The Rise of Placental Mammals
The pace of evolution that is responsible for the rise of placental mammals is gradualism. Gradualism is defined by small and gradual changes that accumulate into significant phenotypic differences over long periods of time. The earliest known placental mammals have been dated back to the start of the Cretaceous period, about 145 million years ago. They can be traced back to a singular species, the therians, who began to diversify as they developed more robust molars. This was the start of the process known as speciation, the divergance of biological lineages and the emergence of reproductive isolation between those lineages. The therians splintered off into placentals and the metatherians who started down separate evolutionary paths. The placentals began to splinter off into major modern subgroups by the end of the Cretaceous period, which included rodents and primates. We know this is gradualism because of the long periods of time (about 145 million years) for placentals to look completely different from the species they both originated and deviated from. The metatherians developed into marsupials, the common ancestor of koalas and kangaroos. We know primates (descended from placental mammals) are a different species from marsupials because we wouldn’t be able to produce viable, fertile offspring with one another, and therefore a result of speciation.