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Complex variation of the East African climate may have played a
key role in the development of our human ancestors.
Scientists have identified extensive lake systems which formed
and disappeared in East Africa between one and three million years
ago. The lakes could be evidence that global climate changes
occurred
throughout this pivotal period in human evolution.
The findings, reported in the journal Science, suggest that
humans evolved in response to a variable climate. Dr Martin Trauth
of the University of Potsdam and his team were able to identify and
date the prehistoric lakes by studying layers of soil along the Rift
Valley in Kenya, Ethiopia, and Tanzania.
Exploring ancient lakes
Layers containing microscopic algae skeletons, called diatoms,
reveal the depth and composition of the ancient lakes. Volcanic ash
in nearby layers provides an estimate of the lakes' ages.
Radioactive elements in the ash act as time stamps because they
decay in a predictable way with time.
By examining soil layers at seven sites throughout East Africa,
Dr Trauth and his collaborators were able to identify three distinct
periods during which extensive lakes covered the region and grew to
depths of hundreds of metres.
They argue that the growth of these lakes resulted from a moist
local climate. The regional wet periods, which may have persisted
for up to 100,000 years, occurred as much of Africa became
increasingly dry.
The periods of wet weather in East Africa might reflect
fluctuations of the Earth's climate as a whole. At the time when the
lakes grew - roughly 2.6, 1.8, and 1 million years ago - glaciers
and the atmosphere were also going through major transformations.
Emergence of humankind
The Science paper states that if the lakes were temporary
features related to the global climate, as the data suggest, they
provide strong support for theories in which early human species
evolved and spread out in response to a rapidly changing
environment.
"These episodes could have had important impacts on the
speciation and dispersal of mammals and hominins," the researchers
write.
Chris Stringer, a leading researcher on early humans in the
department of palaeontology at London's Natural History Museum,
praised the quality of the data, saying that it provides "very good
evidence" of climate change in East Africa.
However, he stressed that more detailed work was necessary to
positively link these environmental changes to the emergence of man.
"What this is showing is that there are fluctuations of the
climatic belts moving up and down," he said. "But if early humans
are able to move around, the effect of varying environment is
reduced. The key issue now is how mobile are these people?"
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