Humans are a cognitive cut above every other animal, primarily thanks to our large neocortices – the part of the brain that coordinates higher-order functions like language and complex thought. In yet another victory for human brain power, researchers from the Max Planck Institute in Germany have used their sizeable neocortices to reveal that the neurotransmitter serotonin, which is known to mediate positive emotions, may be the key to our ability to grow such bulky cerebral apparatus.

The evolutionary expansion of the mammalian neocortex over millions of years has been linked to an increase in cortical stem cells during fetal development. Known as basal progenitors, these stem cells have the capacity to become neurons in the neocortex, and are more abundant in human fetuses than in other mammals.

Describing their work in the journal Neuron, the authors of this latest study explain that serotonin receptors tend to be present in the fetal neocortex of mammals with gyrencephalic brains – meaning those that have folded cortices. In contrast, mammals with smooth – or lissencephalic – brains lack these receptors in their neocortices during fetal development.

Interestingly, however, the placenta of all pregnant mammals produces serotonin, which is then passed on to the brain of the fetus, though its role in fetal development has until now remained poorly understood. In an attempt to solve this long-standing conundrum, the study authors sought to determine if serotonin generates an increase in basal progenitors in the fetal neocortex of mammals that possess serotonin receptors.

Mice, for instance, are lissencephalic mammals that do not express serotonin receptors in their embryonic neocortex, and have therefore not evolved to have particularly large brains. However, when the researchers artificially stimulated the production of serotonin receptors in the brains of mice embryos, they saw a significant increase in basal progenitors.

They then turned their attention to ferrets, which, like humans, have gyrencephalic brains. As such, they do express serotonin receptors in their embryonic brain, and consequently have more basal progenitors than mice, resulting in larger neocortices. By genetically modifying ferrets to lack these receptors, however, the study authors noted a dramatic reduction in basal progenitors in the animals’ embryonic neocortices.

Based on these findings, the researchers suggest that placental serotonin causes the proliferation of basal progenitors in species that express serotonin receptors in their brains during fetal development.

"Indeed, we found that serotonin, by activating this receptor, caused a chain of reactions that resulted in the production of more basal progenitors in the developing brain,” explained study author Lei Xing in a statement. “More basal progenitors can then increase the production of cortical neurons, which paves the way to a bigger brain."

All in all, this research provides yet another reason to be happy because of your serotonin receptors.