We humans are born in a feeble state, very much needing our parents to help us survive. They also play a role in shaping our unripe brains for social life. A major new discovery offers insight into these remarkable neural developments in the first few months of life—and highlights the peculiar evolutionary strategy that allows us to have such big brains.
Scientists have a clear sense of how the brain grows in the womb. A deep “progenitor” zone creates new cells, called neurons, which are guided by molecular signals to various specialized regions. We have long thought that this differentiation in the brain was largely complete before birth.
The new study—published in October in the journal Science and led by Mercedes Paredes of the University of California, San Francisco—changes this thinking. In fact, new neurons fan out all over the frontal lobes after birth. The frontal lobes carry out our most distinctively human functions—speech, reason, planning, the regulation of emotions.
Neurons formed in the progenitor zone must migrate across the brain to the spots where they are needed to form connections. This happens with help from proteins on the cell’s outer walls. The proteins allow a neuron to recognize the surfaces and cells that it needs to slink over—and where it must stop to connect. This circuit formation is key to brain development.
Why are these new cells needed? The scientists identified the presence of gamma-aminobutyric acid (GABA), which suggests one of the roles of this cell migration. In the brain, GABA functions chemically to inhibit a cell from its activities—acting as a kind of “off” switch.