Closing the time gap between then human-chimpanzee divergence

Sudhir Kumar, from the Center for Evolutionary Functional Genomics in the Biodesign Institute at Arizona State, has led a paper published in yesterday’s Proceedings of the National Academy on Sciences redefining the time human lineage diverged from chimpanzee lineage. Previously, there was a wide time range between the divergence… roughly 3 million to 13 million years ago. With new research methods, using comparisons of 167 nuclear protein-coding genes, Kumar et. al. has generated a genomic view of the human-chimpanzee divergence adding another line of evidence in the pursuit of understanding the mechanisms evolution. Furthermore, Kumar says,

“This divergence time also has considerable importance because it is used to establish how fast genes mutate in humans and to date the historical spread of our species around the globe.”

However, this study suggests that bipedalism and large brains and all these other traits that we associate with humans evolved in a short period of time and not over a long period of time. Also this study does not use any fossil evidence because the paleoanthropologists who have discovered the earliest fossils don’t always agree among themselves about whether certain fossils belong to the human branch or the chimpanzee branch or are ancestral fossils…ahem the controversies with Tim White’s fossils. Anyways this is a substantial finding because it was the largest study of its kind ever done — from people and their closest biological relatives, chimpanzees. Macaques, a type of monkey, and mice and it incorporated genetic evidence to recalibrate the molecular clock. Here is the abstract for the article.

Placing confidence limits on the molecular age of the human-chimpanzee divergence
Sudhir Kumar, Alan Filipski, Vinod Swarna, Alan Walker, and S. Blair Hedges
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.

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