Introduction

Several of the cladistic and phylogenetic studies examined in Chapter 3 suggested that the whole of the animal kingdom – including such outlying groups as Porifera – represents a monophyletic clade, which originated from a unicellular eukaryote. The comparative developmental–genetic studies discussed in Chapter 7 confirmed this proposed monophyly; and the idea of a general genetic mechanism underlying the development of all animals is neatly captured in the ‘zootype’ concept (Slack et al 1993).

If this scenario is broadly correct, then all the genes of all animals may have arisen from the gene pool of a single population of a single (unknown) pre-Cambrian species, through the processes of gene duplication and mutation. (The possibility of subsequent ‘injection’ of some other genes into the animal clade through horizontal transfer via transposable elements should also be borne in mind.) The present-day unicellular eukaryote Saccharomyces cerevisiae (baker's yeast) has approximately 6,000 genes (see Chothia 1994). While this is clearly not a candidate for being a very close relative of the ancestor of all animals, it is probable that that ancestor had a comparable gene number – somewhere in the 5,000–8,000 range. So, our own complement of perhaps some 70,000 genes has arisen through an approximately tenfold increase in the number of genes, coupled with mutationally driven divergence.