LOS ANGELES, May 17 (Xinhua)-- Genome analysis revealed that the evolutionary split between human and chimpanzee is actually no more than 6.3 million years ago, scientists reported on Wednesday.

    The split is more recent and more complicated than previously thought, the researchers at the Broad Institute of MIT and Harvard said in the May 17 online edition of the journal Nature.

    Scientists previously estimated that humans and chimpanzees probably split 6.5-7.4 million years ago based on a hominid fossil, which has features thought to be distinctive to the human lineage.The new study indicated the speciation is more recent by about 1 to 2 million years.

    The new study exploited the information in the complete genome sequence to reveal the variation in evolutionary history across the human genome.

    Scientists have known that some genomic regions must be 'older'than others, meaning that they trace back to different times in the common ancestral population that gave rise to both humans and chimps.

    The study, the first to actually measure the range of ages, gave three surprising results:

    First, the time span of the divergence between the two species ranges over more than 4 million years across different parts of the genome. This range is much larger than expected.

    Second, the youngest regions are unexpectedly recent, being no more than 6.3 million years old and probably no more than 5.4 million years old. This finding implies that human-chimp speciation itself is far more recent than previously thought.

    Third, the sexual chromosome X almost entirely falls at the lower end of the time frame. In fact, the average age of the X chromosome is about 1.2 million years "younger" than the average across the 22 non-sexual chromosomes.

    "The study gave unexpected results about how we separated from our closest relatives, the chimpanzees," said David Reich, senior author of the study.

    "We found that the population structure that existed around the time of human-chimpanzee speciation was unlike any modern ape population. Something very unusual happened at the time of speciation."

    Moreover, the speciation process was unusual, the researchers said. The much younger X chromosome indicated that an initial split between earliest humans and chimpanzees was followed by later hybridization before they finally separated.

    "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," said Reich. Enditem