[refurb]

Human biology is made up a number of interacting systems, negative and positive feedback loops and other biochemical pathways.

If you focus on individual mechanisms, it looks like contradictions, but as a whole it makes sense.

Sunlight is beneficial for vitamin D production, but harmful when it comes to UV damage.

Both can be true and not contradictory.

[7952]

And in terms of evolution it makes sense. A lack of vitamin d early in life could effect reproductive success. Skin cancer in your fifties not so much. There just isn't selective pressure.

[refurb]

Another factor to layer on as well!

As humans developed, evolution didn't have to create the optimal system, just a system that gets the organism to reproductive years. After that? Irrelevant.

That could point to the challenge of fixing diseases of aging. A good analogy is programming - if the program you came up with uses 8 bit variables, and that limitation causes problems later, well, you're going to have to start from scratch to fix it.