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Updated: Jun 1, 2026

A Comparative Approach for Quantitative Cell Counting Studies in Widely Different Mammalian Brains
Published on: January 16, 2026
Fossil evidence on origin of the mammalian brain
Timothy B Rowe1, Thomas E Macrini, Zhe-Xi Luo
1Jackson School of Geosciences, University of Texas, C1100, Austin, TX 78712, USA. rowe@mail.utexas.edu
Abstract:
Many hypotheses have been postulated regarding the early evolution of the mammalian brain. Here, x-ray tomography of the Early Jurassic mammaliaforms Morganucodon and Hadrocodium sheds light on this history. We found that relative brain size expanded to mammalian levels, with enlarged olfactory bulbs, neocortex, olfactory (pyriform) cortex, and cerebellum, in two evolutionary pulses. The initial pulse was probably driven by increased resolution in olfaction and improvements in tactile sensitivity (from body hair) and neuromuscular coordination. A second pulse of olfactory enhancement then enlarged the brain to mammalian levels. The origin of crown Mammalia saw a third pulse of olfactory enhancement, with ossified ethmoid turbinals supporting an expansive olfactory epithelium in the nasal cavity, allowing full expression of a huge odorant receptor genome.
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