16 March 2012

Warm sanctuaries key to human evolution

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Modern and ancient humans withdrew to milder sanctuaries during the Ice Ages in Europe and Asia, and these refuges became critical for human evolution, according to a new study.
human dispersal refugees ancient neanderthals

An illustration of Neanderthals at the cave site of Trou Al'wesse in Belgium, clinging on as the climate deteriorated.
Credit: Digital Painting by James Ives

DUBLIN: Modern and ancient humans withdrew to milder sanctuaries during the Ice Ages in Europe and Asia, and these refuges became critical for human evolution, according to a new study.

New models published in a paper in Science today suggest that refugia – locations that harbour relict populations of a once-widespread species – were important in determining the pace and pattern of the massive human migration from Africa, which began approximately 100,000 years ago.

“The refugia would have been in areas such as Iberia, Italy, the Balkans and Israel, and similar places to the east of Eurasia,” said co-author John Stewart, a palaeoecologist at Bournemouth University in England.

For Britain, waves of humans repeatedly colonised the area, only to be driven out in times of severe cold. There are also increasing signs that modern humans and ancient humans mixed, complicating the story of our evolution, the researchers said.

Ancient human ancestors

Modern humans evolved in Africa more than 200,000 years ago; they expanded to all continents bar Antarctica, and the Americas by about 20,000 years ago. But scientists are discovering that a wealth of human species lived in the intervening period, some contributing to our genetic makeup.

Neanderthals likely occupied Europe for more than 200,000 years and were only joined by anatomically modern humans – humans that looked like us – around 50,000 years ago.

The authors compiled genomic data on these two ancient human ancestors and linked them to studies of other organisms that survived many glacial cycles. They used that information to create new models of the mass migration from Africa, around 100,000 years ago.

“We know that Neanderthals and modern humans must have overlapped in western Eurasia in at least a general sense from about 55,000 to 35,000 years, but it is difficult to pin down specific overlaps,” said co-author Chris Stringer, anthropologist at the Natural History Museum in London. But beginning 70,000 years ago, Neanderthals began to suffer population falls due to repeated changes in the climate. “It was a double whammy of rapid climate changes and the arrival of a human competitor. Whether similar processes operated elsewhere, for other humans, we cannot say.”

Siberia’s Denisovans

What we can say is that all modern humans outside Africa have Neanderthal DNA due to inbreeding and, said Stewart, some humans today have DNA from another extinct human group, the Denisovans. This sister group to Neanderthals lived in Siberia at the same time as the Neanderthals were dwindling in their final southern European enclaves.

The team combined new knowledge gained in human evolution with our current understanding of how Ice Age refugia affected evolutionary processes, and considered the how species evolve, hybridise, disperse and become extinct. Since many humans, such as Neanderthals, did not adapt to cold easily, understanding what went on in the warmer enclaves that they withdrew to is critical to understanding human evolution, they said.

Human diversity

“It has become increasingly clear that the evolution and spread of modern humans was much more complicated than we thought. Discovering through fossils and genes that there was so much more ancient human diversity than expected, and that it may have admixed, is an exciting challenge,” commented Robert A. Foley, professor of human evolution at the University of Cambridge in England. “This paper tries to bring some sense to all of this by mapping the ebb and flow of us and our closest relatives onto the rapidly changing climatic conditions.”

Foley said that trying to solve the problems of recent human evolution is like doing a jigsaw puzzle on a table that is constantly moving, and for which we have only a few pieces. “Chris Stringer and John Stewart have given us some important pointers to how some of the pieces can go together.”

Research on human evolution has moved beyond bones and must be multidisciplinary, added José Carrión, evolutionary biologist at the University of Murcia, Spain, who was not involved in the study. “As for classic debates such as the Neanderthal extinction, it is now clear that hypotheses based on environmental change and competitive exclusion alone are not testable. The possibility of early gene flow between Neanderthals and our species, and the appearance of a new actor – the Denisovans – has expanded the puzzle.”

Retreat to warmer places

Human species diversified over the last 100,000 years, and it is possible that earlier species such as Homo heidelbergensis and Homo erectus still survived in parts of Africa and Indonesia, whereas the Hobbit human apparently lived on the island of Flores until about 17,000 years ago.

In northern Europe, Stringer believes that various archaic human species and finally early modern humans went through failed colonisations, though sometimes they persisted in northern Europe. Belgium was home to Neanderthals for longer than expected, and the authors speculate that microclimates in deep valleys offered warmer conditions in an otherwise chilled steppe-tundra landscape.

While northern Europe saw waves of human colonisation, followed by retreat to warmer areas, Australia was inhabited by 50,000 years ago and from that time on.

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