15 Questions for Evolutionists -- #8 How did sex originate?

How did sex originate? That is Question 8 in CMI's '15 Questions for Evolutionists' flyer at the focus of this episode of Genesis Unleashed. Asexual reproduction gives up to twice as much reproductive success ('fitness') for the same resources as sexual reproduction, so how could the latter ever gain enough advantage to be selected? And how could mere physics and chemistry invent the complementary apparatuses needed at the same time.