Mammals' increased brain size may have come from long-ago natural selection for a better sense of smell, suggests a new study published today in Science. By reconstructing in 3D the skulls of two animals far back on the mammal family tree, the researchers saw that growth of smell-related brain regions accounted for much of the early increase in brain size as mammals developed. How the Heck:
What's the News:
The researchers looked at fossil skulls of two ancient animals. The 205-million-year old Morganucodon was a proto-mammal: a reptile with some decidedly mammalian characteristics (it looked a bit mouse-like, researchers say), that is thought to be an ancestor of mammals today. The tiny mammal Hadrocodium---imagine a shrew the size of a paperclip---lived 195 million years ago.
Fossil skulls of these species are rare, and the researchers weren't about to bust them open to examine the brain cavity. Instead, they used high-resolution X-ray computed tomography to make 3D reconstructions of the skulls, inside and out. Based on the size and shape of the skull cavity, and the impressions left by brain tissue, the researchers could make detailed models of the animals' brains.
Morganucodon's brain was 50% larger, relative to its body size, than the brains of the ancient reptiles. Most of this was due to a huge size increase in the olfactory bulb and olfactory cortex, brain areas responsible for smell. Other parts of the brain grew somewhat, as well, and began to look a little more like the brains of modern mammals.
When the researchers looked at Hadrocodium's brain, they found it had grown another 50%, compared to Morganucodon's. Again, much of this increase was due to larger olfactory regions, though the cerebellum---an area of the brain important in motor control---had grown significantly as well. So had areas that processed tactile information, suggesting the animal had an acute sense of touch.
In addition to these two growth spurts, the researchers suggest, olfaction led to a third set of changes in the mammal brain. When modern types of mammals emerged 65 million years ago, they likely had changes in the parts of the brain that integrate sensory information and the odor-sensing tissue lining the nose that helped them take full advantage of their highly developed sense of smell.
"The story of becoming a mammal is the story of developing the most sensitive and high-resolution olfactory system," paleontologist Timothy Rowe, an author of the study, told New Scientist, "and secondary to that is touch and motor skills."
What's the Context:
Researchers have long puzzled over how and why the mammal brain expanded. Many thought the neocortex---the part of the brain involved in higher-order thinking, which is much larger in mammals than other animals---was the first part to grow. This study lets researchers look at which parts of the brain grew when, and how much. “Until now, we could only speculate what changes were occurring and at what rate. Now we have data and can infer what selective pressures were driving brain evolution in the radiation that led to mammals," biologist R. Glenn Northcutt, who wasn't involved in the study, told ScienceNOW.
Reference: Timothy B. Rowe, Thomas E. Macrini, and Zhe-Xi Luo. "Fossil Evidence on the Origin of the Mammalian Brain." Science, May 20, 2011. DOI: 10.1126/science.1203117Image: Matt Colbert / University of Texas, Austin