Re: Why did humans evolve with hair only in certain spots, i.e. head, pubic?

Hair on the head probably serves a dual purpose: preventing heat loss when it's cold (body heat is lost rapidly through the head, which is why mothers caution us to wear a hat in winter) and protecting the scalp from ultraviolet radiation and the sunburn and skin cancer it can cause (bald people are especially prone to skin cancer on the scalp).

Pubic and axillary (armpit) hair are examples of secondary sexual characteristics: external structures (other than genitals) that can be used to distinguish the sexes. Other secondary sexual characteristics include facial hair in males, breast development in females and body structure in both sexes (wider hips in females, broader shoulders in men, etc.) These traits appear at the same time as reproductive maturity and are often under control of the same hormones. In fact, the word "puberty" comes from the Latin root "pubes," which means hair. Today, the amount, appearance and pattern of pubic hair are one of the criteria by which physicians determine what stage of pubertal deveolpment an adolescent is in. For our ancestors, pubic and axillary hair were probably one of several visual signals that an indivdual was fully mature, and therefore could be considered as a potential mate by a member of the opposite sex.

The different patterns of pubic, facial and body hair in humans (along with other secondary sexual characteristics) result in sexual dimorphisms in mature humans: males look different than females. Among our primate relatives, strong sexual dimorphisms most often occur in polygamous species. In monogamous species, such as marmosets, males and females look much more alike. It is hypothesized that our early human ancestors had a polygamous mating system in which males competed for females.

As for why the hair that signals maturity appears specifically on the pubic and armpit regions, we can only speculate. Males of many species rely on odor cues (called pheremones) as signals that a female is fertile, or in the stage of her estrus cycle where conception is most likely to occur. Indeed, for many animals, this is the only phase in which a female will mate. Humans, however, mate during all phases of the menstrual cycle, and for the most part do not seem to rely on odor signals. However, there is some evidence that humans may retain a limited capacity to respond to pheremones. Women housed together (as in a college dorm) tend to have synchronized menstrual cycles. In one experiement, secretions from a woman's armpits were collected and applied to the upper lips of other women, who subsequently began to have synchronous cycles, suggesting that odor is the cause. Men are also reported to prefer the smell of human vaginal secretions from women in the fertile part of their menstrual cycle to over those from other times of the cycle. Perhaps these types of odors were more important to the reproductive success of our ancestors who lived before the days of regular bathing and deodorants. If so, having hair in these regions may have helped the odors to stay around longer and be more easily detected by potential mates.

Source: Human Reprodctive Biology, by Richard E. Jones. 1991