“The nose knows,” or so the expert said. Turns out, he was right. Publishing last week in the journal Science, paleontologists discovered that an improved sense of smell jumpstarted brain evolution in the ancestral cousins of present-day mammals.

The findings may help explain why mammals evolved such large and complex brains, which in some cases ballooned 10 times larger than relative body size. The authors reconstructed fossils of two Early Jurassic Period small shrew-like pre-mammals – Morganuocodon and Hadrocodiumusing a medical imaging technique called X-ray computed tomography or CT.

The 3D images gave the researchers a magnified, inside view of the brain and nasal cavities of the fossils. The team observed that the nasal cavity and related smell regions were enlarged in the pre-mammal fossils, along with areas of the brain that process olfactory information. Both characteristics indicate an improved sense of smell in pre-mammals.

“Now we have a much better idea of the historical sequence of events and of the relative importance of the different sensory systems in the early evolution of mammals. It paints a much more vivid picture of what the ancestral mammal was like and how it behaved, and of our own ancestry,” says lead author Tim Rowe, of the University of Texas.

In fact, comparing the mammal brain endocasts with fossils of other groups, like those of primitive reptiles called cynodonts, revealed that the brains of the Morganuocodon and the Hadrocodium were almost 50 percent larger than the brains of mammal precursors.

Nature News reports that

With this foundation in place, later mammals could have siphoned off some of those resources for colour vision, echolocation and even, in the case of the platypus, the ability to sense electric currents.

Rowe says this is just step one. “Now that we have a general picture of the brain in mammals ancestrally, we plan to explore the subsequent diversification of the brain and sensory systems as mammals evolved and diversified. This will unlock new secrets about how huge brains and extreme sensory adaptations evolved in mammals... It is all very exciting!”

Image: Matt Colbert, Univ. of Texas at Austin

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