Brain organoids—miniature, three-dimensional structures grown from stem cells that mimic the development of real brains—are offering new insights into how different primate species evolved distinct brain shapes. While most primates, including humans, have large brains with complex folds and grooves, common marmosets, a small South American primate, have brains that are smooth and nearly unfolded. Researchers at the German Primate Center (DPZ) in Göttingen have been investigating why marmoset brains develop so differently, focusing on the cellular processes that occur during early brain development. Their findings, published in Science Advances, reveal that specific types of neural progenitor cells—cells that give rise to neurons—develop differently in marmosets, both in structure and timing. These differences lead to fewer neurons and a smaller, less folded brain. The size and complexity of brain folds, known as gyri and sulci, are key features that distinguish primate brains. In humans, these folds dramatically increase the brain's surface area, allowing for the accommodation of billions of neurons, which support advanced cognitive functions. Researchers believe that even the common ancestor of all primates had a moderately sized and partially folded brain. However, some modern primate species, like the common marmoset, have evolved a much simpler brain structure. To understand how this came about, scientists at the DPZ focused on the marmoset's neural progenitor cells, which are critical for generating neurons and shaping brain size and folding. The study found that certain progenitor cells in marmosets divide more slowly than in humans, and others have simpler structures with fewer extensions, making them less efficient at producing neurons. These factors contribute to fewer neurons and, consequently, a smaller and less folded cerebral cortex. Additionally, the timing of certain cellular processes in marmosets is altered compared to humans, reducing the window of time during which progenitor cells can proliferate rapidly. These findings suggest that the unique developmental trajectory of the marmoset brain is the result of multiple, interacting changes at different cellular levels. Much of the research was conducted using brain organoids, which are lab-grown models that simulate early brain development. These organoids allow scientists to study brain development in detail without the ethical or logistical challenges of working with live primates. The researchers confirmed their findings by comparing organoid development with actual fetal brain tissue, ensuring that the results were accurate. The study not only sheds light on the evolutionary diversity of primate brains but also provides a framework for understanding developmental processes in the human brain and potential disruptions that may lead to neurological disorders.