Charles Darwin wrote The Origin of Species in 1859. Various versions of evolution had been proposed many times before, including by Charles Darwin's grandfather Erasmus, but The Origin of Species changed evolution from a speculative musing to a serious scientific inquiry. You would think that in the 141 years since then, scientists would have answered all the critical questions in evolutionary theory. Instead, evolutionary theory is one of the most vibrant and diverse scientific fields today. The greatest of all mysteries, though, is the most surprising.
Sex is virtually ubiquitous.
Sex influences everything from mammals to slime molds. Sex writes poetry. Sex is burned into our brains before we are born. But we don't understand it. Theoretically speaking, sex costs a fortune.