Why does head hair grow long but body hair stops?

Answer 1

We don't really know for certain, but there are a few likely theories.

One is that early humans evolved to have reduced body hair due to parasites similar to lice. This is further supported by the idea that after losing hair, they were exposed to the cold and sun and had to invent clothing from the hides of other animals. As no other animal does this and it's no simple matter, this neccecity is thought to be one of the driving factors behind human intelligence.

Another speculates that s we're one of the few primeapes comfortable swimming, early humans lived near a coast or lake. Having apelike hair on the body proves a disadvantage in the water but on the head it doesn't matter so much (as these people dog paddled, head above the water until they found something worth diving for).

The theory that humans evolved around the african savannahs suggests that body hair is decreased to allow us to dissapate heat faster and that head hair performs a similar function to a hat, protecting the head from direct sunlight. Support for this one can be seen in primitive hunting methods. Early humans would simply chase after their prey until it was exhausted to the point of overheating or suffering a heart attack. While slower than most animals humans can keep up moderate pace for a relatively long time.

It's most likely a combination of many factors, the first and third given here for instance tie into eachother well.

It is coded in our DNA, the original answer is right about that. But every physical trait, excepting those caused by injury is coded in our DNA. DNA stores the information, and mutates to cause random variations, but it is not the reason a given variation becomes widspread or not.