Two roads diverged

The TimeTree site, sponsored by Penn State, Arizona State, the National Science Foundation, and NASA, lets you enter any pair of species you want (common names or Latin) and find out the time since they split from a common ancestor. You get a range of estimates from the scientific literature, along with means and medians. The site lets you can track down sources if you want.

But be careful! There have been some big recalibrations of DNA dates lately that are especially important for human evolution. Human beings and chimpanzees differ at about 1.2% of their DNA sites. Humans and gorillas (and chimps and gorillas) at about 1.6%. Humans and orangutans (and chimps and orangutans, and gorillas and orangutans) at about 3.1%. DNA divergence accumulates a fairly steady rate, so this level of divergence has been used to estimate a chimp/human split around 7-6 million years ago.

But lately we’ve got direct estimates of DNA divergence rates, based on measuring mutation rates in living populations, instead of estimates based on calibrating genes with the fossil record. These suggest that the DNA “clock” is running more slowly than we thought. A report from just three years back based on the new estimates puts the chimp/human split around 13 million years. But there’s disagreement how reliable the new methods really are.

In a sense it may not be that big a deal knowing when the human lineage (the hominins) split from our closest living relatives, the chimpanzees (including bonobos). It’s possible that even after the split the two species looked similar for a long time before hominins started on a distinctive adaptive pathway – in particular, before they started being bipedal. Still, it would be interesting to know. The fossils put some kind of lower bound on the time of the split. – probably at least 6 million years ago. But the upper bound is now in dispute.


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