Should I be wearing sunblock?
We've spent our entire evolutionary history under the sun, but also aging quickly and dying at young ages. The evolutionary history argument just isn't very compelling.

The current consensus is that sunscreen helps prevent skin cancer, and skin cancer is indeed common. On the other hand, I have seen references to theories suggesting that sunscreen might increase a certain type of cancer or decrease vitamin D production, but these do not seem to be widely accepted. An in-depth discussion of this is probably more appropriate for Skeptics.SE than Bicycles.SE.

In the meantime, just use sunscreen.

EDIT: As heltonbiker pointed out, burning and cancer susceptibility depend on your skin, and most people develop some protection from tanning. I personally wear sunblock during mid-day rides (between about 11:00 and 3:00), when the sun is directly overhead, but I'll leave it off for evening and early morning rides.

If you're burning instead of tanning (even if you don't mind the burns), you're probably getting too much exposure and should use sunscreen more often.