Fluctuating
environment may have driven human evolution
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The researchers
examined lake sediments from Olduvai Gorge in northern Tanzania, looking for
biomarkers -- fossil molecules -- from ancient trees and grasses [Credit:
Gail Ashley]
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A series of rapid environmental changes in East Africa
roughly 2 million years ago may be responsible for driving human evolution,
according to researchers at Penn State and Rutgers University.
"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."
According to Katherine Freeman, professor of
geosciences, Penn State, the current leading hypothesis suggests that evolutionary
changes among humans during the period the team investigated 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."
According to Magill, many anthropologists believe that
variability of experience can trigger cognitive development.
"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," he 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 researchers -- including Gail Ashley, professor of
earth and planetary sciences, Rutgers University -- examined lake sediments
from Olduvai Gorge in northern Tanzania. They removed the organic matter that
had either washed or was blown into the lake from the surrounding vegetation, microbes
and other organisms 2 million years ago from the sediments. In particular, they
looked at 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.
The team used gas chromatography and mass spectrometry
to determine the relative abundances of different leaf waxes and the abundance
of carbon isotopes for different leaf waxes. The data enabled them to
reconstruct the types of vegetation present in the Olduvai Gorge area at very
specific time intervals.
The results showed that the environment transitioned
rapidly back and forth between a closed woodland and an open grassland.
To find out what caused this rapid transitioning, the
researchers used statistical and mathematical models to correlate the changes
they saw in the environment with other things that may have been happening at
the time, including 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 the
environment and sea-surface temperature in the tropics.
"We find complementary forcing mechanisms: one is
the way Earth orbits, and the other is variation in ocean temperatures
surrounding Africa," Freeman said. The researchers recently published their results in the
Proceedings of the National Academy of Sciences along with another
paper in the same issue that builds on these findings. The second paper shows
that rainfall was greater when there were trees around and less when there was
a grassland.
"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."
Source: Penn State
via EurekAlert! [December 27, 2012]
