Mankind has evolved over millions and millions of years from single-celled, water-bound organisms, to primitive primates, and then to apes. Venturing out of the forests, early man started to walk on two feet and exposed the scalp to the harsh rays of the Sun.
Radiation from the Sun is harmful and can cause free-radical damage. Free radicals are highly reactive substances which occur in the body under certain conditions such as over-exposure to UV light, and which are thought to be responsible for cancers and a general speeding up of the ageing process.
Thick hair of the scalp on a person signifies much protection from solar radiation and therefore a minimised risk of free radical damage and premature aging. Our prehistoric ancestors were very attracted to someone with thick hair as a potential mate. The likelihood is that the person with the thick hair had the genes for thick hair as well, and therefore passed this natural Sun protection to his or her offspring, thereby increasing their chances of a long life in which to procreate.
If, however, the person displaying a thick head of hair did not have the genes for thick hair, this is not to say that he or she is less of an attractive offer. Hair is made of protein, predominantly one called keratin. Rich sources of protein include meat and eggs. If a person had thick hair, it meant they had a protein rich diet, and the only people with a protein rich diet were the best hunter-gatherers. They also had the best overall health since their diet was good. They made brilliant mates since it would mean that the offspring would be well fed.
The pigmentation in the hair is also a protein, called melanin. Therefore, people with very dark hair (for their hair colour) were the people with a protein rich diet. Prehistoric man found dark hair very attractive because of this sign of a rich diet. Melanin is the substance in the body that prevents UV light damaging the skin, so dark haired people had extra protection and were likely to live longer and have more children.
One thing that humans still did in their early development, was social grooming, just like our ape cousins. Social grooming cemented relationships between all members of the family and contributed to lower stress levels and overall well-being. Of course, playing with someone's hair could lead to other things, like sex, because its tactile nature can be sexually stimulating. Thick hair on the scalp lends itself well to social grooming and of course increased the likelihood of sex and therefore more offspring were produced.
So, why can a thick head of hair play an important role in human evolution? Well, it impresses the animal urges in us and can lead to a long and healthy sex life.