Image: Mouse brain
M. Florio and W. Huttner / Max Planck Institute

This embryonic mouse cerebral cortex was stained to identify cell nuclei (in ) and a marker for deep-layer neurons (in red). The human- gene known as ARHGAP11B was selectively expressed in the right hemisphere: Note the folding of the neocortical .

Excerpt from  nbcnews.com  

By Tia Ghose

ave the way for the of human intelligence by dramatically increasing the number of neurons found in a key brain region. 
This gene seems to be uniquely human: It is found in modern-day , Neanderthals and another branch of extinct called Denisovans, but not in chimpanzees. 
By the brain region called the neocortex to contain many more neurons, the snippet of DNA may have laid the foundation for the human brain’s expansion.
“It is so cool that one tiny gene alone may suffice to the phenotype of the , which contributed the most to the expansion of the neocortex,” said study lead author Marta Florio, a doctoral candidate in molecular and cellular biology and genetics at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden,
She and her colleagues found that the gene, called ARHGAP11B, is turned on and highly activated in the human neural progenitor cells, but isn’t present at all in mouse cells. This tiny snippet of DNA, just 804 genetic bases long, was once part of a much longer gene. Somehow, this fragment was duplicated, and the duplicated fragment was inserted into the human genome. 
In follow-up experiments, the team inserted and turned on this DNA snippet in the brains of mice. The mice with the gene insertion grew what looked like larger neocortex regions. 
The researchers reviewed a wide variety of genomes from modern-day and extinct species — confirming that Neanderthals and Denisovans had this gene, while chimpanzees and mice do not. That suggests that the gene emerged soon after humans split off from chimpanzees, and that it helped pave the way for the rapid expansion of the human brain. 
Florio stressed that the gene is probably just one of many genetic changes that make human cognition special.
The gene was described in a paper published online Thursday by the journal Science.