Mutation Rate in Humans Has Slowed Down

I am not quite sure what this means, yet. Researchers at the Aarhus University, Denmark, and the Copenhagen Zoo have discovered that, when compared to our nearest taxonomic relatives, our mutation rates have slowed down. Science Daily has the scoop:

“Over the past six years, several large studies have done this for humans, so we have extensive knowledge about the number of new mutations that occur in humans every year. Until now, however, there have not been any good estimates of mutation rates in our closest primate relatives,” says Søren Besenbacher from Aarhus University.

The study has looked at ten families with father, mother and offspring: seven chimpanzee-families, two gorilla families and one orangutan family. In all the families, researchers found more mutations than would be expected on the basis of the number of mutations that would typically arise in human families with parents of similar age. This means that the annual mutation rate is now about one-third lower in humans than in apes.

Why is this important for the study of human origins?

The higher rates in apes have an impact on the length of time estimated to have passed since the common ancestor of humans and chimpanzees lived. This is because a higher mutation rate means that the number of genetic differences between humans and chimpanzees will accumulate over a shorter period.

If the new mutation rates for apes are applied, the researchers estimate that the species formation (speciation) that separated humans from chimpanzees took place around 6.6 million years ago. If the mutation rate for humans is applied, speciation should have been around 10 million years ago.

The six-to-eight million year point for the LCA never made a whole lot of sense to me. If the fossil material from Orrorin, at 6 mya really does reflect bipedality, then the split has to have been much earlier.  The material from Ardipithecus kadabba is very sketchy with regard to bipedalism (one toe bone found ten miles away), but the fragmentary post-cranial bones can be confidently identified as being hominin, in nature.  Furthermore, the fossil material is dated to between 5.6 and 5.8 mya.  That would leave a very short period of time.  It cannot be pre-split because the fossil material exhibits derivations in the hominin direction, rather than the modern ape direction. 

If this study holds up, it will change how we view the search for the LCA.