Wednesday, May 6, 2009

Genetic 'Adam never met Eve'

The study confirms the Out of Africa hypothesis

The most recent ancestor of all males living today was a man who lived in Africa around 59,000 years ago, according to an international team of researchers.

The scientists from eight countries have drawn up a genetic family tree of mankind by studying variations in the Y chromosome of more than a thousand men from different communities around the world. The Y chromosome is one of the two sex chromosomes (X and Y) which only men carry (women carry two X chromosomes).

The new research confirms the Out of Africa theory that modern humans originated in Africa before slowly spreading across the world.

But the finding raises new questions, not least because our most recent paternal ancestor would have been about 84,000 years younger than our maternal one.

The team believes there is an explanation. They propose that the human genetic blueprint evolved as a mosaic, with different pieces of modern DNA emerging and spreading throughout the human population at different times.

Origins of man
Evidence from the fossil record suggests that modern man originated in Africa about 150,000 years ago, before moving steadily across the globe.
This Out of Africa hypothesis has been confirmed by studies of mitochondrial DNA, the segment of genetic material that is inherited exclusively from the mother.

Based on these studies, our most recent common ancestor is thought to be a woman who lived in Africa some 143,000 years ago, the so-called Mitochondrial Eve.

To find the common paternal ancestor, the team drew up a genetic family tree of mankind. They mapped small variations in the Y chromosomes of 1,062 men in 22 geographical areas, including Pakistan, India, Cambodia, Laos, Australia, New Guinea, America, Mali, Sudan, Ethiopia and Japan.
The new genetic family tree supports the Out of Africa scenario. But it suggests that our most recent paternal ancestor would have been about 84,000 years younger than our maternal one.

Regions of the genome
"You can ultimately trace every female lineage back to a single Mitochondrial Eve who lived in Africa about 150,000 years ago," said Dr Spencer Wells of the Wellcome Trust Centre for Human Genetics in Oxford, UK, who was part of the team.

"The Y chromosome we trace again back to Africa but the date is about 80,000 years ago.

He told BBC News Online that the two studies could be reconciled. "There's a different evolutionary history for each region of the genome but they all are consistent in placing the ancestor of all modern humans alive today in Africa."

The research, published in the journal Nature Genetics, gives an intriguing insight into the journey of our ancestors across the planet, from eastern Africa into the Middle East, then to southeast and southern Asia, then New Guinea and Australia, and finally to Europe and Central Asia.

Some modern-day men living in what is now Sudan, Ethiopia and southern Africa are believed to be the closest living descendants of the first humans to set out on that great journey tens of thousands of years ago.

Mitochondrial Eve

Y-chromosomal Adam