used in so many contexts toward so many ends. They define the envelope of possibilities of what regulatory change can achieve.
Parenthetically, though, some core components and processes have admittedly evolved since the Cambrian, and these, too, have become conserved. Appendage and limb formation (arthropods and tetrapods, respectively) would be developmental examples. These complex processes are, we argue, combinations of different conserved core processes linked in new regulatory configurations, conserved in their entirety. Others appear to entail protein evolution and new functions combined with old conserved processes, such as the SCPP proteins of bone formation, or keratins of hair and skin cells, or various myelin proteins of glial cells, or neural crest cells, or the adaptive immune system, all evolving in early vertebrates. These entail significant additions to the toolkit. And of course, protein evolution was very important in the four episodes of pre-Cambrian innovation described previously. For the most part, though, animals since the Cambrian have repeatedly reused the processes and components that had been evolved long beforehand to generate novel traits of anatomy and physiology.
Recent genome analysis has brought quantification to the impressions about conservation. More than 80 metazoan genomes have now been sequenced, and a typical case is the mouse (Mouse Genome Sequencing Consortium et al., 2002). Of its total set of gene sequences, 23% are shared with prokaryotes, a further 29% are shared with non-animal eukaryotes (protists, fungi, and plants), and a further 27% are shared with nonchordate animals. Thus, 79% of mouse genes retain pre-Cambrian sequences. Reciprocally stated, only 21% of its functional components are unique to chordates, much less vertebrates, mammals, or mice. Such DNA sequence conservation among life forms conveniently allows the rapid identification of genes in new genomes by equating them with proteins or RNAs of other animals or yeast or bacteria where their function has been elucidated. As examples, the actins and β-tubulins of yeast and humans are 91% and 86% identical in amino acid sequence, respectively, and the otoferlins (a sensory cilium protein) of human hearing and Drosophila sensilla are 80% identical.
A complementary finding of genomics is the less-than-expected number of genes in animal genomes compared with bacteria and single-celled eukaryotes. The gene range from sea anemone (Nematostella) to human is 20–25,000 (Putnam et al., 2007), with some exceptions reflecting gene loss (honey bee, 10,000; Drosophila, 13,600). These numbers are but two to five times the inventory of Escherichia coli (4,600) or yeast (6,400), even though animals seem much more complex in their anatomy and physiology. One way out of the seeming paradox both of an embarrassingly small gene number in animals and of the widespread sharing of gene sequences with