Common Ancestry: We Come From One

My first print article has been published by Significance, the statistics popularization magazine of the Royal Statistical Society and the American Statistical Association. Titled We come from one I tackle a statistical analysis that conclusively point towards a common origin of life on Earth rather than multiple ancestors.

The tree of life that most taxonomists currently accept is the three-domain system which Carl Woese introduced in 1990. According to this the first, most ancient and most fundamental division of the tree of life is into three domains: Archaea, Bacteria and Eukarya. Archaea are ancient single-celled organisms once thought to inhabit only extreme conditions, such as salt lakes, but now realised to be extremely common. Bacteria are ubiquitous in nature and, for the sake of illustration, there are around 10 times more bacterial cells residing in the human body than there are human cells. Eukarya account for the rest of life on earth and are divided into up to six kingdoms, depending on which authority you follow. All animals (including humans), plants and other organisms such as fungi and algae are Eukaryotes and share a common ancestor. And universal common ancestry would have it that all three domains themselves stem from a single root.

As it turns out though, universal common ancestry has never been properly tested before. Instead, it has just been widely assumed as correct by the scientific community.

In the past, scientists have attempted to test UCA by comparing the similarities that are found in the DNA sequences, and in the protein sequences, of different living creatures. (We share vast sequences of our DNA with chimps; we share some sequences with the banana.) However, such a strategy alone is not enough. Taking only sequence similarity into account cannot disprove conflicting hypotheses; identical sequences in different life-forms could come from convergent evolution due to selection, or from structural constraints on sequence identity; these sequences may be the only ones that can work, so all living organisms would have to have them, no matter what their origins. Mutation bias could be another explanation as it is known that some building blocks in DNA may change into different specific others, irrespective of species. Even pure chance is possible. If the odds are 50-50 of any new life-form having left- or right-handed amino acids, the odds of three different forms, Archaea, Bacteria and Eukarya, all having the same handed forms are one in four - which is hardly impossible. An appropriate method to test UCA would in fact be one that does not assume that sequence similarity is equivalent to genealogical relationship at all.

The splendid thing about common ancestry is that it unifies us all. It is a message hidden in the heart of Mother Nature herself. From the Trochetia to the dog. From the bacteria to the banana. We are all family. We all originally came from one. And I think it is important to remember this.

--

It's ironic that now that I can actually brag about being a published science writer, I find myself struggling with a bit of a writer's block! Hence the lack of posts on the blog. Apologies.

Image credit: Ivica Letunic and Mariana Ruiz Villarreal (wikimedia commons)