( A team of researchers has proposed new ...)Scientists narrow the time limits for the human and chimpanzee split

A team of researchers has proposed new limits on the time when the most recent common ancestor of humans and their closest ape relatives -- the chimpanzees -- lived. Scientists at Arizona State and Penn State Universities have placed the time of this split between 5 and 7 million years ago -- a sharper focus than that given by the previous collection of molecular and fossil studies, which have placed the divergence anywhere from 3 to 13 million years ago.

The scientists analyzed the largest data set yet of genes that code for proteins and also used an improved computational approach that they developed, which takes into account more of the variability -- or statistical error--in the data than any other previous study. Gene studies are needed to address this problem because the interpretation of the earliest fossils of humans at the ape/human boundary are controversial and because almost no fossils of chimpanzees have been discovered. "No study before has taken into account all of the error involved in estimating time with the molecular-clock method," said Sudhir Kumar, lead author on the report, which was published early online in the journal, Proceedings of the National Academy of Sciences. The team describes its new statistical technique as a "multifactor bootstrap-resampling approach."

Penn State evolutionary biologists Alan Walker and Blair Hedges also took part in the collaborative effort. The team examined 167 different gene sequence sets from humans, chimpanzees, macaques, and mice. "In order to reduce the variability in this time estimate as much as possible, we needed the largest amount of data available," said Kumar.

"There is considerable interest in knowing when we diverged from our closest relative among animal species," said Kumar, who is director of the Center for Evolutionary Functional Genomics in the Biodesign Institute at Arizona State University. "This divergence time also has considerable importance because it is us
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Source:Penn State

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