A rose by any other name would smell as sweet; the saying is perhaps a testament to the acute sense of smell that is unique to mammals. Paleontologists have now discovered that an improved sense of smell jump-started brain evolution in the ancestral cousins of present-day mammals. The research appears in the 20 May issue of the journal Science.
The findings may help explain why mammals evolved such large and complex brains, which in some cases ballooned 10 times larger than what might be expected for their body size. By reconstructing fossils from two Early Jurassic Period mammals—Morganuocodon and Hadrocodium—the authors provide new evidence that the mammalian brain evolved in three major stages: first by improvements in sense of smell or olfaction; next by an increase in touch or tactile sensitivity from body hair; and third by improved neuromuscular coordination or the ability to produce skilled muscle movement using the senses.
“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,” said lead author Tim Rowe, director of the Vertebrate Paleontology Laboratory at the University of Texas at Austin. “It paints a much more vivid picture of what the ancestral mammal was like and how it behaved, and of our own ancestry.”
The study used a medical imaging technique called X-ray computed tomography or CT to reconstruct brain molds or endocasts of the 190 million-year-old Morganuocodon and Hadrocodium fossils from China. These tiny, shrew-like critters are thought to be precursors to existing mammals, or “pre-mammals.” A brain endocast is a mold of the space or cavity that encloses the brain. The endocasts used in this study occurred naturally through fossilization.
CT technology is indispensible for analyzing fragile fossils because it allows researchers to create precise, three-dimensional images of a fossilized brain cavity without having to destroy the fossil to expose the endocast.
Rowe’s team spent several years CT scanning over a dozen pre-mammal brain endocasts at the High-Resolution X-ray Computed Tomography Facility at The University of Texas at Austin. The scans are archived online and are freely available.
The three-dimensional 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.
The study also looked at the influence of body hair development on brain size. For example, the paper clip-sized Hadrocodium sported fur, and evidence from fossilized pelts or skins of closely related animals hints that Morganuocodon likely had hair too. The authors speculate that hairy early mammals were quick to develop a keen sense of touch or tactile sensitivity, along with enhanced motor coordination.
Rather than being used for warmth, body hairs initially served as tiny air traffic controllers, allowing pre-mammals to navigate small crevices and avoid harm. This heightened tactile sensitivity eventually led to the formation of intricate sensory fields in the neocortex of the mammalian brain, the authors propose.
Since the necortex is involved in tasks like sensory perception and the generation of motor commands, improvements in its function likely led to the fine-tuning of early mammals’ motor skills and neuromuscular coordination. In both fossils, the cerebellum (the region of the brain responsible for sensory-motor integration) grew so large it began to ripple over into folds; this increase in size supports the idea that early mammals developed advanced neuromuscular coordination.
Comparing the pre-mammal brain endocasts with fossils from other groups, like those of primitive reptiles called cynodonts, the researchers revealed that the brains of Morganucodon and Hadrocodium were almost 50% larger than the brains of mammal precursors. Taken together, the results hint that the ability to exploit a world of information dominated by odors and smells made early mammals extraordinarily different from even their closest extinct relatives.
“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,” Rowe said. “This will unlock new secrets about how huge brains and extreme sensory adaptations evolved in mammals, such as electroreception in the platypus, and sonar in whales and bats. It is all very exciting.”
Read the report, “Fossil Evidence on Origin of the Mammalian Brain” by Tim Rowe and colleagues.
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