The Surprising Secret Behind Your Brain’s Wrinkles

Key takeaways

• Researchers compared developing brain models from humans and common marmosets to investigate why one species has a heavily folded brain while the other has a nearly smooth one.

• Marmoset brain-building cells divided more slowly, had simpler structures, and had less time to multiply rapidly—all contributing to lower neuron production.

• The findings suggest that major differences in brain size and shape can emerge from several coordinated changes, rather than a single developmental switch.

• This is research on brain development and evolution—not evidence that increasing brain folds or neuron production would improve cognition in adults.

A small monkey, a big question

The human brain resembles a walnut. The common marmoset’s brain is almost smooth.

Both are primate brains, and early in development, the marmoset brain has features associated with the construction of a larger, folded cortex. So what changes along the way?

Researchers at the German Primate Center looked for the answer in the cells that build the brain. Their findings suggest that the difference comes partly from how quickly those cells work—and how long they have to finish.

The construction crew sets the scale

During development, neural progenitor cells generate the neurons that populate the cerebral cortex, the brain’s outer layer.

To compare this process across species, researchers grew brain organoids: small, three-dimensional tissue models that reproduce important aspects of early brain development.

They identified three differences that help limit growth in marmosets:

• Some progenitor cells divided significantly more slowly than their human counterparts.

• Others had fewer branching extensions and were less able to multiply.

• The period available for rapid cell multiplication was shorter.

Together, these differences reduced neuron production, helping explain the marmoset’s smaller, smoother cortex.

The central idea is timing: even a productive cellular workforce builds less when it works more slowly and has a shorter construction window.

Why the wrinkles matter

Folding allows a larger cortical surface to fit within the skull. Understanding how that surface expands helps researchers investigate how different primate brains evolved.

This study suggests that a relatively smooth brain does not require a completely different developmental blueprint. Several adjustments to a shared set of cellular processes may be enough.

That is a more nuanced explanation than a single “big-brain gene” or one switch that determines whether a brain becomes folded.

It also does not make the number of folds a simple score of intelligence. The researchers investigated how brain structure develops, not how folding alone determines cognitive ability.

Miniature brains need a reality check

Organoids let researchers examine more samples than would be feasible using primate brain tissue alone. But they are models, not complete brains.

The team therefore repeated key experiments in fetal brain tissue to check its findings.

They also established that a 50-day-old marmoset organoid corresponded closely to a natural marmoset brain at approximately day 90 of development.

That calibration matters. Comparing models by their age in a laboratory dish could otherwise mean comparing different developmental stages—and mistaking a timing mismatch for a biological difference.

What this means for longevity

This is not a study of brain aging, and it offers no intervention for preserving memory.

Its relevance is more foundational: understanding how the brain is built helps scientists distinguish normal developmental variation from disrupted growth and create more reliable research models.

Those models are useful only if researchers understand what they reproduce—and where their developmental clocks differ from the real thing.

The takeaway

The difference between a smooth brain and a folded one may depend partly on small changes in cellular speed, structure, and opportunity.

The study offers a striking reminder that biology does not always need a new blueprint to produce a different result. Sometimes, changing the construction schedule changes the entire building.

References:

1. Lidiia Tynianskaia, Cesar Mateo Bastidas-Betancourt, Esther Marie Grewe, Julia Marie Kniep, Neringa Liutikaite, Nesil Esiyok, İrfan Burak Göloğlu, Sabrina Heide, Nancy Rüger, Dimitri Lindenwald, Charis Drummer, Stoyan Petkov, Nataliya Di Donato, Michael Heide. Coordinated neural progenitor adaptations contribute to primate neocortical downscaling. Science Advances, 2026; 12 (39) DOI: 10.1126/sciadv.aeg5563