An international research team led by Professor Dr Frank Schabitz has published a climate reconstruction of the last 200,000 years for Ethiopia. This means that high-resolution data are now available for the period when early Homo sapiens, our ancestors, made their way from Africa to Europe and Asia. Schabitz and his colleagues determined the dates using a drill core of lake sediments deposited in southern Ethiopia's Chew Bahir Basin, which lies near human fossil sites.
Temporal resolution of the samples, reaching nearly 10 years, revealed
that from 200,000 to 125,000 years before our time, the climate there was
relatively wet, providing enough water and thus abundant plant and animal food
resources in the lowlands of East Africa. From 125,000 to 60,000 years ago, it
gradually became drier, and particularly dry between 60,000 to 14,000 years
ago. The data now obtained fit well with genetic findings, according to which
our direct genetic ancestors ('African Eve') left Africa 'successfully' during
a wet phase about 70,000 to 50,000 years ago.
Scientists collect information about the environment from lake
sediments because, in the best case, sediments are flushed into lakes
relatively continuously from the catchment through erosion. In addition to
mineral components, sediments include organic material and remains of organisms
living in the lake. If lake sediments from suitable lakes can be drilled, these
'proxy data' can be used to draw conclusions about environmental conditions at
the time, and thus help to reconstruct the climate.
From November to December 2014, the researchers recovered an
approximately 300 metre long drill core from the Chew Bahir Basin in southern
Ethiopia, which dries out during the dry season. In its entirety, the drill
core dates back to about 620,000 years. "This enables us to
chronologically cover the entire evolutionary history of Homo sapiens in
Africa. The work now published on the last 200,000 years of this drill core
thus provides very good evidence of the environmental and climate history
during the migration of our ancestors," Schabitz explained.
"Some of our proxies allow time resolution for specific decades in
large sections of the core, which has not been done before for this part of
Africa. That way we can capture very short-term climate changes representing
less than a human lifetime," he said. The drill core reveals that the
climate of East Africa was largely influenced by changes in solar insolation,
which led to either wet or dry climate conditions.
a) Map of Greenland and location of NGRIP ice core; b) Map of northeastern Africa, the Near East, and southeastern Europe showing important fossil and archaeological sites; c) Map of northeastern Africa showing the location of the Chew Bahir drill site as well as important fossil and archaeological sites [Credit: Frank Schaebitz et al. 2021]
From 200,000 to 125,000 years ago, the climate was generally relatively
favourable, i.e., the lowlands provided enough water and thus abundant plant
and animal food resources for our ancestors. Under such conditions, people
could move relatively easily over long distances and even reach the Arabian
Peninsula, as evidenced by the oldest fossil finds there (about 175,000 years
ago). From 125,000 to 60,000 years ago, however, it gradually became drier, and
then particularly dry between 60,000 to 14,000 years ago, with the lake drying
up completely several times.
"However, during this period in particular, quite striking,
short-term moisture fluctuations can also be observed, the temporal patterns of
which are reminiscent of cold-warm climate fluctuations known from Greenland
ice cores. So the people who lived in East Africa at that time were exposed to
extreme changes in their environments," Schabitz said.
"It is interesting that just in the period from 60,000 to 14,000
years ago, when the lowlands of East Africa were repeatedly particularly dry,
numerous archaeological findings in the high altitudes of the Ethiopian
mountains bear witness to the presence of our ancestors there." In
addition, the weapons and tools of these people also evolved during this time
period (transition from Middle to Late Paleolithic in Africa). "We suspect
that the greater "environmental stress" at lower elevations forced
this development," the scientist noted.
Furthermore, the scientists noted that the last major wet phase which
we can see in the core fits well in time with the genetic findings: It shows
that our direct genetic ancestors 'successfully' left Africa about 70,000 to
50,000 years ago. Their descendants probably reached southeastern Europe 50,000
to 40,000 years ago, where they encountered Neanderthals.
"We hypothesize that the evidence of dry-humid climate
fluctuations in East Africa found in our drill core had a significant impact on
the evolution and mobility of our ancestors," said Schabitz.
"Migration out of Africa was possible several times during the last
200,000 years, during periods when the climate was wetter, and has led to the
spread of our ancestors as far as Europe. During the particularly dry phases of
the recent past, starting around 60,000 years ago, Homo sapiens groups
repeatedly managed to survive in the high altitudes of mountainous
Ethiopia."
The study has been published in Nature Communications.
Source: University of Cologne
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