If you like your big human brain, you can thank your nose — or at least the noses of your earliest mammal cousins.
That’s because 200 million years ago, a keen sense of smell drove an explosive growth in brain size that pushed mammals out from under the shadow of dinosaurs and other creepy crawlies, a new study concludes.
Enhanced smell probably gave the first mammals — tiny shrewlike critters — the ability to sniff out insects and grubs at night while allowing them to avoid stinky, predatory dinosaurs, the study authors suggest.
That enhanced sense of smell “gave mammals our head start,” said Timothy Rowe, the paleontologist who led the work, published Thursday in the journal Science.
For the study, Rowe and his colleagues deployed powerful X-ray machines to scan the fossilized skulls of two of the earliest, and tiniest, mammals. Although the brains of these early human cousins disappeared long ago, the organs left imprints on the inner surface of the skull. Reading those impressions, called endocasts, revealed for the first time the size, shape and structure of early mammalian brains.
These brains were much bigger, relative to body size, than those of the squat, reptilian ancestors of mammals, Rowe said. Particularly enlarged was the olfactory bulb, a dual-lobed structure at the front of the brain that processes odors. An enlarged olfactory bulb implies an exquisitely sensitive sniffer.
Many modern mammals retain this keen sense of smell, as dogs can distinguish between millions of scents, for instance, and bats can sniff and identify their offspring from thousands of others in a dark cave.
For the tiny shrewlike creatures that scurried underfoot of giant reptiles, a sense of smell was vital. It allowed them to navigate the world at night, when dinosaurs were probably asleep, Rowe said, while steering them away from danger and toward their kin.
“Being able to smell a lot better than your reptile counterparts would be a huge advantage,” said Anjali Goswami, a University College London paleontologist not involved in the study.
In addition to the larger olfactory bulb, the Hadrocodium brain sported a wrinkly, topmost layer missing in reptiles, the neocortex. The neocortex processes tactile information from the skin — and from hair, which the earliest mammals almost certainly possessed, said Thomas Macrini, who worked on the study.
While providing warmth, individual hairs also act like whiskers, sensing the wind and helping animals navigate tight spaces, such as underground dens. It takes a lot of brain power to process all that sensory information, brain power located in the neocortex.
A large neocortex also implies improved agility and better motor coordination, Rowe said, other skills handy for scurrying away from predators. As the neocortex grew, so did cognitive abilities, thinking power.
And so with their noses — and pelts — leading the way, the brains of our tiniest, most distant cousins laid the groundwork for an explosion in ever-larger mammalian brains that evolved after the dinosaurs died out.
