May 17, 2006

Genetic study reveals surprises in human evolution

LONDON (Reuters) - 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 on Wednesday.

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

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 analyzing 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 hybridization 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.

Hybridization 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 hybridization
between the emerging species," he added.