- The Paleontological Society
Morphologically complex metazoans appear abruptly during the Cambrian explosion. Suggested measures of metazoan complexity include number of cell morphotypes and aspects of the genome such as the amount of DNA, the number of genes, and the information content of the genome or egg. Estimates of gene numbers are now available for metazoan species belonging to five different phyla or subphyla. There is little correlation between gene number and morphological complexity in the invertebrates: relatively complex forms can have fewer genes than relatively simple forms. Presumably, the more complex forms use more gene-expression events during development, implying that, on average, cis-regulatory elements of more complex invertebrates are richer in binding sites than are those of simpler forms. Vertebrates have many more genes than invertebrates and therefore have more total gene-expression events during development, although they may have, on average, fewer expression events per gene than the invertebrates. There are thus two genomic pathways in the evolution of metazoan complexity: one involves increasing the number of genes, the other involves increasing the number of cis-regulatory binding sites. Both modes were associated with the origin of bodyplans that first appear as fossils during the Cambrian explosion.