Human & Chimp Genes May Have Diverged Twice As Long Ago As We Thought

Human & Chimp Genes May Have Diverged Twice As Long Ago As We Thought - The ancestors of humans and chimpanzees may have begun genetically diverging from one another 13 million years ago, more than twice as long ago as had been widely thought, shedding new light on the process of human evolution, researchers say.

Scientists also discovered that male chimps pass on far more genetic mutations to their offspring than male humans do, revealing previously unknown evolutionary differences between the species.

The number of genetic differences between two species reveals how closely related the species are. By estimating the rate at which mutations occur, researchers can then determine when the ancestors of species such as humans and chimpanzees may have diverged. Here, estimates of mutation rates act like "molecular clocks" that help scientists pinpoint when key moments in evolution occurred.

But, calibrating how fast these molecular clocks actually tick can be challenging; the molecular clock of one species might conceivably tick faster or slower than that of another species, the scientists said. Researchers usually try to overcome this challenge by comparing molecular clocks to the fossil record to see when species diverged. Yet, ages gleaned from the fossil record are often somewhat imprecise.

One way to directly pin down the rate of mutation in a species is to compare members of that species with their progeny. The genes that children get from their parents may possess mutations caused by factors such as radiation, mutation-triggering chemicals or errors during cell division. By counting the number of genetic changes that accumulate over generations, scientists can estimate the rate at which mutations occur in that species.

Past estimates of when the ancestors of humans diverged from chimps suggested the most recent common ancestor of both species lived about 6 million years ago. However, in the past decade or so, genetic analyses revealed the human mutation rate is actually half as fast as was previously thought, suggesting the most recent common ancestor of humans and chimps actually lived at least 12 million years ago.

The chimp and human split

Now a new study of chimp mutation rates appears to confirm that the most recent common ancestor of humans and chimps lived about 13 million years ago.

"Our results add substance to the idea that the human-chimpanzee split was considerably older than has been recently thought," said study co-author Gil McVean, a geneticist at the Wellcome Trust Centre for Human Genetics in Oxford, England.

In humans, the average mutation rate is about one mutation per 2 billion base pairs per year. (The spiraling double strands of DNA are made of pairs of molecules known as bases.) Each person inherits, on average, about 70 new mutations from his or her parents.

To see if chimpanzees have similar patterns of mutation, scientists analyzed...