>>339567I see you are asking for a reason why our head hair grows continuously instead of groing to a given length and then stopping like all other hair on our body. The best explanation so far is sexual selection. The trait for continuous hair growth may hare appeared through genetic drift, but the ability to take care of a head full of continuously growing hair (not to mention the continuous growth itself) would be a good indicator of fitness. Certainly hair has been important to human cultures for as long as there is any evidence of hair, often to signify status. It may also be important to note that while men often get bald (a signal of age and maturity) human females very rarely do. This is not so in chimpanzees, who also lack continuous hair growth.
The other known examples of animals with continuous hair growth are of no use in this question. In horses it seems to have been nothing more than a chance mutation that arose during domestication. In musk oxen hair is part of an insulating fur very unlike our head hair. There are also sheep in which the trait has been selected for by breeders.