Each of the four species of great apes - gorillas, bonobos, chimpanzees, and orangoutangs - have 24 chromosomes. In a rare stable chromosomal disruption, some 7-8 million years ago, two chromosomes fused in our common ancestor, giving rise to the homo genus with our 23 chromosomes. Additionally astounding, breeding had to have occurred between different species with a different number of chromosomes.