Human ancestors in Eurasia earlier than thought
Mon Jun 13 2011 (Arabian Standard Time) Oman Time
Human ancestors in Eurasia earlier than thought
Reid Ferring, an anthropologist at the University of North Texas in Denton,
and his colleagues excavated the Dmanisi site in the Caucasus Mountains of
Georgia.
They found stone artefacts ‘ mostly flakes that were dropped as hominins
knapped rocks to create tools for butchering animals ‘ lying in sediments
almost 1.85 million years old. Until now, anthropologists have thought that
H. erectus evolved between 1.78 million and 1.65 million years ago ‘ after
the Dmanisi tools would have been made. Furthermore, the distribution of the
122 artefacts paints a picture of long-term occupation of the area.
Instead of all the finds being concentrated in one layer of sediment, which
would indicate that hominins visited the site briefly on one occasion, the
artefacts are spread through several layers of sediment that span the period
between 1.85 million and 1.77 million years ago. The findings are published
in the Proceedings of the National Academy of Sciences.
`This is indeed suggestive of a sustained regional population which had
successfully adapted to the temperate environments of the southern
Caucasus,’ explains Wil Roebroeks, an archaeologist at Leiden University in
the Netherlands.
Eurasian ancestry?
The presence of a tool-using population on the edge of Europe so early hints
that the northern continent, rather than Africa, may have been the
evolutionary birthplace of H. erectus.
Unfortunately, the fossils of the hominins responsible for making the tools
are not proving very helpful to the debate. Fossilised bone fragments found
in the same sedimentary layers as the Dmanisi artefacts are too weathered to
be identified as belonging to any one species, so it is impossible to say
for sure whether the tools were made by H. erectus. Neither do fossil skulls
previously retrieved from later sediments at the site help to resolve the
controversy.
These fossils, dating from 1.77 million years ago, had brains between 600
and 775 cubic centimetres in volume, whereas H. erectus is generally thought
to have had an average brain size of around 900 cubic centimetres. For
comparison, modern humans have a brain capacity of around 1,350 cubic
centimetres. `Many people call those Dmanisi fossils the earliest H.
erectus, but there is still frequent debate about this,’ explains Ferring.
There and back again
Even if the ancient inhabitants of the Dmanisi site were not early members
of H. erectus, there is still a problem: anthropologists have previously
thought that no hominins existed outside of Africa as early as 1.85 million
years ago.`Anthropology textbooks of the 1990s often showed maps with large
arrows indicating migration of early H. erectus from its inferred core area
of eastern Africa to other parts of the Old World,’ explains Roebroeks. The
findings in Dmanisi make such an explanation look faulty. Ferring and his
colleagues propose that some ancestors of H. erectus might have travelled to
Asia and possibly Europe, done a bit of evolving, then wandered back to
Africa.
Climate change guide to authors
`Remember, it would not have been obvious to the hominins they were leaving
Africa. There were no signs saying `You are leaving Africa now ‘ come and
visit us again!” says Bernard Wood, an anthropologist. But Wood admits that
it is unclear why the hominins might have made these movements. `It
perplexes me,’ he says.
Ferring suggests that ancient hominins might have been following their food
source ` animals. `My hunch is that the migrations relate to the rise of
carnivory and a sudden flexibility to live and eat meat anywhere,’ he says.
Vegetarians, he explains, are limited to the specific plants that sustain
them and cannot travel from tropics to deserts to mountains nearly as easily
as predators can. Wood agrees. `My guess is that hominins were following
game,’ he says.
Other possibilities also exist. `We tend to think of hominins as living in a
disease-free world, but maybe they were eliminated in some places by an
epidemic, and the only healthy ones left were at the edges of their
distribution,’ who could then move back into the vacated areas, says Wood.
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