May 19, 2011

Improved Sense Of Smell Helped To Grow Mammal Brains

Paleontologists have discovered that an improved sense of smell jumpstarted brain evolution in the ancestral cousins of present-day mammals. 

The findings help explain why mammals evolved such large and complex brains, which in some cases ballooned 10 times larger than relative body size. 

The researchers constructed fossils of two Early Jurassic Period mammals to provide evidence that the mammalian brain evolved in three major stages.  First by improvements in sense of smell or olfaction, second by an increase in touch sensitivity, and third by improved neuromuscular coordination.

"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," lead author Tim Rowe, Director of the Vertebrate Paleontology Laboratory at the University of Texas at Austin, said in a statement.

"It paints a much more vivid picture of what the ancestral mammal was like and how it behaved, and of our own ancestry," said Rowe.

Zhe-Xi Luo of the Carnegie Museum of Natural History said, "Our new study shows clearly that the olfactory part of the brain and the part of the brain linked to tactile sensation through fur were enlarged in these early mammals. A sophisticated sense of smell and touch would have been crucial for mammals to survive and even thrive in the earliest part of our evolutionary history."

The researchers used X-ray computed tomography (CT) to reconstruct brain molds of the 190 million year old Morganuocodon and Hadrocoium fossils from China.  These creatures are thought to be precursors to existing mammals or "pre-mammals."

CT technology is indispensable for analyzing fragile fossils because it allows researchers to create three-dimensional images of a fossilized brain cavity without having to destroy the fossil in order to expose the endocast.

The team spent years scanning over a dozen pre-mammal brain molds, making 3D images to give the researchers an inside view of the brain and nasal cavities of the fossils.

The researchers discovered the nasal cavity and related smell regions were enlarged in the pre-mammal fossils, along with areas of the brain that process olfactory information.

"This is the most comprehensive study yet undertaken using computed tomography to study the evolution of the mammalian skull," Thomas E. Macrini, assistant professor of biological sciences at St. Mary's University in San Antonio, said in a statement. "And it is exciting to see these new insights emerging from years of intense labor."

The team also scanned over 200 living species over the past 10 years.  The scans are available to the public along with nearly 1,000 other specimens on digimorph.org.

Results of the study are published in Science.


Image 1: Skull of Hadrocodium wui. Photo: Mark Klingler and Zhe-Xi Luo, Carnegie Museum of Natural History.

Image 2: CT scans of modern short-tailed opossum (upper left) and Hadrocodium (bottom right) brains (pink) through cut-away skulls. Olfactory bulbs are at front of brain (reddish pink). Credit: Matt Colbert, Univ. of Texas at Austin.


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