Biologically speaking, there are only two human sexes, male and female. However, did you ever stop and think about why reproduction occurs only between two sexes and not, say, four, eight or even 30? Surely having more sexes to reproduce with would increase an organism’s chances of creating more offspring, right? As explained in a recent Draw Curiosity video, it doesn't quite work like that.

According to Science Magazine, some scientists believe that there was once only one sex, and living organisms were able to mate with whomever they chose and still produce offspring. Having two distinct sexes rather than one universal sex cuts an organism’s reproductive possibilities in half. However, this is the system that has persisted in nearly every species on earth. Why? No one is quite sure, although there are a few theories. For example, some think that two sexes, male and female, are less likely to breed with family members than one sex, and therefore create stronger offspring, the magazine reported.

Read: What Is Intersex? Model Hanne Gaby Odiele Reveals Rare Sexual Condition, Urges Further Acceptance

Other theories on the rise of “two sexes” include the Hurst Hamilton Hypothesis, which, as explained in the video, posits that mutational dynamics of the mitochondrial genome would have favoured the evolution of sexual reproduction. Whatever the reason for creating two sexes, it seems to work and is likely here to stay as the main way new life emerges.

While sex, as defined by the two groups organisms are split into in order to reproduce, may be stagnant, the concept of gender is more fluid. Some researchers have even suggested that the social idea of “gender” may even be based on a spectrum from which individuals may adjust throughout their lives.

To understand more about theories on the sexes, watch the video below.

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