Neuroscientists have tapped into what may represent a fundamental difference in brain development between people and other mammals and may offer insight into how humans evolved an enlarged frontal cortex capable of supporting symbolic thought and language use.In the new study, researchers injected a dye into the telencephalon of living tissue slices taken from the brains of 15-to-26-week-old human fetuses, as well as from monkey and mouse fetuses of comparable development. Over a period of 36 to 48 hours, the dye unveiled a contingent of human neurons that were migrating from a spot on the telencephalon to a destination on the thalamus. This activity was absent from the other creatures’ brains, the researchers report in the September Nature Neuroscience.

Only in people did thalamic neurons at the end of the migratory stream attract telencephalic neurons taken from its source. The new data suggest that specific developmental innovations fostered human brain evolution, says Stewart A. Anderson of the Weill Medical College of Cornell University in New York City. Since the migrating neurons head to a part of the thalamus that connects to the frontal cortex, this link “may provide one mechanism for the remarkable flexibility of human cognition,” he says. It also supports a theory that disturbed thalamic development contributes to thought disorders such as schizophrenia, he holds.