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We humans often brag about our large brain. In fact, most mammals - from aardvarks to zebras - have big brains for their body size. Scientists wonder what evolutionary forces pumped up the mammal brain so much. NPR's Christopher Joyce reports there's a new explanation. Mammals needed to smell better.

CHRISTOPHER JOYCE: Not smell better as in, hey, I need a deodorant. But smell better, as in...

(Soundbite of sniffing)

JOYCE: ...hmmm. I think there's food nearby. Or, hmm, gee, is that a hungry dinosaur just upwind of me?

Tim Rowe, a paleontologist at the University of Texas, says part of the mammal brain grew larger as mammals became better sniffers. The evidence comes from the skulls of two kinds of reptile that lived about 200 million years ago. The skulls are the size of paper clips. They're from China and they're a kind of reptile believed to have evolved into the first mammals.

Dr. TIM ROWE (Paleontologist, University of Texas): We looked at them and went, wow, I mean, these are crown jewels. But they were so tiny and all that we could do is put them behind a microscope and study their external surfaces.

JOYCE: That was over two decades ago. Rowe asked if he could break open the crania and make a cast of the inside, to see the shape of the brain that once sat there. The fossil owners said, no way. So Rowe waited decades - until he developed a way to do make a digital image of the inside of the cranium, using a CT scanner like those used in medicine.

Rowe made brain images of several specimens that lived millions of years apart. And he noticed that the brains changed over time. The smelling part - the olfactory bulb - kept growing.

Dr. ROWE: We could see a huge increase in the size of the olfactory bulbs. And behind the olfactory bulbs...

JOYCE: Where the brain processes signals from the bulb.

Dr. ROWE: ...we could see this region of the brain also got really, really large.

JOYCE: Over millions of years, the whole olfactory apparatus got bigger. And so did parts of the brain that process tactile information, not from skin, but from hair, one of the things all mammals have. Hair wasn't just for warmth. It provided mammals with important information.

Dr. ROWE: They were moving in and out of burrows and up and down on trees and their hair offered a lot of sensory information that helped them balance and navigate and move through this environment.

JOYCE: So mammals whose brains made them better smellers and movers, survived better. But then, why did mammals develop super-smell and touch? I mean, everyone needed a leg up in that dog-eat-dog world.

Well, Rowe suggests this: it was a dinosaur's world back then. Dinosaurs probably hunted during the day, so it behooved tasty little mammals to stay out of sight 'til it got dark.

Dr. ROWE: So once the dinosaurs had bedded down for the night, that's when the mammals came out and started sniffing around for insects, insect larvae and things that lived in the soils, like worms.

JOYCE: At night, smelling and feeling were more useful than, say, good vision.

Glenn Northcutt, a neurobiologist at the University of California at San Diego, notes that other parts of the brain in these reptiles were growing too, areas for motor coordination, for example. He says all these changes actually may be part of the evolution of something bigger than smell or touch - warm bloodedness. Warm-blooded animals didn't need the sun.

Dr. GLENN NORTHCUTT (Neurobiologist, University of California San Diego): The advantage is that it allows you to operate, to use the world at times that reptiles can't.

JOYCE: Northcutt says questions about the deep past like this are hard to answer. But they're fun.

Dr. NORTHCUTT: We all are interested in where we came from and how we came to be.

JOYCE: The research appears in the journal Science.

Christopher Joyce, NPR News.

(Soundbite of music)

MONTAGNE: This is NPR News.

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