Pedopmorphy refers to an evolutionary process in which adult animals maintain the traits of juveniles. One fairly easy way for genetic mutations to produce physical changes in the animal is to adjust the timing of maturation. No new physical structures or behaviors need to be introduced; the animal simply maintains its youthful traits instead of shedding them when it reaches maturity. There is evidence that many advances in human evolution involve pedomorphy. For example, we are one of the few mammals that retain a high level of playfulness throughout adulthood. Secondly, the shape of our skull mirrors that of juvenile apes. The skulls of juvenile chimpanzees look more like the skulls of adult humans than the skulls of adult chimpanzees. Adult chimpanzees have small sloping foreheads, prominent jaws, and more horizontally-aligned faces. Adult humans, on the other hand, have high foreheads, small jaws, and flat, vertically-aligned faces, similar to juveniles of both species. Of note, juvenile chimpanzees have a larger brain to skull ratio than do adult chimpanzees. In humans, this favorable ratio is retained into adulthood.