How the Climate Changed Human Evolution

JIM LEHRER: Now another climate change story, this one reaching back to the earliest origins of human history.

Tonight's NOVA explores a provocative new theory about those possible connections.

Here's an excerpt. The narrator is Lance Lewman.

LANCE LEWMAN: Two million years ago, what jump-started human evolution?

Scientists all over Africa looked for clues. Here in Kenya, they found some at the southern end of the Great Rift Valley. It's a hotbed of tectonic activity, where ancient layers are forced to the surface.

Ten million years ago, Africa was a much wetter place, a tropical jungle which has been slowly drying out ever since. But these rocks in Kenya show that Africa's gradual drying trend was punctuated by bursts of wild climate fluctuation.

Rick Potts is an expert in reading the rocks.

RICK POTTS, paleoanthropologist, Smithsonian Institute: This layer right here represents about 1,000 years of environmental stability. But then we had an abrupt volcanic eruption. And then the lake was around for perhaps 500 years before a drought, and the lake came back.

So, in some cases, we saw this through layer after layer of environmental change.

LANCE LEWMAN: With his trained eye, Rick could see some layers were once lake beds. Others desert sands. Still, others came from volcanic eruptions, a snapshot of a million years of climate history. This observation led him to an amazing new idea: rapid change as a catalyst for our evolution.

RICK POTTS: And I began to think that, well, maybe it's not the particular environment of a savanna that was important, but the tendency of the environment to change.

LANCE LEWMAN: Against the backdrop of a slow drying trend, Africa was periodically pulsing with climate change, wet, dry, then wet again, sometimes in the space of 1,000 years.

Punishing drought alternated with storms and monsoons. Rivers and forests sprang up, then turned to dry grassland, all in the evolutionary blink of an eye.

MARK MASLIN, University College London: So, we have a complete change of our ideas, from this slow drying-out, to this incredible change between wet and dry, wet and dry.

LANCE LEWMAN: What effect did that have on our ancestors? Could these periods of climate instability be the key to understanding the evolutionary leap from small bipedal apes to the larger-brained toolmaker, Homo habilis?

JIM LEHRER: And beginning tonight and over the next three weeks, "NOVA" will answer that question and others about human origins. Please check your local listings for the time.

And you can trace the origins of man in an interactive family tree. That's on "NOVA"'s Web site. Follow a link from ours, NewsHour.PBS.org, for that.