From ScienceDaily: Did Burst Of Gene Duplication Set Stage For Human Evolution?
This is pretty crazy stuff. Around 10 million years ago, the common ancestor of chimpanzees and humans started undergoing gene duplication at a much faster rate than normal, while other mutations like a single nucleotide change became less prevalent.
Evan Eichler, a geneticist with the University of Washington says, "There's a big burst of activity that happens where genomes are suddenly rearranged and changed." Duplication slowed down after the chimp and human lines diverged. '"You might like to think that humans are special because we have more duplications than did earlier species," he says, "but that's not the case."'
Apparently, though, this crazy burst of duplication is what makes humans human. It "creat[ed] an instability that persists in the genome of modern humans and contributes to diseases like autism and schizophrenia. But that gene duplication also may be responsible for a genetic flexibility that has resulted in some uniquely human characteristics."