One hundred eighty million years ago, a small, hairy animal resembling a shrew or a vole evolved a new way to care for her developing offspring. Instead of laying eggs and incubating them in an uncertain outside world, she retained her embryos and allowed them to develop inside her body. Her evolutionary invention earns her the name ancestral therian, the common ancestor of all placental and marsupial mammals Her innovation ranks with such evolutionary breakthroughs as the development of feathers in dinosaurs and the emergence of aquatic animals onto the land.

The ancestral therian is generally thought to have descended from a creature that had evolved the ability to delay laying her eggs. This still earlier animal could retain her developing eggs in her oviducts, while she chose the best time and place to lay them. The initial advantages of the added internal incubation would have been substantial. The mother would have had more time to find a suitable nest site. Her offspring would have gained a major protective advantage against drastic weather changes and other instabilities. And internal embryonic development would have freed the mother from most of the restrictions of sitting on the nest.