Mammals evolved at roughly the same time as dinosaurs, around 225 million years ago in the Triassic. For 160 million years they remained small, nocturnal, and insectivorous — because the large ecological niches were occupied by dinosaurs. The K-Pg impact overturned that equilibrium in a single geological instant.

When the Chicxulub asteroid struck the Yucatán peninsula around 66 million years ago — an event covered in detail under 66-mya-kpg-extinction — most terrestrial vertebrates, including non-avian dinosaurs, disappeared. Small mammals that survived thanks to their size and nocturnal habits found themselves in a vacated world.

Through the Palaeocene (66–56 million years ago) mammals diversified rapidly. Primitive groups collectively called condylarths filled the niches of large herbivores; carnivores grew larger; the first climbing and grasping adaptations evolved. The Palaeocene-Eocene Thermal Maximum (PETM) at 56 million years ago expanded forests and produced the earliest clear fossil record of primates.

Marine mammals also returned to the water in this period: Pakicetus, known from around 50 million years ago, is the earliest known ancestor of whales — itself descended from a land mammal. Bats were flying by 50 million years ago. Elephant ancestors, horse ancestors, pig and rhinoceros relatives all took shape across the Eocene and Oligocene.

This acceleration was not accidental: mammals were warm-blooded, allowing activity across varied climates; placental reproduction and direct parental care protected offspring; larger brains, longer lifespans, and learning capacity combined to produce what is today the dominant vertebrate class in terms of ecological range — and the one that eventually turned its gaze on its own origins.