Better dates for Human/Chimp split

With all the posting about the Dover decision, it is always good to remember that scientific problems are solved by scientists and aired in scientific journals, not in the legal arena. Investigators at Arizona State and Penn State Universities have placed the time of the human/chimp 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.

From the press release:

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."

The abstract reads:

Molecular clocks have been used to date the divergence of humans and chimpanzees for nearly four decades. Nonetheless, this date and its confidence interval remain to be firmly established. In an effort to generate a genomic view of the human-chimpanzee divergence, we have analyzed 167 nuclear protein-coding genes and built a reliable confidence interval around the calculated time by applying a multifactor bootstrap-resampling approach. Bayesian and maximum likelihood analyses of neutral DNA substitutions show that the human-chimpanzee divergence is close to 20% of the ape-Old World monkey (OWM) divergence. Therefore, the generally accepted range of 23.8-35 millions of years ago for the ape-OWM divergence yields a range of 4.98-7.02 millions of years ago for human-chimpanzee divergence. Thus, the older time estimates for the human-chimpanzee divergence, from molecular and paleontological studies, are unlikely to be correct. For a given the ape-OWM divergence time, the 95% confidence interval of the human-chimpanzee divergence ranges from -12% to 19% of the estimated time. Computer simulations suggest that the 95% confidence intervals obtained by using a multifactor bootstrap- resampling approach contain the true value with >95% probability, whether deviations from the molecular clock are random or correlated among lineages. Analyses revealed that the use of amino acid sequence differences is not optimal for dating human-chimpanzee divergence and that the inclusion of additional genes is unlikely to narrow the confidence interval significantly. We conclude that tests of hypotheses about the timing of human-chimpanzee divergence demand more precise fossil-based calibrations.

See Sudhir Kumar, Alan Filipski, Vinod Swarna, Alan Walker, and S. Blair Hedges, "Placing confidence limits on the molecular age of the human-chimpanzee divergence" PNAS published December 19, 2005, 10.1073/pnas.0509585102/ [available online]