Brain evolution: climate shifts into gear.

Scientists have generally assumed that stable savanna environments allowed human ancestors to evolve the big brains that made possible such mentally complex feats as tool making and organized foraging. Now, new evidence indicates that our Stone Age forebears encountered a surprising amount of climate change, forcing them to cope with unfamiliar habitats.

Frequent, jarring environmental shifts over the past 2 million years may have provided the fundamental evolutionary spur to human brain expansion, contends Richard Potts, an archaeologist at the Smithsonian Institution in Washington, D.C.

"I'm suggesting that the genus Homo evolved by a process of accommodating to habitat variability and disruption," Potts asserts. "This accounts for increased density and plasticity of neural connections, as well as specialized brain functions that enable mental and social flexibility."

Potts presented data in support of this theory at the annual meeting of the American Anthropological Association in Washington, D.C., last week.

Analyses of oxygen isotopes in soil extracted from the ocean floor and of fossil pollen found in Europe and China indicate that a gradual large-scale shift to a cooler, drier climate, which led to savanna conditions, began around 5 million to 6 million years ago. At this time, the first members of the human evolutionary family appeared. Many substantial climate fluctuations occurred within that overall trend, Potts argues. The largest oscillations in global climate within that period coincided with the evolution of Homo species over the past 2 million years, he maintains.

Research at Olorgesailie in southern Kenya, for example, has uncovered evidence of dramatic environmental changes negotiated by hominid tool makers who inhabited the area from 1.2 million to 500,000 years ago. Climate shifts and earthquakes took particular liberties with a large lake that now dominates the site, says Potts, who directs the Olorgesailie project.

Excavations suggest that the lake expanded significantly around 1 million years ago, but over the next 500,000 years it shrank, shifted its position numerous times, and even disappeared occasionally. Many animals, such as elephants, zebras, and baboons, that had previously flourished near the lake met extinction in that period, Potts notes. However, abundant stone tools and debris reveal that hominids survived those habitat shifts.

The ratio of brain size to body size in early hominids had remained similar to the ratios for other primates, Potts notes. But then, as a result of the repeated climate and habitat shifts, hominid brains began to bulge. "Hominids had to adapt to environmental extremes that altered the conditions of natural selection under more stable conditions," says Potts.

This conclusion dovetails with preliminary evidence that Stone Age groups responded to recurring crisis situations by pooling information and making effective collective decisions (SN: 11/18/95, p.328).