Reservoirs of evolution: rainy periods linked to human origins in Africa.

A massive, earthquake-induced gash that cuts through eastern Africa contains evidence of three rainy phases during the Stone Age. They might have spurred the evolution of modern humanity's direct ancestors as well as of many other mammal species, scientists suggest.

Extended intervals of heavy rains created deep lakes in several parts of eastern Africa at times critical in human evolution, according to a team led by geologist Martin H. Trauth of Potsdam (Germany) University. Ancient lakes formed between 2.7 million and 2.5 million years ago, between 1.9 million and 1.7 million years ago, and finally between 1.1 million and 900,000 years ago, the scientists report in an upcoming Science.

These watery eras correspond, respectively, to the times when the Homo genus originated, when the species Homo ergaster (sometimes called Homo erectus) first evolved, and when several ensuing Homo species debuted.

The periods of lake formation also roughly correspond to three pronounced global shifts to a colder, drier climate that have been identified in an independent analysis of dust layers in ocean-floor sediments. Trauth and his coworkers propose that those climate transitions exerted a different effect in parts of Africa, triggering periods of substantial rainfall.

The team examined previously dated soil layers in 10 rift basins located in Ethiopia, Kenya, and Tanzania. Sediment containing large amounts of fossilized algae indicated an ancient lake. Each site contained a large lake during one of the three ancient time spans, the researchers say.

In Trauth's view, evidence of recurring rainy periods in Stone Age Africa supports the idea that human ancestors and other animals evolved to deal with a merry-go-round of novel environments spawned by frequent climate changes (SN: 7/12/97, p. 26). The leading proponent of that view, anthropologist Richard Ports of the Smithsonian Institution in Washington, D.C., has dubbed this controversial process variability selection.

"The new findings indicate that East African lakes were created by monsoon rains followed by periods of drought," Potts remarks. "But I think climate change was more frequent than Trauth and his colleagues suggest it was."

For instance, Trauth's team suspects that eastern African lakes may often have sprung up during roughly 20,000-year-long stretches of heavy rainfall. At Olorgesailie, a Kenyan site where Potts has directed research for the past 20 years, Trauth's team identified algae-laden sediment from a lake that they conclude existed between 992,000 and 974,000 years ago.

However, a thin layer of white soil runs through the middle of the algae-bearing Olorgesailie sediment layer, Potts says. So, a drought emptied water from the lake for a period of several hundred years, he argues.

Human ancestors didn't necessarily respond to such changes via variability selection as Potts theorizes, comments anthropologist Mark Collard of the University of British Columbia in Vancouver. Human ancestors might have evolved primarily in response to the global shift to a drier, cooler climate or to climate-related changes in the population mix of animals inhabiting eastern Africa, Collard says.