When injected with bits of human DNA, mouse embryos grew brains that were 12 percent larger than those of embryos injected with the same genes from chimpanzees. Scientists at Duke University, writing in the journal Current Biology, say the experiment demonstrates the role that a particular gene sequence, HARE5, plays in the development of the human brain, which is far heavier and more complex than the brains of our closest animal cousins. The research also may help reveal why humans develop conditions like Alzheimer’s and autism, but chimpanzees do not.