Scientists have long analyzed the brains of chimpanzees and men to determine what caused the increase in brain size of humans during the evolutionary process. The genetic makeup of man and chimpanzee is virtually identical, so why did man’s brain grow larger enabling advanced capabilities such as mastering abstract language and complex math? Scientists at Duke University believe they have a major part of the answer. They used mouse embryos. When they were injected with a specific regulator DNA from homo sapiens, they called HARE5, an increase in brain size of 12 percent occurred. The embryos injected with chimpanzee DNA did not have an increase in brain size.
The Scientists are anxious to see if the mice will display a change in behavior after they are born.
More evidence that HARE5 is the reason human brains became larger is based on the differences between humans and chimpanzees during the gestation period. Although the length of time from conception to birth is relatively the same, the growth of the brains of chimpanzees levels off at 22 weeks.
The scientists believe that there are other factors for the difference in brain size but this is an important piece of the puzzle.
The Duke researchers were astounded when near the end of the gestation period brain size differences among the mice was visible to the naked eye.
Sources:
Tre'vell Hill 
Jeanette Smith