Big Smell, Big Brain
By Lindsay Patterson
New research from the University of Texas at Austin might explain how mammals have evolved the largest brains of any living creatures. According to a study published Thursday in the journal Science, an improved sense of smell was the important first step in the evolution of mammals’ big brains.
A mammal’s brain occupies more space in its head than any other living animal’s, relative to body size. Over the years, scientists have considered theories to explain this unique feature, and. Tim Rowe, the director of the University of Texas’ Vertebrate Paleontology Lab, suspected that the answers were hidden within the very tiny skulls of our earliest ancestors. He began looking at fossil specimens of pre-mammals in the 1980s. These fossils are so rare, he wasn’t allowed to cut them open to get a good look inside.
“It was kind of frustrating,” Rowe said, “because I knew that these specimens held answers to the questions that we’d been wondering about for eons.”
That changed when paleontologists appropriated the technology used for medical CT scans to image the interior of fossils. The UT High Resolution CT Facility is in the basement of the Jackson School of Geosciences. Matthew Colbert, a research associate, scans the fossils.
“Let’s take a peek inside the scanner here,” Colbert said as he slid the door to the scanner open. “Right now, I actually have the ear bone of a fossil whale mounted. What we’re seeing here is an X ray source on one side, and detectors on the other side, and the object, in this case, sits on a turntable, and spins 360 degrees while the X ray shines through it.”
Rowe scanned the skulls of two pre-mammal species that lived during the Early Jurassic Period, about 190-million years ago . The CT scans image the brain cavity as if it was cut into thin slices.
“I’ll take that and use fancy computer programs to visualize it as a three dimensional object. And that’s really the power, is to see it in three dimensions,” he said.
Rowe and his research team saw that the olfactory bulbs, the part of the brain that processes smell, was much bigger in the pre-mammal species than its earlier relatives. That, they concluded, jumpstarted the evolution of other senses.
“These fossils that we scanned give us the first really clear picture, first solid picture, of what the brain was like in the ancestral mammal, and some of the sequences of evolutionary events leading up to the origin of mammals,” said Rowe.
He added that a keen sense of smell gave early mammals an advantage in the dangerous world of dinosaurs. Today, mammals like dogs retain their sensitive olfactory bulbs, while in other mammals, like dolphins, they’ve disappeared completely.