finally starting to fill in,” Kappelman said.

Samuel Barker noticed something key: The rivers are dry for most of the year, but every summer the water rushes back “like freight cars,” Kappelman said. The torrent of water gouged out deep holes that retained water even during the dry season, leaving a necklace of isolated pools.

And the pools were full of fish. “The fish were literally in a bucket,” Kappelman says. If early humans stayed near these water holes, they could feast all through the dry season without working too hard.

“We think of dry seasons as a time of adversity. We’re proposing that these were the easy times,” Kappelman says.

Kappelman and his team found double-edged blades that were probably used as arrow heads and evidence of hearth fires in several sites around the Nile. He thinks using these water holes could have taught early humans crucial skills, like fishing with nets or bow and arrow, that helped them survive seasonal and climate changes after migration to other parts of the world.

“It honed the behavioral foraging habits of early humans, and taught them to exploit a wide range of food,” Kappelman said.

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