Humans and chimpanzees, two very closely related species, share a common ape-like ancestor that lived on Earth millions of years ago. But when did this ancestor begin to diverge genetically, splitting into the different lineages that would eventually give rise to these two species?

Previous estimates using the fossil record suggested that the last common ancestor of humans and chimps lived around 6-7 million years ago, which has been widely accepted by the scientific community. However, a new study may challenge this as the findings suggest that the common ancestor could have begun diverging as far back as 13 million years ago. The report has been published in Science.

Chimpanzees are our closest living genetic relatives; our DNA is around 99% similar. Comparing the DNA of these two species can therefore unearth clues about their evolutionary history. By determining the mutation rates of both species, which serve as evolutionary “clocks”, it is possible to determine when they last shared a common ancestor.

Earlier estimates suggested that the most recent common ancestor of humans and chimps lived around 6-7 million years ago; however, recent evidence has suggested that the mutation rates may have been vastly overestimated and that the most recent common ancestor (MRCA) in fact lived 12 million years ago. Now, a new study investigating the mutation rates of chimpanzees has provided further evidence to support this notion, suggesting that the MRCA lived around 13 million years ago.

“Our results add substance to the idea that the human-chimpanzee split was considerably older than has been recently thought,” study co-author Gil McVean told Live Science.

The average mutation rate of humans is approximately one mutation per two billion base pairs of DNA per year. In order to investigate whether chimpanzees experience similar rates, the research team sequenced the genomes of nine Western chimpanzees; two fathers, two mothers, and five of their offspring.

The researchers found that humans and chimpanzees share very similar mutation rates and from this data the team were then able to calculate divergence rates. “Our results indicate that human and chimp ancestors’ genomes would diverge by about 0.1 percent every million years, so when we see a divergence of 1.2 percent, we infer that it must have been about 12 million years – 13 million years is our actual estimate,” McVean told Live Science.

However, as paleoanthropologist John Hawks pointed out, this estimate is only the average time for divergence and it is therefore still possible for the split to have occurred around 7 million years ago if the ancestral population was very large. This is because a large population would provide ample opportunity for the gene pool to begin diversifying well in advance of the actual split.

While the team found that humans and chimps share similar rates of mutation, they discovered that males contributed seven to eight times the number of mutations to offspring than females. In humans, however, males contribute only three to four times as many as females.

Males pass on more mutations to offspring than females because males constantly produce sperm throughout their lives. As a cell divides, there is a chance that a mistake will be made during DNA replication which could give rise to a mutation if it is not fixed by cellular processes. Females, however, are born with a finite number of eggs that do not divide and are therefore less likely to experience mutation.

Another interesting finding was that male chimps contribute three further mutations to offspring with every year of the father’s age, whereas male humans contribute only two more per year. So what is causing these differences in mutation rates and patterns between these two very closely related species? The scientists believe it could be down to sex.

Chimpanzees are a highly promiscuous species; far more promiscuous than humans. Consequently, they have evolved a rather impressive pair of testicles to keep up with all of the sperm that is needed for lots of mating. Relative to body size, the testes of a male chimpanzee are ten times larger than the testes of a male human. While all this extra sperm might be great for the females, it unfortunately increases the likelihood of mutations arising.

While the study size was small, the findings are important nonetheless and seem to add to the growing body of evidence that the ancestors of humans and chimps may have begun diverging long before earlier estimates. 

[Via Science, LiveScience and National Geographic]