ELI5: Why humans are relatively hairless?

It is difficult to ELI5 because no one actually knows the answer for sure. Every answer presented as fact is really a hypothesis. More than that, they are just-so stories, because they are almost untestable and thus unfalsifiable. All of that being said, there are three major hypotheses, which are not mutually exclusive:

• The running man hypothesis: Walking on two-legs helped us throw spears and see far, and also let us separate our breathing from our stride. When most four-legged animals sprint, their bodies expand and contract such that their breathing is forced to follow their stride; we can decouple those two motions, which is a luxury. Furthermore, hairlessness helps us to sweat, as hair would slow down evaporative cooling.

• The aquatic ape hypothesis: Another idea holds that humans became bipedal because an elevated head helped them when wading and fishing. Aquatic mammals tend to either have very dense hair or no hair at all (whales, dolphins, pigs - kinda, etc.). This idea is not as crazy as it sounds, and some random observations support that we evolved to be in or near wet environments. For example, you know how your fingertips get wrinkly when they’re in water for a while? Well, that reaction is regulated by your nervous system, and is not a direct effect of wetness. Furthermore, those wrinkles have been demonstrated to aid your ability to grip wet rocks.

• The filthy fur hypothesis: Fur is not as good as clothing, because you can remove and clean clothing. Fur, on the other hand, is always full of parasites. Consider the two hairiest parts of the body, the scalp and the crotch; both are subject to lice. This argument holds that we lost fur because of the terrible parasite load associated with dense fur. It also argues that the few remaining hairs can help you feel crawling parasites and impede their progress (I have a hairy back, and can attest to this. Good luck, ticks!) We either replaced fur with clothing gradually, or else picked it up later to cover our nakedness, especially as we went into colder climates, depending on the timeline (which I will admit isn’t known to me).

The remaining hair may serve a number of purposes, but it seems to help prevent sunburn, demonstrate sexual maturity, channel water flow, filter air, increase sensation and sensory range, and possibly trap aroma (while many probably no longer find this desirable, body odor was considered sexy even in historical times, and still is in some places). Some people here have asked if (or argued that) a trait must have been selected for if we see it today, but that’s not always the case. As hard as it is to accept, some things are the way they are purely by chance. Red hair is frequent in Ireland in spite of no known selective benefit. Eyebrow shapes could be in the same category. Again, no one knows.

Explain Like I`m Five: good questions, best answers.