Fluctuating environment may have driven human evolution
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 three 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 two 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.
- ScienceDaily
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