Tuesday, 28 August 2001

By: Ivan Noble

Modern humans migrated out of Africa into Central Asia before spreading both east and west into North America and Europe, says an international team of scientists who have used modern DNA analysis to trace ancient migrations.

"Then, as desertification came in, some people moved west into Europe and some moved into Siberia and on into North America," she told BBC News Online.

Her colleague Professor Ruslan Ruzibakiev organised the collection of thousands of blood samples across Central Asia and the Caucusus.

Shuffling DNA

They are now working with Spencer Wells at the Wellcome Trust Centre for Human Genetics, Oxford, UK, using telltale mutations of the Y-chromosome to trace the paths of the ancient travellers.

Dr Wells explained why the Y-chromosome holds the key to tracking lines of descent.

"The cool thing about the Y-chromosome is that it doesn't recombine," he said.

When a child is created, its DNA comes from both its parents.

"The DNA is shuffled like a pack of cards," Dr Wells said.

Family tree

But if it the child is a boy, his Y-chromosome can only come from his father, since only men have Y-chromosomes.

So any mutations to the Y-chromosome, which take place naturally over time, are passed directly from father to son without any shuffling.

Researchers like Dr Wells and Dr Yuldasheva have used these inherited markers to trace a family tree of human history.

And by comparing this tree with known archaeological and linguistic facts, they believe that they have developed a more detailed understanding of how anatomically modern humans moved around the world.

Regional 'Adams'

"It looks like Central Asia was settled really early, around 40,000 years ago, as humans came out of Africa. We can trace back to regional 'Adams'," said Dr Wells.

"Our regional Adam's descendants moved up to the steppe lands - probably because of climate change and then went west.

"These would have been the Cro-Magnons, the people who correspond to our popular image of the cave man," he said.

"A second wave moved along the steppe belt well to the east into the Americas," he added.

'Aryan' expansion

Dr Wells and his colleagues believe that their work also traces the expansion of the Indo-Iranian people known as the Kurgan civilisation, or more popularly Aryans.

"We have a diagnostic Indo-Iranian marker," he said, referring to one of the Y-chromosome mutations.

This marker shows the progress of the 'Aryans' into India and beyond. These Indo-Iranians spoke a language which is believed to be the forerunner of many modern tongues.

Some people living high in the mountain valleys of Central Asia still speak a form of Sogdian - the oldest living Indo-Iranian tongue.

The study also shows how successful emigrants from Central Asia were able to spread their language further than their genes.

DNA samples from Iran show far fewer Indo-Iranian markers in the west of the country, despite an Indo-Iranian language being dominant across the region.

Modern diversity

One explanation, said Dr Wells, could be that the incomers were so successful that the original inhabitants of the region began to adopt the newcomers' language.

Modern Central Asia's diverse genetic mix is explained by the migrations that came much later, when the Silk Road carried wealth and trade goods from China to Europe and back.

These migrations are reflected in the DNA, too, and it is clear that despite the majority of modern Central Asians speaking Turkic languages, they derive much of their genetic heritage from the conquering Mongol warriors of Genghis Khan.

"Central Asia is revealed to be an important reservoir of genetic diversity, and the source of at least three waves of migration, leading into Europe, the Americas and India," the researchers wrote in the Proceedings of the National Academy of Sciences.

Source: BBC