How to realistically increase hair growth rate

In the answer to this question, it is said that that is needed to be done to make the solution feasible is to increase the growth rate of human hair. But the problem is that I cannot think of any evolutionary explanation for hair growth to be that quick. What evolutionary history would support quicker hair growth (let's say 2 cm a week). I only have 2 constraints:

1. They must have evolved this in, around or near jungles (for story reasons).
2. I would prefer explanations other than sexual selection; that is an obvious answer that is over used by worldbuilders.

posted about 9 years ago

Uncle Tres‭

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