Molecular biologists can now tell us who is related to whom in the chain of lifeby John Maddox / July 20, 1996 / Leave a comment
Palaeontologists, the people with hammers who look for fossils, may be the latest group to feel the hot breath of the molecular biologists on their necks. Within a few years of the discovery of the structure of DNA in 1953, it was realised that the differing structure of proteins could be used to learn something about the relationship between organisms back into the mists of time. Forty years on, it seems the ambition is being realised.
Scientific journals are slowly filling up with little family trees relating different species with each other simply on the basis of the differences between proteins performing the same function in all of them.
Conceptually, it is quite simple. Protein molecules are all constructed from simple chemicals called amino acids strung together. Only 20 different units turn up in natural proteins, and the order in which they occur is genetically determined. So it is simply necessary to count the number of places along the length of a protein molecule at which the amino acids differ, and that will be a measure of their taxonomic distance from each other. The virtual identity of proteins with similar functions in human beings and the great apes is a constant reminder of our common origin.
Constructing family trees showing the divergence between species is the standard use of this technique, but there are difficulties. One protein (insulin, say) may give one tree, another (haemoglobin, say) another. That does not invalidate the technique, but makes people wary of relying on a single result.
The variations of protein structure arise from changes in the structure of the corresponding DNA, which happen at random. These are the “inheritable small variations” at the root of Darwinism. The molecular taxonomists hope that they can avoid discordant family trees by assembling enough data about different proteins to iron out the randomness of the variations.
Russell Doolittle, one of the pioneers of molecular taxonomy, has now gone a big step further by using the techniques to chart the course of the evolution of living things back for at least 2 billion years. Earlier this year, Doolittle and colleagues at the University of California at San Diego published their results. Perhaps inevitably, they have now run into trouble.
First, for stripling molecular taxonomy, the good news: it seems to work if there is enough data to play with. Doolittle chose no fewer than 57 proteins with…