Biochemical Homology

QUESTION: What is biochemical homology?

ANSWER:

One prediction which can be drawn, if indeed all forms of life have evolved from a common ancestor, is that the basic biochemical machinery -- such as genetic mechanisms and metabolic pathways -- which would have been present in the primitive, single-celled, common ancestor, ought to have been inherited by successive organisms, eventually leading up to higher organisms. Thus, the fact that many biochemical features are exhibited in all forms of life is often hailed as decisive evidence for common ancestry.

Is such a conclusion warranted? One of the more intriguing discoveries of modern science is that there is a distinct similarity, not only in terms of the biochemistry within the cell, but also in many of the genes which are used to implement the developmental pathways of multicellular life in radically different groups of animals. One example of this is Hox genes, a series of genes which occur in both invertebrates and vertebrates, that facilitate the formation of different anatomical structures during the early phases of embryological development. These genes code for proteins that act as transcription factors, controlling the expression of batteries of genes which implement particular features of the animal’s body. This is also often hailed as evidence for the common ancestry of all forms of life. The underlying assumption is that Hox genes originated in some early multi-cellular animal from which all animal phyla have subsequently developed. But why should evolution -- having no foresight -- have utilised the same developmental genes for the production of very different morphological structures?

And what of cases in which the same developmental genes are responsible for controlling the development of organs which have a very similar function but have arisen independently? One case of this is a gene known as distalless, which is involved in the development of appendages/projections in various phyla such as fins, limbs, arthropod legs, etc. It is widely held that divergence of animal phyla occurred so early that the last common ancestor did not have appendages. While it might be reasonable to suggest that the distalless gene arose before divergence and was present in a very early common ancestor -- such that the genes in the various phyla are homologous of the early gene -- it cannot be maintained that distalless already had a function in the development of appendages.

To sum up, although some biochemical similarities support the notion of a common evolutionary origin, it is becoming increasingly evident that other similarities are very difficult to accommodate within that sort of explanation, and may indeed even militate against it. It appears to be a remarkable coincidence -- from a Darwinian perspective -- that essentially the same gene is used for structures which are analogous, but not homologous.