Common descent and biochemistry

Despite the dizzying array of life on Earth – if you doubt that, watch the BBC’s excellent Planet Earth series – there is a remarkable degree of biochemical consistency between all living things. This is one of the strongest arguments in favour of common descent: the idea that all living things are descended from the first replicators so evocatively described in the opening chapter of Dawkins’ The Selfish Gene. The very strongest evidence of that thesis comes not from the universality of the really essential mechanisms of life, but from the universality of arbitrary conventions common to all living things.

Some of the more astonishing elements of life are universal: the storage of constitutive information in strands of DNA or RNA, the use of three nucleotide codons to refer to amino acids, and the dominant role of proteins in cellular architecture. These are common to animals and plants, fungi and bacteria and archaea. It is difficult to imagine how living things would look if they were based on alternatives to this basic system. Then, there are elements of common biology which need to be in place, but are somewhat arbitrary. For example, there is the metabolization of glucose for energy and the use of adenosite triphosphate (ATP) as an energy carrier. Something needs to play these roles, but there are presumably other molecules that could serve the purpose. Also, unlike the consistencies in the first category, life would not be staggeringly different if different molecules served these purposes. Finally, there are what might be considered arbitrary conventions – things that were established at the origin of life, are common to all life, but which could just as well be another way or a patchwork of different ways. This includes the use of only 20 amino acids to make proteins, and the fact that the L-isomers of these acids are used. This also includes how cells establish a lower concentration of sodium inside themselves than exists in the surrounding matter, with a higher concentration of potassium inside. It could just as well have been the other way.

In a sense, it is the third category that provides the best evidence of common descent. It is like language: pretty much any language will need a way to refer to objects and to actions performed upon them. As such, the inclusion of these aspects in different languages isn’t really evidence of relation. When you find a language that has a number of arbitrary conventions in common with another (say, an alphabet), you have more reason to think they both evolved from something older.

While statistics suggests that it is highly likely, it would nonetheless be rather thrilling to find life that emerged entirely independently, somewhere out among other planets or distant stars.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here. View all posts by Milan