Climate change aids evolution

Climate change led to the appearance of the first humans

By the BBC's Andrew Craig

There is a new theory about what caused the change in the East African climate that may have led to the evolution of humans.

It was clearly a complex combination of circumstances that created the conditions in which our ancestors appeared

Two American researchers say that the catalyst may have been the gradual drifting of the Indonesian islands, rather than the North Atlantic currents which have usually been held responsible.

The first creatures that are usually described as human are thought to have evolved in East Africa after it became drier between four million and two-and-a-half million years ago.

Although there are rival theories, it is widely believed that as forests gave way to grassland, early hominids adapted by spending more time on their two feet, and scavenging meat as well as gathering fruit and roots.

Rainfall change

But why did the African climate change? Mark Cane and Peter Molnar, writing in the journal Nature, say the answer may lie far to the east in Indonesia, whose islands divide the Indian Ocean from the Pacific.

Five million years ago, they believe, much of the water flowing into the Indian Ocean came from the Pacific's southern, warmer half. That warm water led to more rainfall in East Africa.

Our ancestors are thought to have evolved in East Africa

But, as Australia and New Guinea drifted northwards, the warm southern current was blocked by such Indonesian islands as Halmahera, which grew much bigger at the time.

The result was a much higher proportion of cold, North Pacific water reaching the Indian Ocean. That meant less evaporation, and less rain in East Africa.

Until now, the dryer African climate has been ascribed to the closing of the gap between North and South America, and the subsequent flow of the Gulf Stream across the North Atlantic.

It was clearly a complex combination of circumstances that created the conditions in which our ancestors appeared.