Timings - a review

I'm sure many of you reading this are aware of the magazine New Scientist. It covers stories which are newly emerging and occasionally controversial; in last weeks issue (24th November) they ran a story which suggested that we have been miscalculating the age of our split with the last common ancestor of chimpanzees; a result which would mean the re-writing of textbooks and a shift in the way we view our evolution. Pretty revolutionary stuff right? That's what I thought; lets take a look at the evidence.

The way we have always been told in schools and lectures is that the timing of the split between us and chimpanzess was 5 million years ago. This age was a fork in the evolutionary road, along one branch an ancestor evolved into modern day chimpanzee, along another the ancestor became us. The technique of working out this date lies in our DNA; with each generation new mutations appear, these occur randomly and many do not cause any significant changes affecting our likelihood to survive, they just sit there hanging out and can be picked up upon DNA analysis. The rate these mutations accumulate remains relatively stable through the generations and was thought to be 75 per generation, this allows us to calculate how many generations there have been since our ancestors split with the chimp's ancestors, all we have to do is add up the mutations.

Sounds pretty simple, however a recent study by Kong et al. proves that this method needs some corrections. His study was focused on mutations observed between human parents and offspring, after analysing the genetics of 78 'parent offspring trios' (child with both parents) they found that each child had on average 36 mutations which  were new and not shared with either parent. This is clearly half what we have always thought; the mutations accumulate much more slowly Now if we add up all the mutations it appears that the acutal age of split between us and chimps is more like 7-13 million years.

Figure 1 Old 5my split versus the newer estimates of 7-13my

If this more ancient split is to be believed then Australopithicus afarensis may not be the oldest ancestor we can identify as hominin, other species may have to be brought into the family such as Sahelanthropus tchadensis at 6-7mya and Orrorin tugenensis at 6mya .Other ramifications include an older Homo sapien migration out of Africa date at around 90,000-130,000 years ago (a topic I haven't covered yet but when I do I will try to give a scenario based on both dates).

All of this is enough to get very excited, Kong's paper was published in August of this year (2012) in Nature, a journal which undergoes heavy peer reviewing, something which helps with the reliability of a paper. However some anthropologists in the field such as Matt Skinner from UCL, suggest caution should be used when reading such revolutionary claims. Public interest is focused on the more sensationalist stories and scientists are therefore pressured into making claims they themselves would rather not make in order to get their articles to be published. Whether or not this is the case with this story is up for debate and I'd be interested in hearing your opinions.