Jussi Lehtonen, Geoff A. Parker

Males and females are a fundamental aspect of human reproduction, yet procreation is perfectly possible without this division into two sexes. Biologically, males are defined as the sex that produces the smaller gametes (e.g. sperm), implying that the male and female sexes only exist in species with gamete dimorphism (anisogamy). Our ancestors were isogamous, meaning that only one gamete size was produced. The question of the evolutionary origin of males and females is then synonymous to asking what evolutionary pressures caused gamete sizes to diverge. Studying the ancestral evolutionary divergence of males and females relies largely on mathematical modelling. Here, we review two classes of models explaining the evolutionary origin of males and females: gamete competition and gamete limitation. These seemingly alternative explanations are not mutually exclusive, but two aspects of a single evolutionary process. Once evolved, anisogamy and the two sexes are evolutionarily very stable. This explains the maintenance of anisogamy in organisms with internal fertilization, which can cause large decreases in both gamete competition and gamete limitation. The ancestral divergence and maintenance of gamete sizes subsequently led to many other differences we now observe between the two sexes, sowing the seeds for what we have become.