Why are there only two sexes in humans and animals? In this episode, we explore the fascinating evolutionary and mathematical reasons behind the existence of just two sexes, and why sexual reproduction is so much more complex than simple bacterial splitting. You’ll discover how variation and diversity drive survival, why eggs and sperm evolved to be so different, how mitochondria shaped this process, and why evolution picked exactly two sexes instead of three or more. We also look at examples from yeast and plants, the problem of self-pollination, and how men and women’s bodies diverged through evolutionary pressures including the surprising Fisherian runaway effect. If you’ve ever wondered “Why does sex even exist?” this episode has the answers.