[Jimbee (OP)]

Every human being seems to have head hair, or at least be born with it. What purpose does it serve? Does it have a practical, physiological purpose? Or like many things in human evolution, do we have it because we thought it was attractive at one point?

[Petrochemicals]

To keep your head warm. What is a beard for or ear hair?

[Zer0]

Soo...
Global Warming will turn future Humans..
Bald?

ps - what about hairy species that cannot trim it or shave it off?
Must be feelin Hawt!

[Halc]

Every human being seems to have head hair, or at least be born with it. What purpose does it serve?

Cooling and sunburn protection is a fair guess. Back when such things were evolving we were semi-aquatic, losing hair most other places for swimming/wading, but keeping it on the part that sticks out of the water.
Why does it grow to unlimited length?  Very few species have unchecked hair growth. A cat for instance always has nicely uniform thickness of fur. Human head/beard hair is not fur since it grown unchecked, unlike groin and armpit hair.

Or like many things in human evolution, do we have it because we thought it was attractive at one point?

That's getting it backwards. One evolves to be attracted to a fit mate. A good head of hair spells health over those missing it. A strong beard spells masculinity. I don't think the beard much serves to cool and such since there's hardly any surface blood flow at the chin.

Why does the male lion grow a mane?  If it was heat, the females would need it too. I think the lions need it for combat protection.

To keep your head warm.

If warmth was wanted, we'd not have lost the hair everywhere else. Humans mostly evolved in very warm climates. Those migrating further to colder climates started getting some of it back, but lacked the time to complete the task.
So you put hair in the paces you want kept cool, nappy hair especially which stays wet longer.

Global Warming will turn future Humans..
Bald?

Humans are not currently being eliminated due to excessive hair, even in the warmest places. But take away our grip on technology and humans on warm beaches may well reverse the current trend.