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Did Climate Change Trigger Human Evolution?

James Owen
for National Geographic News
February 2, 2006
 
It may be a threat to humans' long-term future on the planet, but climate change may have helped bring us into being in the first place, some scientists say.

Some human-origins theories suggest that ancient climate changes acted as powerful evolutionary drivers, spurring our ancestors to stand tall on two legs, grow large brains, and develop other human traits (related reading: "Was Darwin Wrong?").

The evolution of early human species, so the theories go, was concentrated in periods marked by fluctuating environmental conditions.

Other theories suggest that humankind emerged independently of climatic swings, with adaptations arising, for example, out of competition between or within species.

While more evidence is needed to settle the debate, experts say the answer may lie at the bottoms of ancient African lakes.

The main challenge for researchers is to find out whether prehistoric shifts in climate coincided with key stages of development in the early human fossil record, writes ancient-biology expert Anna K. Behrensmeyer in tomorrow's issue of the journal Science. Behrensmeyer is a paleobiologist at Washington, D.C.'s National Museum of Natural History.

Records of ancient global climate change come mainly from ocean sediments. These suggest cooler, drier, and more variable conditions kicked in some three million years ago.

Early Human Fossils

Differing amounts of dust blown from the land into these seabed sediments indicate that continents were affected as well as the oceans.

But, Behrensmeyer said, "in the continental basins that preserve hominid [early human] fossils, the record of climate change is much harder to decipher."

Geological forces, including erosion and tectonic movements, have largely obscured or erased such evidence, she said.

Humans are thought to have evolved in the Great Rift Valley region of East Africa. Studies have linked our forebears' emergence to drier conditions, as indicated by the ocean records.

Researchers say the growth of arid grasslands at the expense of tropical forests may have prompted the first humans to split off from other primates.

For instance, a study in the journal Nature in 2004 identified adaptations for running in human fossils more than two million years old. These adaptations may have enabled early humans to chase down prey on the open plains of Africa, researchers said. (See "Humans Were Born to Run, Fossil Study Suggests.")

More recent research, published last year in Science, suggested that a period of more sudden, regional climate fluctuation played a key role in human development.

Analysis of soil layers in the Great Rift Valley showed evidence for three unusually wet periods between 2.7 and 1 million years ago.

Fossils of aquatic algae indicated the presence of extinct lakes, some more than 328 feet (100 meters) deep, which quickly formed then disappeared.

Researchers say the lakes are evidence of the type of rapid climate swings that might have driven human evolution, forcing populations to adapt and readapt to cope with fast-changing environmental conditions.

"Strong Evidence"

"These temporary humid periods would have imposed huge impacts on early humans, and our research provides strong evidence for theories in which early human species evolved and spread out in response to a rapidly changing environment," said Mark Maslin, a co-author of the new study and a prehistoric-climate scientist at University College London, England.

But Behrensmeyer, the ancient-biology expert, said it remains unclear how fluctuating lake levels "could have exerted selective pressure on the immediate ancestor of Homo and resulted in the emergence of [the first humans]."

There is also uncertainty over the precise time and place of humanity's origin, she adds. Estimates range from 2.6 million to 1.7 million years ago. (Read "Did Early Humans First Arise in Asia, Not Africa?")

Behrensmeyer says samples from the bottoms of some of the world's oldest lakes, in East Africa, should provide more complete data "and a stronger bridge between oceanic and continental climate records."

Unlike sediment records found on land, the beds of deep tropical lakes are thought to hold relatively undisturbed records of climate change laid down in layers over thousands of years.

Now an international team of scientists hopes to get a better picture of African and global climate history from mud drilled up from the bottom of Lake Malawi in the Great Rift Valley.

The samples, dated to as many as 1.5 million years ago, should enable researchers to make detailed comparisons with climate change records from temperate and polar regions, according to lead researcher Christopher A. Scholz, of the Department of Earth Sciences at Syracuse University in upstate New York.

The project should uncover "an archive of environmental change that occurred in concert with human evolution," Scholz said.

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