December 25, 2012
Rapid Environmental Changes May Have Help Drive Human Evolution
April Flowers for redOrbit.com - Your Universe Online
A research team from Penn State and Rutgers University found that roughly 2 million years ago in East Africa, a series of rapid environmental changes may be responsible for driving human evolution."The landscape early humans were inhabiting transitioned rapidly back and forth between a closed woodland and an open grassland about five to six times during a period of 200,000 years," said Clayton Magill, graduate student in geosciences at Penn State. "These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years."
The current leading hypothesis, according to Katherine Freeman professor of geosciences at Penn State, suggests that evolutionary changes among humans during the period of investigation were related to a long, steady environmental change or even one big change in climate.
"There is a view this time in Africa was the 'Great Drying,' when the environment slowly dried out over 3 million years," she said. "But our data show that it was not a grand progression towards dry; the environment was highly variable."
The variability of experience can trigger cognitive development according to many anthropologists.
"Early humans went from having trees available to having only grasses available in just 10 to 100 generations, and their diets would have had to change in response," Magill said. "Changes in food availability, food type, or the way you get food can trigger evolutionary mechanisms to deal with those changes. The result can be increased brain size and cognition, changes in locomotion and even social changes -- how you interact with others in a group. Our data are consistent with these hypotheses. We show that the environment changed dramatically over a short time, and this variability coincides with an important period in our human evolution when the genus Homo was first established and when there was first evidence of tool use."
The research team examined lake sediments from Olduvai Gorge in northern Tanzania, first removing the organic matter that had washed or blown into the lake from the surrounding vegetation, microbes or other organisms 2 million years ago from those sediments. They were especially interested in biomarkers — fossil molecules from ancient organisms — from the waxy coating on plant leaves.
"We looked at leaf waxes because they're tough, they survive well in the sediment," said Freeman.
To determine the relative abundances of different leaf waxes and the abundance of carbon isotopes for different leaf waxes, the team used gas chromatography and mass spectrometry. The data obtained enabled the team to reconstruct the types of vegetation present in the Olduvai Gorge area at very specific time intervals.
The findings of this study show that the environment of the Gorge transitioned rapidly back and forth between a closed woodland and an open grassland.
To understand this rapid transitioning, the research team used mathematical and statistical models to correlate the environmental changes with other things that may have been occurring at the time. These things include changes in the Earth's movement and changes in sea-surface temperatures.
"The orbit of the Earth around the sun slowly changes with time," said Freeman. "These changes were tied to the local climate at Olduvai Gorge through changes in the monsoon system in Africa. Slight changes in the amount of sunshine changed the intensity of atmospheric circulation and the supply of water. The rain patterns that drive the plant patterns follow this monsoon circulation. We found a correlation between changes in the environment and planetary movement."
The team also found a correlation between changes in sea-surface temperatures in the tropics and changes in the environment.
"We find complementary forcing mechanisms: one is the way Earth orbits, and the other is variation in ocean temperatures surrounding Africa," Freeman said.
"The research points to the importance of water in an arid landscape like Africa," said Magill. "The plants are so intimately tied to the water that if you have water shortages, they usually lead to food insecurity."
"Together, these two papers shine light on human evolution because we now have an adaptive perspective. We understand, at least to a first approximation, what kinds of conditions were prevalent in that area and we show that changes in food and water were linked to major evolutionary changes."
The results of this new study have recently been published in Proceedings of the National Academy of Sciences, along with a second paper describing the relationship of rainfall abundance to forest and grassland prevalence.