Genetic study reveals surprises in human evolution

LONDON - Humans' evolutionary split from their closest relatives, chimpanzees, may have been more complicated, taken longer and probably occurred more recently than previously thought, scientists said today.

After comparing the genomes, or genetic codes, of the two species they suggest the initial split took place no more than 6.3 million years ago and probably less than 5.4 million years ago.

The process of separation may have taken about 4 million years and there could have been some inter-breeding before the final break.

"The study gave unexpected results about how we separated from our closest relatives, the chimpanzees," said David Reich of the Broad Institute and Harvard Medical School's Department of Genetics in Massachusetts.

Instead of analysing genetic differences between humans and chimpanzees, Reich and researchers from the Massachusetts Institute of Technology (MIT) and the Broad Institute of Harvard and MIT looked at variations in the degree of divergence between the two in different regions of the genomes.

The analysis, published in the journal Nature, shows some regions in the human genome are older than others which means they trace back to different times in the common ancestral population of the two species.

The youngest regions are unexpectedly recent, according to the researchers, which means the separation between the two species was more recent than previously thought.

"A hybridisation event between human and chimpanzee ancestors could help explain both the wide range of divergence times seen across our genomes, as well as the relatively similar X chromosomes," Reich explained.

Hybridisation refers to the initial separation of two species followed by interbreeding and then the final split.

The findings also raise questions about the 7 million year old fossil of a skull called "Toumai" which was thought to be the earliest member of the human family.

The skull, which has a mixture of primitive and human-like features and dates, was hailed as probably the most important fossil discovery in living memory because it was thought to belong to an ancient ancestor of modern humans.

Some scientists had argued it was a fossil of a female ape.

"It is possible that the Toumai fossil is more recent than previously thought," said Nick Patterson, of the Broad Institute and a co-author of the study.

"But if the dating is correct, the Toumai fossil would precede the human-chimp split. The fact that it has human-like features suggest that human-chimp speciation (separation) may have occurred over a long period with episodes of hybridisation between the emerging species," he added.

- REUTERS