Climate Change Fueled Human Evolution?
Aug. 18, 2005 - Climate changes that affected Africa over a million years ago may have profoundly influenced human evolution, according to a new study published in this week's Science.
Overall, the findings support the "variability hypothesis" of human evolution, which holds that alternating humid and dry periods provided the stresses essential for species divergence, meaning that our ancestors had to change and adapt so much over time that they evolved into new species.
Previously, some researchers theorized that modern humans, and our ancient hominid relatives, evolved in an increasingly arid environment in East Africa.
The researchers discovered that large, deep lakes, which are indicators of humidity and higher rainfall patterns, existed between 2.7 to 2.5 million, 1.9 to 1.7 million and 1.1 million to 900,000 years ago in East Africa.
"We believe that these humid episodes could have had important impacts on the speciation and dispersal of hominins because they correlate with important events in human evolution," said Martin Trauth, who led the research.
Trauth, a paleoclimatologist at the University of Potsdam in Germany, explained to Discovery News that around 2.6 million years ago, Australopithecines, or our earliest known direct ancestors, emerged in Africa. Homo erectus, the first upright human ancestor, emerged 1.8 million years ago and hominids began their "out of Africa" migration to settle other parts of the world.
During the final humid period, at around 1 million years ago, Homo erectus began a second major migration and Paranthropus, a small-brained descendent of Australopithecines, went extinct.
Trauth and his team discovered the climate changes by studying diatoms, which are tiny silica algae that live in bodies of water off of sunlight. The researchers analyzed sediments from ancient lakebeds in East Africa that contain diatom fossils at various levels.
Since diatoms need sun, they are not present at the bottom of deeper lakes. The researchers, however, found a lot of evidence for surface-dwelling diatoms in the deep ancient lake sediments over the three identified periods. That suggests that the lakes fluctuated widely during times of drought or heavy rainfall.
The researchers ruled out volcanic and tectonic causes for the lake depths, which leaves climate change as a possible reason for the increased water levels.
Richard Potts, who formulated the "variability hypothesis" and is director of the Human Origins Program at the Smithsonian Institute's National Museum of Natural History, told Discovery News that he agreed the climate history of East Africa has been complex, but that more work was needed to prove that environmental changes in that region were due to broad climate change.
Potts, however, does still believe that environmental changes fueled human evolution.
"Environmental variability leads to the expansion, contraction, and fragmentation of habitat zones," Potts explained. "This process of habitat change tests the ability of populations to diverge, experiment in their adaptations, and then meet up again - at which time populations of what may have once been the same species could have become distinct enough to hinder effective reproduction between members of those populations."
[edit: related article on the BBC]
Overall, the findings support the "variability hypothesis" of human evolution, which holds that alternating humid and dry periods provided the stresses essential for species divergence, meaning that our ancestors had to change and adapt so much over time that they evolved into new species.
Previously, some researchers theorized that modern humans, and our ancient hominid relatives, evolved in an increasingly arid environment in East Africa.
The researchers discovered that large, deep lakes, which are indicators of humidity and higher rainfall patterns, existed between 2.7 to 2.5 million, 1.9 to 1.7 million and 1.1 million to 900,000 years ago in East Africa.
"We believe that these humid episodes could have had important impacts on the speciation and dispersal of hominins because they correlate with important events in human evolution," said Martin Trauth, who led the research.
Trauth, a paleoclimatologist at the University of Potsdam in Germany, explained to Discovery News that around 2.6 million years ago, Australopithecines, or our earliest known direct ancestors, emerged in Africa. Homo erectus, the first upright human ancestor, emerged 1.8 million years ago and hominids began their "out of Africa" migration to settle other parts of the world.
During the final humid period, at around 1 million years ago, Homo erectus began a second major migration and Paranthropus, a small-brained descendent of Australopithecines, went extinct.
Trauth and his team discovered the climate changes by studying diatoms, which are tiny silica algae that live in bodies of water off of sunlight. The researchers analyzed sediments from ancient lakebeds in East Africa that contain diatom fossils at various levels.
Since diatoms need sun, they are not present at the bottom of deeper lakes. The researchers, however, found a lot of evidence for surface-dwelling diatoms in the deep ancient lake sediments over the three identified periods. That suggests that the lakes fluctuated widely during times of drought or heavy rainfall.
The researchers ruled out volcanic and tectonic causes for the lake depths, which leaves climate change as a possible reason for the increased water levels.
Richard Potts, who formulated the "variability hypothesis" and is director of the Human Origins Program at the Smithsonian Institute's National Museum of Natural History, told Discovery News that he agreed the climate history of East Africa has been complex, but that more work was needed to prove that environmental changes in that region were due to broad climate change.
Potts, however, does still believe that environmental changes fueled human evolution.
"Environmental variability leads to the expansion, contraction, and fragmentation of habitat zones," Potts explained. "This process of habitat change tests the ability of populations to diverge, experiment in their adaptations, and then meet up again - at which time populations of what may have once been the same species could have become distinct enough to hinder effective reproduction between members of those populations."
[edit: related article on the BBC]