Why Did Sex Evolve?
Our biological sex is one of the defining attributes of our physical, social, and mental lives. Sticking with us from birth until death, sex shapes our relationships with others, our goals for the future, and our roles in society. This is on top of more mundane things like which bathrooms we use, which sports teams we can play on, and what clothes we wear.
It’s no wonder that sex plays such a large role in our lives. Biological sex determines reproductive strategy, which is one of the fundamentally most important things for any life — humans included. But why did human sexuality come to look the way it does now? And why does life bother having sex at all?
On the one hand, sex seems quite expensive. A sexual organism needs to maintain all the biochemical machinery to allow for genetic mixing to happen in a controlled way. Maintaining this machinery requires extra energy and resources. In addition, finding a mate is an expensive and risky process. It’s expensive because it takes a lot of energy to select the right mate, and risky because in some cases it may be hard to get anyone to mate with you.
Of course, we do see that most life is sexual. So what gives? Why did life evolve sex despite the two drawbacks outlined above?
Why do bacteria have sex?
Imagine a bacteria which can’t have sex. This would mean that no gene sharing would happen between any two bacteria in the population. In other words, the code that defines how the bacteria functions would not be shareable. Of course, random mutations would accumulate in the bacteria as genetic information is passed from generation to generation. However, an individual bacterium’s genetic mutations would only be shared with its direct ancestors. This process can be pictured below where each arrow indicates asexual reproduction.
In the diagram above, the newly evolved green bacteria are unable to share their new genetic information with the blue bacteria because they cannot have sex. The same is true for the blue bacteria with a thicker cellular membrane. Given that both of these new variations are able to survive and reproduce better, wouldn’t it be great if we could have a bacterium with both thick cell membranes and the green color? Unfortunately, without sex, we will need to wait for either lineage to independently evolve the other’s traits to get a blended bacterium.
Of course, if these bacteria were to invent sex, DNA could be passed from one bacterium to another freely. This would allow for a newly evolved cellular trait to quickly spread to the entire population. And, as you can imagine, populations of bacteria that adopt this strategy would evolve much, much faster. Evolving faster would allow this bacteria to more quickly fill evolutionary niches and more readily adapt to environmental perturbations like famine, parasites, or changing atmospheric conditions.
Taking a look at the archaeological data, it approximately lines up with the model outlined above. Life has been around for about 3.7 billion years. About 1.7 billion years into life’s history, bacteria started having sex via horizontal gene transfer — a process where one bacteria directly injects a sequence of DNA into another bacteria. Over the next 2 billion years, an explosion of complex life began to emerge. Life’s new sexual behavior rapidly increased the pace of evolutionary progress. Trait sharing allowed a new gene to quickly be combined with all existing genes in the population and so fantastic new organisms were produced. These organisms are what we see around us today — including ourselves.
Why do humans have two sexes?
Of course, complex multicellular life does not use horizontal gene transfer to have sex. Instead of directly injecting DNA from one organism to another, most multicellular organisms use specialized sex cells and organs. These cells and organs come in two flavors: male and female. Males produce many small and relatively cheap sperm. And females produce a small number of more expensive eggs. The sperm and egg are combined to form a zygote or fertilized egg. This zygote divides to form a full-grown organism.
In humans and other mammals, the female is responsible for growing the zygote inside of her womb. Once the child is born, she is then responsible for providing food via breastfeeding during its early life. The female may also provide relatively more direct parental care during the child’s life. As you can imagine, birthing, feeding, and raising a child is very metabolically expensive for the female. So, what’s the male doing during all of this? Well, typically, he provides food and other resources while the female is pregnant and breastfeeding. In addition, given his larger size and greater athletic ability, he protects the mother and child. In a non-parasitic relationship, the male and female will make equal but different investments into raising the child.
But why on earth did it come to be this way? Why not just have one sex which provides equal support in all aspects of the young’s development? Having only one sex would make it easier for each parent to keep track of how much investment was given by the other. This would make it easier to ensure equal input as it’s easier to mentally compare similar types of support. In addition, because there is only one sex, it would allow for members of a species to mate with all other members of the species. As compared to a sexually dimorphic species, this would double an individual’s mate choices. With that said, why do we observe two sexes?
To put it simply, two sexes has the benefits of specialization. Specialization into two sexes allows for females to develop the necessary assets to do their job well (e.g. breasts, wider hips for birth, better color vision for gathering), and for males to develop the necessary assets to do their job well (e.g. larger muscles, greater athleticism, superior navigational abilities for hunting). This specialization allows for the assets of each sex to be maximally utilized.
To make this point clear, imagine what life would be like for a single-sex humanity. This one sex would be responsible for sharing pregnancy, breastfeeding, hunting, gathering, and protection responsibilities. Now, quite clearly, this would cause some issues. For instance, having to both breastfeed and hunt might be a bit inconvenient. Larger breasts may make it difficult to throw a spear or run to chase down prey. On the other hand, stronger, metabolically expensive muscles may be very unnecessary if you’re staying home and breastfeeding a child.
Of course, this specialization should only go so far. Males and females still need to maintain some level of competence in each domain of child-rearing. For example, mothers still need to aggressively protect their young in light of danger. And, in rare cases, men may need to lactate in light of the mother dying. The point here is that specialization will only go as far as is evolutionarily beneficial.
Following the logic above, natural selection preferred effectively specialized couples. In other words, the couples that were specialized survived better and produced more healthy offspring. This bias towards specialization created the two sexes that we are now a part of today.
