Women are from Venus, men are from Mars. Countless films, books, plays and poems have focused on this hyperbole. But why are men and women different? What is the origin of the differences between the sexes?

Sex evolved early in the history of life, and has many benefits. Sex helps to increase the spread of new positive mutations, and thereby increase the pace by which species can adapt to their environment. It is also key to removing negative mutations from the population more rapidly, and is thought to provide particular advantages in the evolutionary arms race of host-parasite interactions. Almost all animals have sex, in some sort of way, and even bacteria have found creative ways to ensure genes are mixed within a population. Although some sexual species have only a single sex, or have many sexes, it is far more common to have two. And the creation of two sexes, and a system whereby opposites attract, naturally leads to divergence – it is almost inevitable that the two sexes will not remain identical.

The exact reasons why there are two sexes, rather than zero, five, or infinity, and why those sexes rarely remain identical, are still under debate among scientists. Having any sexual incompatibility at all makes finding a mate more difficult, though it may also help with incest avoidance. But why just two sexes? Any mutant ‘3rd sex’ individual appearing in the population would be at an advantage because it could mate with anyone. One possible explanation for having only two sexes is that this configuration makes it easiest to coordinate uniparental investment of cytoplasm, believed to be key to preventing intracellular conflict. All cells contain cytoplasm, made up of various organelles such as mitochondria, as well as structural elements. Mitochondria and some other cytoplasmic elements contain their own DNA, which is responsible for controlling very early foetal development. If a newly fertilized egg contained cytoplasmic DNA from both parents, competition would ensue, leading to selfish mutations which reduce productivity. It is so essential to prevent this,that it is worth one sex relinquishing its control and passing down no cytoplasmic DNA at all.

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