Figure 1.

Simplified phylogenetic tree illustrates relationships of representative extant Mammalian species. Estimates in millions of years ago (MYA) of origin of each major branch were derived from Bininda-Emonds et al. [2]. The two earliest splits established monotremes, (166.2 MYA), and marsupials and placentals (147.7 MYA). Approximately 50 million years pass before the origination of any extant groups, and then the four placental superorders (italicized capitals) arose within 2.4 million years of each other.