LIFE × EVOLUTION

The Rise of the Mammals

~66 to 56 million years ago

Phanerozoic · Cenozoic · Paleogene · Paleocene

Placental mammals came through the extinction at the end of the Cretaceous in fewer than ten lineages, and within a few million years their descendants grew from the size of a rat to the size of a bear.

The rise of the mammals is the diversification of placental mammals, whose young grow in the womb fed through a placenta, after the mass extinction that ended the Cretaceous period. Fewer than ten placental lineages are reckoned to have survived that extinction. Within a few million years their descendants grew from the size of a rat to the size of a bear, and their anatomy changed about three times faster than it had through the Cretaceous. Bertrand and colleagues (2022) measured the encephalisation quotient, the ratio of brain size to the size expected for the body. Brains first became relatively smaller, because the body grew first, and became larger afterwards.

Why it matters

A mass extinction empties ecosystems of many of their species, and such an event is rare, so occasions to observe its effects on survivors are few. After the end-Cretaceous extinction, body size increased before brain size. As long as ecosystems had room, a larger body was the first advantage. The brain developed later, in the Eocene, a later geological epoch, once ecosystems had filled with species and competition had increased.

Dating & uncertainty

Researchers who date the mammalian family tree place the origin of placentals in the Late Cretaceous, before the impact at the end of that period. The diversification of body forms comes after the impact. In the five million years following the extinction, anatomy changed about three times faster than the Cretaceous background, the ordinary rate of change during that period.

Earth history as one day

23:39:0320 min 56 s before midnight

if Earth’s 4.54 billion years were compressed into a single day