Remember that there is no particular expectation of good design in something that wasn't designed in the first place and that a pre civilized human would have e

Remember that there is no particular expectation of good design in something that wasn't designed in the first place and that a pre civilized human would have encountered only a small number of other humans and would have been primarily considered a success or failure as far as fitness depending on ability to breed in your 20s not living to 85. What is optimal for us may differ substantially from what might have been optimal for our ancestors. Also consider that vitamin D from sunlight would likely have been a bigger component and that is naturally slackened by tanning reducing vitamin d production/need to stay out of the sun to avoid getting burned/overheated. Basically it would be harder to get too much.