Genome studies reveal that we have more than 98% of our DNA sequences in common with our closest relatives, the chimpanzees. These seemingly miniscule differences are still ten times larger than differences between any two humans. A powerful explanation for the significant physical differences between chimpanzees and humans is neoteny, the retention of juvenile features into adulthood.

In the early embryos of chimps and humans, corresponding neotenous features appear identical. The overt differences that appear later depend on differentiated rates of prenatal development. This is an elegant evolutionary trick. The selective timing of gene expression can be determined by a limited number of genes working to spectacular effect. Baby chimpanzee features retained in adult humans include: much larger brain to body size ratio, curiosity and playfulness, flat face with small teeth, upright rather than forward facing positioning of the skull and downward vaginal orientation. It has been proposed that our lack of hair is an embryonic feature.