What do scientists talk about when they get together? All sorts of random things. Welcome to the first installment of Science With Friends, a new series where friends write about science. I hope it is both entertaining and informative. Enjoy!

Why do men and women exist? Why are there not just…people? Peace, dear reader—I’m not channelling Yoko Ono’s Twitter feed here, but addressing the biological question of why so many creatures (but not all!) are divided into male and female.

The simplest way to reproduce is to just make an identical copy of yourself. This is mainly how bacteria, the oldest and most numerous organisms on the planet, get it done. But plenty of more complex beings, such as plants and animals, can reproduce the same way, through cuttings, budding or other similar means. This asexual sort of reproduction has many advantages—there’s no need to wander around searching for a mate and all one’s tried-and-true genes get passed on, instead of taking a chance and ending up mixed and matched with some rando’s janky DNA.

Asexual reproduction does not result in absolutely unvarying organisms—there’s inherent randomness in the biochemical processes governing life and errant radiation or chemicals can also come by and scramble things. These generate genetic variation, on which the evolutionary processes of selection and drift can act. But this way is slow and conditions, whether they are climatic or pathogenic, can change fast. When a new critter comes along that wants to hitch a ride on you/feed on you/liquefy you from the inside out, it would helpful to have the tools to deal with it sooner rather than later. This is where grabbing some DNA from another individual of your species can come in handy—they just might have what you need to fend this threat off. This is sexual reproduction.

So, exhanging DNA—that gives us a functional definition of sex but it does not necessarily give us sexes. Two organisms that are of the same type can (and do) have sex. This is a rather efficient way of doing things, since everyone around you can be a potential mate. And yet, perhaps because it increases the likelihood of inbreeding (and thus reduces the advantages of sexual reproduction), individuals of the same species are often split into different types.

One way to distinguish types is for cells to put certain molecules on their surface, or secrete them into the surrounding environment. These molecules can be controlled by just one gene, or a small cluster of genes, and define the mating type of the individual. Individuals will then have sex with others of a different mating type, but not with those of the same mating type. For example, the sort of yeast that makes bread rise has two mating types, a and alpha, which usually mate with each other but not with themselves. But other organisms can have three, four, or more mating types, even up to hundreds in some types of fungi. In these species, an individual can mate with another individual of any other mating type, providing a large variety of mates.

While mating types do determine reproductive compatibility, they do not lead to differences in anything other than a few molecules and the genes regulating them—individuals of varying mating types continue to look and act quite the same in all other aspects of their lives. In order to have different sexes, we need more of a difference. That’s where gametes come in.

Gametes are sex cells. They can arise from single-celled or many-celled organisms, and they are specialized for reproduction—their goal is to find and fuse with another gamete and create a new organism with mixed genetic material. While gametes of some species look much the same (though they might have different mating types), in other species they are quite different. Evolutionary pressures seem to push them into one of two directions: either to be larger, fewer in number and relatively stationary; or to be smaller, more numerous and relatively mobile. The first type is called eggs, and the second, sperm. And this distinction forms the delineation of sexes: an individual that produces eggs is called female, and one that produces sperm is called male. Although most organisms we know only have sexes, in a further twist, mating types and sexes can exist in the same species, creating a multiplicity of allowed and disallowed reproductive interactions.

The basic distinction of female and male is established by which gametes they produce, but the pressures of evolution and the resulting requirements of growth, physiology, and behavior often lead to greater differentiation between the sexes. In many mammals, this leads to males being larger than females, but the inverse is true for many insects. And though some birds display striking differences between males and females, others require close anatomical examination to distinguish the sexes. Just because the sexes can look different doesn’t mean they must look different.

It is important to note that female and male are far from exclusive states. Many sexually reproducing species exist as hermaphrodites, either producing both eggs and sperm at the same time, or producing only one type of gamete at a time, and either switching once during their life or being able to switch back and forth. Those that do produce both gametes at the same time can be further divided into those whose eggs and sperm are not fertile between each other, and those whose are. These latter species can literally go fornicate themselves.

So why do men and women exist? For the same reason that birds and bees, fish and fowl, and turtles and trees exist: the vagaries of evolution. At some point a split happened by chance, it worked well enough for the purposes of reproduction, and now we’re stuck with it. Vive la différence!, sure, but in the end it’s just gametes, and we’re just people.

Further reading, suitable for those with some background in biology:

Signs of sex: what we know and how we know it.

Origins of eukaryotic sexual reproduction.

Sex Determination: Why So Many Ways of Doing It?

[Update on 09/02/15] Bdelloid rotifers are a type of tiny animal that were thought to have managed without sexual reproduction for a long time. They were thus considered highly interesting for somehow avoiding the negative consequences of asexual reproduction. But it turns out that much like Ashley Madison users, they were just sneaking around and getting busy in secret. Also like Ashley Madison users, they’ve now been found out.

Praveer Sharma obtained a PhD in Molecular Genetics at the University of Toronto, where he studied how tissues are organized in developing animals. He is now a post-doctoral fellow at the University of California, Irvine, where he studies craniofacial skeletogenesis, which frequently involves shooting lasers at innocent little fish. He tweets once in a while @writermagant.