After killer asteroid, mammals got bigger before they got smarter

In the aftermath of the asteroid impact 66 million years ago that doomed the dinosaurs, it appears that brawn was more important than brains for the mammals that managed to survive the calamity and conquer a changed world.

Researchers said on Thursday an analysis of fossils of mammals from the Paleocene Epoch - spanning the 10 million years after the asteroid wiped out three-fourths of Earth's species - found that while their bodies got much bigger, their brain size relative to body mass actually declined.

The findings contradict the notion that it was intelligence that drove mammals - bit players during the age of dinosaurs - to become the planet's new rulers following the mass extinction at the end of the Cretaceous Period.

"The process to get large brains to emerge in mammals after the extinction was much slower than we previously thought," said Ornella Bertrand, a postdoctoral researcher in mammal paleontology at the University of Edinburgh and lead author of the study published in the journal Science.

The researchers performed CT scans on fossils of 28 Paleocene mammal specimens and 96 from the subsequent Eocene Epoch, spanning 56-34 million years ago. They assessed brain size and the development of specific cerebral components. Brain growth, they found, kicked in during the Eocene, along with a change in the importance of various functions.