53 What is the Evidence for Sexual Selection in Humans?
53 What is the Evidence for Sexual Selection in Humans?
Evolution by sexual selection has been invoked to explain a number of human anatomical features, which appear useless or detrimental to human survival.
Biology is Sexy!
Why do human men have nipples?
Early evolutionary biologists were keen to observe that humans, especially compared to nearly all other mammals, were much less hairy. Many hypothesized that because human females were much less hairy than their male counterparts, the loss of hair was due to selection by pre-historic human males for less hairy female partners. But if this were the case, and men were selecting for less hairy women, than why would hairlessness be present in human males, and not just human females? The answer will show up often as we discuss evolution- by-products of similar biological pathways in male and female animals can give rise to traits that are only selected for in one sex, but will often show up in the other sex. An interesting example of this is the presence of nipples in mammals. In most mammals, including human females, the nipple is important for transporting milk from inside to outside of the body. Given that human women are solely responsible for breast feeding, why do male animals, like human men, have nipples? Maybe this is not a question you have ever thought about; after all, it would be quite startling to suddenly see a handful of animals, including humans, without nipples. But evolutionarily, this question is quite interesting and the answer will come up a lot in our discussions of human evolution, so it’s worth spending some time on it.
Have you ever noticed most animals have a similar body plan? For example, most animals have a set of eyes somewhere on their head, the head rests atop a thorax or torso, and legs protrude downward from the thorax. While aberrations from this body plan can happen, often as the result of mutation, most animals have this body plan. The reason is that all animals, in fact, all living things, evolved from a common ancestor. As such, the developmental pathways that give rise to the traits we see in animals are highly conserved. This means that across a wide variety of animal species, pathways sharing a common function, also share a common origin in the biological past. Evolution typically takes the path of least resistance and which organisms may not be completely optimized, we are often just “good enough” to survive and reproduce.
This same logic applies to the case of nipples in humans. The developmental pathway from embryo to fetus is highly conserved, with little variation in the early stages of embryonic development. Because nipples are not a trait that is determined by male or female-specific genes, all human fetuses will develop them. While they do not serve the function of nourishing offspring in males, they also do not affect survival and reproduction. Because they are not costly, nipples persist in men as it is not an evolutionary priority to get rid of