Most primates have relatively large brains covered in distinctive folds and grooves. However, common marmosets (Callithrix jacchus) are an unusual exception. These small monkeys have brains that are almost entirely smooth, with very little folding.
Scientists at the German Primate Center (DPZ), the Leibniz Institute for Primate Research in Göttingen, have now identified several cellular processes that help explain this difference. Using miniature brain structures known as organoids, the researchers discovered that certain neural progenitor cells, which produce neurons, develop differently in marmosets than in humans. These differences involve the cells' structure, the speed at which they divide, and the timing of their development.
Together, these changes reduce the number of neurons produced, contributing to the smaller size and smoother surface of the marmoset brain. Published in Science Advances, the findings provide new insights into primate brain evolution and could eventually improve our understanding of human brain development disorders. Why Some Primate Brains Are Smooth Instead of Folded Brain size and the extent of surface folding are important features that distinguish different primate species.
The human brain, for example, has a highly folded surface resembling the ridges and grooves of a walnut. This arrangement greatly expands the brain's surface area, creating room for billions of neurons that support complex thinking and other advanced mental abilities. Scientists believe that the common ancestor of modern primates already possessed a moderately sized brain with at least some folding.
Yet not all primates living today retain these characteristics. Common marmosets (Callithrix jacchus), small monkeys native to South America that are becoming increasingly important in biomedical research, have brains with almost no visible folds. Researchers at the German Primate Center set out to determine why their brains develop so differently from those of humans and other primates.
"We wanted to understand which changes at the cellular level cause the marmoset brain to grow less and form fewer folds," explains Lidiia Tynianskaia, one of the study's two first authors and a PhD student in the Junior Research Group Brain Development and Evolution at the DPZ. "At the beginning of development, the common marmoset brain exhibits the typical structure and composition of a large, folded primate brain. As development progresses, processes must therefore occur that effectively slow down the production of nerve cells." Scientists Discover What Slows Brain Growth To investigate these developmental differences, the team compared brain organoids grown from human and common marmoset cells.
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