Deep TimeLife & Evolution Placental mammals 160 million years agoNotable Mammals that nourish unborn young through a placenta evolve; humans are among them. ▲ MattersSaveShareTrace roots Longest chain: The Big Bang → First stars ignite → Supernovae seed heavy elements → The Sun forms → Earth forms → First oceans → First life → Photosynthesis evolves → Great Oxidation Event → Complex cells (eukaryotes) → Multicellular life → First animals → Cambrian Explosion → First vertebrates → Jawed fish → Fish walk onto land → The amniotic egg → The Great Dying → First mammals → Placental mammals Community notesLoading notes… Learn more ranked by votesBookSteve BrusatteThe Rise and Reign of the Mammals ↗Editor's pickVideoSearch on YouTubeorigin of placental mammals ↗Editor's pick