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'Out of Africa' theory backed by DNA
31/10/2000 09:46 - (SA)
Paris - The groundbreaking theory that Homo sapiens originated in Africa before slowly spreading across the world has been powerfully backed by new research into variations in the male sex chromosome.
The so-called "Out of Africa" hypothesis, sketched in 1987, is based on mitochondrial DNA - scraps of genetic tissue only
inherited from the maternal side - that were found in ancient fossils.
This suggested that modern man first appeared on the scene in eastern Africa about 150 000 years ago, leaving between 35 000 and 89 000 years ago on a relentless push in which the species
eventually conquered the planet.
A major research effort from scientists in eight countries, published in November's issue of the specialist US journal Nature Genetics, has now validated the theory - and in so doing
has devised a potent tool to probe the very earliest origins of mankind.
The team drew up a genetic family tree of mankind thanks to small variations in the genes of 1 062 men in communities around the world.
They identified 167 markers - specific genetic sequences called alleles located in the Y chromosome, one of the two sex chromosomes (X and Y) which only men carry (women carry two X chromosomes). Variations in these markers corresponded astonishingly to the geographical location of where the men live.
In other words, the markers reflected the waves of human migration that unfolded across the world over tens of thousands of years.
Each ripple caused a tiny disturbance in the male gene pool as the species intermingled and the Y chromosome adapted to the process of natural selection.
Samples were taken from men in 22 different geographical areas, in countries that included Pakistan and India, Cambodia and Laos, Australia and New Guinea, America, as well as Mali, Sudan, Ethiopia and Japan.
Their allele mutations were then assembled into 10 types, called haplogroups. Like branches off a family tree, they show a migration from eastern
Africa into the Middle East, then southern and southeast Asia, then New Guinea and Australia, followed by Europe and Central Asia.
Among the findings:
Some modern-day men in latter-day Sudan, Ethiopia and southern Africa are the closest lineal descendants to the first Homo sapiens
who set out on that great trek.
"A minority of contemporary East Africans and Khoisan (southern Africans) represent the descendants of the most ancestral patrilineages of anatomically modern humans that left Africa
between 35 000 and 89 000 years ago," the team writes.
New Guinea and Australia were settled early in the process. This could be supported by the finding of a Homo sapiens burial site in
Australia believed to 60 000 years old.
Japan has remained in remarkable genetic isolation. The mutations are strikingly different from those of surrounding populations and account by themselves for a specific haplogroup.
Native Americans have a common ancestry with Eurasians and East Asians, raising intriguing questions about the first peopling of North America.
The findings "takes historical population genetics, or 'archaeogenetics,' a quantum leap forward," says a commentary in Nature Genetics by a team from the McDonald Institute for
Archaeological Research in Cambridge.
The research was especially important given that it came from DNA of living populations rather than genetic material teased out of rare fossils, they said.
The technique was to take samples of genetic tissue, amplify them and then search for the markers using a chromatographic analysis.
The study was led by Peter Underhill of California's Stanford University. - Sapa-AFP
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