Continental rift
It is generally accepted that early man began colonising the planet from Africa, but how were those first settlers replaced: by evolution or invasion? Robin McKie reports
Robin McKie The Guardian, Tuesday April 29 2008
This article appeared in the Guardian on Tuesday April 29 2008 on p8 of the Part IV: Humans section. It was last updated at 00:02 on April 29 2008. The origins of Homo sapiens have been a source of dispute among scientists for decades. Some say our species is old and that its racial divisions are deep and profound. Others claim we are a young species and that differences among races are superficial and unimportant. Only in the last few years has a consensus emerged. Studies of DNA, and of skeletons, now point clearly to the fact that modern humans are recent emigres from Africa. "All men and women are Africans under their skin. It's as simple as that," says Chris Stringer of the Natural History Museum, London, a founder of the Out of Africa theory.
Most scientists accept that humanity originally evolved in Africa. About 2m years ago, our predecessors Homo erectus - tall, tool-making, small-skulled apemen - emerged from the continent and began spreading around the Old World. But what came next is hotly disputed.
One group of scientists - known as the multi-regionalists, led by Milford Wolpoff from Michigan University - claim these ancient humans began to evolve in these new homelands and that the Chinese, aborigines from Australia, the Inuit and other races today are the direct descendants of these ancient emigres. An example is provided by the Neanderthals who dominated Europe from about 250,000 years ago until they disappeared around 35,000 years ago. In their place, a race of sophisticated craftsmen and hunters called the Cro-Magnons began to flourish. Multi-regionalists claim Neanderthals vanished for the simple reason they had evolved into Cro-Magnons, who then became the founding stock of Europeans today.
But supporters of the Out of Africa theory say Cro-Magnons were not descendants of Neanderthals but invaders from another continent. And for evidence they point out that Cro-Magnons had tall and cylindrical skeletons, an adaptation which helps radiate excess heat and is found in individuals from hot climates. By contrast, Neanderthals had burly, spherical skeletons and had clearly evolved in a cold, or at least, cool climate - for spherical bodies are best at retaining heat.
Thus there was no evolution of one to the other. Instead, Neanderthals - adapted to the cold of Europe - were replaced by a heat-adapted Cro-Magnons from a hot equatorial region, Africa. This second wave of African invaders appeared in chilly Europe about 45,000 years ago and quickly replaced the Neanderthals, possibly violently or merely by being more successful at monopolising resources. Men and women of recent African origins then became the founding stock of Europeans, and of all the other peoples of the world today.
This theory has since been given powerful backing by studies in other disciplines. For example, in Nature last year, Andrea Manica of Cambridge University revealed that his studies of genes and physiques from people across the world showed human variation diminishes the further their homeland is from Africa. Indigenous people in Australia and South America - the most distant from Africa - have the lowest amount of genetic and physical variation of all, while people living in Africa have the greatest. This finding is important because variations accrue within a stable population over time, which means that people with the greatest levels of variation must have been living in a particular area for the longest period. Findings such as Manica's therefore add powerful support to the idea that the origins of modern mankind are African and that Australia and South America were the places settled in at the end of our great journey out of Africa.
Most scientists now date those African origins at about 200,000 years ago. After that, Homo sapiens spread through the continent until one group spilled out into Asia between 50,000 to 100,000 years ago and then spread to Europe, Australia and finally the Americas. Crucially, this diaspora was very recent in biological terms, which means humans have not had time to evolve racial differences that are anything other than superficial, such as those affecting hair or skin colour.
http://www.guardian.co.uk/science/2008/apr/29/fossils.evolution2?gusrc=rss&feed=science
