Fossil evidence shows mammal ancestors gave birth 236 million years ago

New research is shedding light on a pivotal moment in mammalian evolution: the transition from egg‑laying to live birth. Scientists analyzing fossilized embryos and genetic markers have identified a timeline that places the emergence of viviparity in mammals earlier than previously thought, suggesting that the majority of modern mammals inherited this reproductive strategy from a common ancestor that lived roughly 160 million years ago, during the late Jurassic period.

The study, which examined well‑preserved specimens of early mammaliaforms and compared key reproductive genes across extant monotremes, marsupials, and placentals, found molecular signatures consistent with internal gestation emerging shortly after the split between monotremes and therians. These findings align with newly discovered fossil evidence of embryonic development within the womb of a Jurassic mammal, providing concrete support for the hypothesis that live birth became the dominant mode of reproduction long before the rise of dinosaurs’ extinction. The results refine our understanding of mammalian ancestry and underscore the deep evolutionary roots of viviparity in the class.