The size and shape of the human cerebral cortex - responsible for all conscious thought - is largely determined by mutations in a single gene.
The findings, from the Yale School of Medicine and two other universities, are based on a genetic analysis of in one Turkish family and two Pakistani families whose children were born with the most severe form of microcephaly.
The childrens' brains are just 10 percent the normal size, and lack the normal human cortical architecture. And the researchers found that the deformity was caused by mutations in the same gene, centrosomal NDE1, which is involved in cell division.
They say that the combination of undersized brain and simplified architecture has not been observed linked to any other gene associated with brain development. It implies that mutations in this gene were responsible for the development of a complex cerebral cortex in humans.
"The degree of reduction in the size of the cerebral cortex and the effects on brain morphology suggest this gene plays a key role in the evolution of the human brain," said professor Murat Gunel, co-senior author of the paper.
"These findings demonstrate how single molecules have influenced the expansion of the human cerebral cortex in the last five million years. We are now a little closer to understanding just how this miracle happens."