Human Brains May Take Unique Turn

Two neuroscientists have tapped into what may represent a fundamental difference in brain development between people and other mammals.

If the findings hold up, they’ll offer insight into how humans evolved an enlarged frontal cortex capable of supporting symbolic thought and language use, conclude Kresimir Letinic and Pasko Rakic of Yale University School of Medicine.

Researchers have identified many commonalties in how brains of various animal species develop. In contrast, facets of brain growth unique to any one species, including humans, have been elusive.

“There must be small differences between brain development in humans and in other animals,” Rakic says. “These small differences can have a big functional impact on how the brain works.”

In 1969, Rakic and his coworkers first used brain slices from human fetuses–obtained following second-trimester death–to identify remnants of a stream of neurons that traveled from the telencephalon to the thalamus. The telencephalon is a structure that gives rise to the cortex, and the thalamus is a relay center for messages to and from the cortex.

In the new study, Letinic and Rakic 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.

In further tests on human and mouse brain tissue, Letinic and Rakic found that only in people did thalamic neurons at the end of the migratory stream attract telencephalic neurons taken from its source. In mice, neurons located along the migratory path chemically repelled telencephalic neurons, the scientists report.

Although other scientists need to confirm that this neural migration occurs only in people, 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.

Barbara L. Finlay of Cornell University in Ithaca, N.Y., regards the new data as intriguing evidence for an evolutionary change in brain development. However, the same neural migration that Letinic and Rakic observed in human tissue may also occur in apes, which the researchers didn’t test, she notes.

Moreover, there are few examples of evolution molding specific brain areas independently, Finlay notes. In a controversial analysis of data from 131 mammalian species’s evolution, her team finds that within a species, most brain areas grow or shrink in unison according to overall changes in brain size.