for National Geographic News
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.
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