According to this article from the BBC's Science Focus
Charles Darwin thought that the 28-day human menstrual cycle was evidence that our ancestors lived on the seashore and needed to synchronise with the tides.
Did Darwin ever say such a thing?
The closest I have been able to find from a cursory search is this letter from T.H. Huxley in the Darwin Correspondence Project, where Huxley writes
Fancy lunacy & menstruation coming out of the primary fact that one’s nth. ancestor lived between tide-marks! I declare it’s the grandest suggestion I have heard of for an age
Is there any evidence that Darwin suggested this?