MOLECULAR EVIDENCE FOR THE INCLUSION OF CETACEANS WITHIN THE ORDER ARTIODACTYLA

The transition in the cetaceans from terrestrial life to a fully aquat ic existence is one of the most enduring evolutionary mysteries. Resol ving the phylogenetic relationships between Cetacea and the other orde rs of eutherian mammals may provide us with important clues to the ori gin of whales and may help us date the evolutionary transition to aqua tic life. Previous paleontological and molecular evidence has indicate d that cetaceans and artiodactyls constitute a natural clade within su bclass Eutheria. Our present phylogenetic analyses of protein and mito chondrial DNA sequence data indicate that cetaceans are not only intim ately related to the artiodactyls; they are in fact deeply nested with in the artiodactyl phylogenetic tree; i.e., they are more closely rela ted to the members of one suborder of artiodactyls, the Ruminantia, th an either ruminants or cetaceans are to members of the other two artio dactyl suborders: Suiformes and Tylopoda. On the basis of the rate of evolution of mitochondrial DNA sequences and using paleontological ref erence dates for calibration, we estimate that the whale lineage has b ranched off a protoruminant lineage <50 Mya. By implication, the cetac ean transition to aquatic life is inferred to be a relatively recent e volutionary event.