Universal Common Grandma

A profoundly interesting and important question is whether or not all of the life we see on Earth today has one common ancestor - we'll call that organism 'Grandma' - or whether the great domains of bacteria, archaea, and eukarya (such as ourselves) have a number of deep, ancient, and unrelated ancestors. This is a big question from the point of view of understanding the history of life on this planet, and also possibly for understanding more about the origins of life - at what point did recognizable things arise? A brand new study, delving deeply into the statistical relationships of key bits of genetic code seen across modern organisms, seems to point to a single 'Grandma'.

This work, hot off the presses at Nature is by Douglas Theobald, together with a nice opinion piece by Steel and Penny, is quite a tour-de-force of advanced statistics incorporating Bayesian methods to look at correlations in amino acid sequences between different species. The bottom line is that a single 'Grandma' is about 100,000 times more likely than multiple grandmas. The author and commentators point out that this is not really anything terribly shocking, the received wisdom has long been that all modern life has a common 'Grandma' ancestor. It is however a very nice piece of quantitative analysis that breaks through some significant  hurdles - such as the horizontal transfer of microbial genes that hugely complicates this type of detective work.

Does this mean that life only arose once on the Earth? Well, as Steel and Penny discuss, no it doesn't. It only means there's a high probability that the three great domains that we see today have the same 'Grandma'. If there were other, genuinely distinct branches of early life then their genetic imprint may be gone forever. What I find remarkable, in retrospect, is that the incredible diversity of species we see today could indeed come from just one, with a few billion years of natural selection thrown into the mix. So much for life being prone to evolutionary bottlenecks....