The discovery adds weight to the theory that climate change played a significant part in the evolution of early humans.

Dr. Jeremy Marlow, of Newcastle University's Department of Fossil Fuels and Environmental Geochemistry, who led the team of English, American and German scientists, said:

"There have been arguments for many years about whether the emergence of our ancestors was linked to climate change. By looking at the molecular fossils of microscopic marine algae, we began to discover evidence of a 10 degree fall in temperature in the region of Africa where much of the early human fossil evidence has been discovered.

"We didn't believe it at first, but further tests kept producing similar results until we had to conclude that temperatures really had decreased so dramatically."

The scientists, from the Universities of Newcastle, Durham, California and Bremen, found that cooling was particularly rapid about 2 million years ago -- at the time when the first ancestors of modern humans emerged in sub-tropical southern Africa.

The research also sheds new light on the mechanisms that may cause climate change.

By examining the rate of sediment deposition and the levels of organic carbon within the sediments, the researchers obtained evidence of a well-defined cycle in which a cooling atmosphere causes increased upwelling of nutrient-rich deep waters in specific parts of the oceans, leading to increased biological uptake of carbon dioxide from the atmosphere, which then cools further, causing more upwelling and uptake of carbon dioxide.

This mechanism took hundreds of thousands, or even millions, of years to have an effect on climate. But it could be reversed far more rapidly through the burning of this type of locked-up carbon as fossil fuels, Dr. Marlow said.

(Editor's Note: This news story is based on an article in Science concerning factual findings. That article did not discuss the implications for human evolution.)

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