Climate swings shaped human evolution, researchers claim

The evolution of our earliest human ancestors was driven by wild swings in eastern Africa's ancient climate, scientists claim today.

The rapidly changing climate reshaped the landscape, leaving once plentiful food and water resources in scarce supply and placing enormous pressure on early humans to adapt.

The sustained upheaval drove some species to the brink of extinction, while other better-suited relatives emerged and flourished, the scientists believe.

Researchers identified several extreme shifts in climate dating back millions of years to when humans were first emerging on the continent. Three of the greatest periods of climatic change took place around 2.5m, 1.5m and 1m years ago. These roughly coincide with the appearance of Homo habilis, the first human species; Australopithecus afarensis, a sturdy primitive ape, and the later human species, Homo erectus, which became adept at stone tool use and hunting.

Researchers led by Mark Maslin, director of the Environment Institute at University College London, conducted geological surveys of ancient lakes throughout eastern Africa. They found evidence that over the past 3m years, giant lakes up to 300 metres deep formed and then vanished with the changing climate. The disappearances of the lakes were followed by periods of extreme drought.

"At one extreme, the landscape would have been a true Garden of Eden, with beautiful freshwater lakes, beautiful shorelines and forests along the rivers. There would have been open spaces allowing early humans to exist easily, with water and lots of resources," said Maslin. "But occasionally, these quickly flipped into bone dry periods, where it's 45C in the middle of the day and no natural water resources."

Early humans and other primate species, collectively known as hominids, were forced to adapt to the new environment. Humans developed larger brains, evident in Homo habilis and Homo erectus, enabling them to fashion simple stone cutting tools and form effective hunting groups. Other ancient primates, such as the squat Australopithecus afarensis, emerged with powerful jaws that allowed them to chew and get nutrition from tough roots and vegetables.

"If you look at the new species of hominid that evolved, 80% of those, or 13 out of 15, appeared during these pulsed climate periods. It suggests new human species evolved when the climate was highly variable. We don't know if it's the wet period, the dry period or the transition that triggers this, but we can say that when the climate is highly variable, you get a big change in species."

The research, conducted with scientists at Adelphi University in New York, was published today in the Journal of Human Evolution.