Humans are significantly different from other species when it comes to childhood and development. Humans take roughly three years longer to mature than any other species. This extra time and extra development shows that we have adapted to an increase in the complexity of social and behavioral competencies needed to function in adulthood and reproduction. The differences in the physical, and social development of boys and girls are influenced by adult sex differences in sexual selection, intra-sexual competition, and parental investment.
Physical development results in the physical changes associated with adult reproduction, as well as those physical changes that directly facilitate intra-sexual competition or influence mate choice. Men go through prolonged puberty to grow larger than women. This evolvement must have derived from sexual selection, because there are not many species in which males are larger than females. Because sexual reproduction results in diversity among the individuals of a sexually-reproducing species, there is also variability in the "mate value" of individuals (Geary).
This results in competition for the most desirable mates, and the exercise of choice among mates with the best characteristics. The name Darwin gave to this process is "sexual selection" (Geary). The most prominent sex differences to emerge during puberty are a widening of the hips and pelvis in girls and a widening of the width of the shoulders in boys. Other sex differences include greater changes in the facial features of boys and greater increases in body fat in girls. The former reflects the surfacing of those facial features that members of the opposite sex find attractive, that is, a masculine jaw in men and a youthful appearance in women. Many of the physical changes associated with puberty, such as the relatively large breasts in women compared to other mammals, are due to mate choice preferences,