On the importance of Sexual Selection in the Evolution of Human Intelligence

The question of how mankind got here is a curious one. How is it that we differ so profoundly from all the other animals on the planet? How is it that we are so (relatively speaking) unreasonably smart?

We evolved so we could better read texts about evolution on our phones while walking. Photo by Johannes Plenio on Unsplash

Darwin’s idea of Natural Selection is so deeply ingrained in our mindset (perhaps not so much in the US but definitely in Germany) that it’s become tempting to think that one cannot be wrong about it.
But as always with most complex scientific theories, things can be more difficult than they seem, and the devil is in the detail.

The concept of Sexual Selection was already introduced back in the day by Darwin.

But, as pointed out by Geoffrey Miller in The Mating Mind, it was overlooked in the field for a long time, which almost exclusively focused on the more narrow perspective of Natural Selection instead.
Natural Selection, roughly speaking, is the whole “survival of the fittest” deal. You live and you reproduce. If you are stronger than the others, your chances of survival increase, increasing the likelihood of reproduction.

Charles Darwin (source: wikipedia).

The strong live, the weak die. Surviving long enough for reproduction is an organism’s sole evolutionary target. After sucessful reproduction, it becomes superfluous, and dies soon enough.

Who or what survives isn’t completely obvious. Biologists like Richard Dawkins can be credited for shifting the focus away from the survival of the organism onto the survival of the gene. In his book The Selfish Gene, making altruistic behaviour, among other superficially puzzling phenotypes, more plausible.

That idea was always really appealing to me because it emphasizes the role the tiniest organizational structures of life play, and therefore approaches evolution from a “purer” and almost mathematical perspective.

Schrödinger adressed similar questions in What is Life, where the genes (and the then unknown DNA) were described as an aperiodic crystal that carries structure and information downwards in time as the tiniest possible unit of a stable, self-reproducing entity.

While Schrödinger made some conceptual errors, the “Martian” mind of John von Neumann came up with an even more sophisticated theory, where he predicted precisely how DNA needed to be in order to do the job it was doing. All of that by thinking about life from the perspective of Turing machines and computer science… and all of that before anyone even knew how DNA worked!

Schrödinger’s influential What is Life from 1944.

These things aside: genes self-reproduce, but how are they selected?

It’s not the most naively obvious route to build a full grown dinosaur or monkey around your genes for them to survive longest. But there are some environements where it turned out (over the course of absolutely unimaginable time scales) to be optimal to have the scales of a fish, the eye of a tiger, or to experience the thrill of the fight when involved in physical conflict.

Evolving the eyes of a tiger allowed them to better rise up to the challenges of their rivals. Photo by Sander Wehkamp on Unsplash

The setup of the environements determnes what makes an organism fit within the respective environement, and the genes that reliably provide these properties useful for survival are selected.

If you have the best eyes or use all the best words, you get an edge in survival, so the eyes or language faculties tend to improve (if there is pressure from the outside world). If they are sufficiently well adapted and evolution has found the optimal niche, stagnation can be perfectly alright, as then an equilibrium is reached between cost for building the respective organs and their usefulness. This is exemplified by the fact that many animal species, such as alligators, haven’t really changed in millions of years.

Alligators have both looked like this for millions of years and look like they are millions of years old. Photo by Vish K on Unsplash

When environements change, new adaptive pressures are applied, and species can change, sometimes dramatically.

When the rainforests in parts of Africa began to recede and turned into savannah, the primates that were our ancestors were still used to living in trees and having access to an abundance of food.

But soon enough, they were driven out in the open. They needed to develop new strategies and completely reinvent their way of life and social structure.

A pressure for natural selection was exerted, as the species as a whole had to rise to the new challenges it was presented with. But did they really have to turn into something as smart and as strange as us in order to survive?

Maybe different kinds of pressures played their part.

Imagine your genes lead to a phenotype that does not necessarily improve your chance of survival, but just looks very attractive to your sexual partners?
Famous examples of this are the peacock’s tail, or the antlers of a stag. Or, as I have been told, a beard (Darwin might have thought the same if you remember that magnificent mane).

Having a peacock’s tail doesn’t make peacocks, on average, more likely to avoid being eaten. But it sure as hell makes the individual peacock look fly af. Photo by Jakob Owens on Unsplash

These are superficially useless things, and many times can actually complicate survival siginificantly (looking flashy or having an enormous and useless piece of bone on your forehead isn’t always good for that).

But sometimes they nevertheless persisted because they signal to the opposite gender that the individual is so healthy/strong that is has resources to spare.

These ornaments are calling out: mate with me, I can afford to have this useless thing, therefore I have the best genes.

Vulnerability becomes virtue.

One very relevant application for us homo sapiens, which I assume my average readers to be, is that it might help explain the evolution of something as bizarre as the human intellect.

As the view on the past is limited, it has been a source of great puzzlement to many scientists (leading to something as absurdly wonderful Terrence McKenna’s Stoned Ape hypothesis) why it looks like the brain size of our ancestors doubled in a very short evolutionary time span, with only limited obvious benefits associated with it.

Apes might have already liked getting stoned and thinking about philosophy under the shower millions of years ago. Photo by Kelly Sikkema on Unsplash

Cars and guns haven’t been around that long, and the chances of survival of our ancestors does not seem to have changed sufficiently to justify all the complications that came with increased brain size: from the simple increase in energy costs to larger head sizes forcing the premature birth of utterly incompetent babies to the ridiciulous amount of time humans need to invest to foster their children until their brain is fully developed (this takes humans until their 25th birthday!), it sure is a big burden.

One can argue that brain size did have to increase when our ancestors got driven out of the rain forest, when they had to start hunting in groups, had to have more complex social interaction, depended on each other more and more.

These are good reasons to have a larger brain. But was it really necessary to build something as large as our brain solely in order to survive? Probably not.

A big part of reproduction is concerned with keeping genetic mutations at a minimum (this actually gives us a wonderful mathematical reason why sexual reproduction is necessary, and why there are two sexes with two sexual partners that share 50/50 in the DNA of the offspring in almost all species we find on earth).

When you build something as fancy as a mind, then those with a healthy and working mind are, evolutionarily speaking, more likely to have healthy genes.
It’s kind of like with cell phones: compare an iPhone to an old Nokia.

The fancier it is built, the more you have to be on the look-out that everything is working well, and complicated things usually get broken more easily.

Therefore, another way one can think of the mind is as a highly sophisticated fitness indicator, like an internal peacock tail.

We value the proper workings of a mind: our idols are sport stars, musicians and writers (and sometimes even scientists), all superficially useless activities that nevertheless show off specific cognitive and physical skills working in tandem.

A second aspect is that sexual selection means that the group you are competing in determines your standards you have to live up to.

As William von Hippel says in his wonderful book The Social Leap: you probably wouldn’t want to take a pill to double your IQ if that meant everyone else tripled their IQ.

So if sexual selection starts going for smart people, pressure builds up within the group and the mating hierarchy to select for even smarter people. This, in turn, makes smart people more likely to be selected in future generations.

Maybe the mind has more advantages than being good for hunting: maybe it’s there simply because it is attractive.

In the most extreme case, this can cause something called a Runaway process. This happens when someone develops something arbitrary that someone else thinks is hot, and then they have kids that share the genes of finding that peculiar thing hot.

Suddenly that thing, which can be somewhat pointless, spreads through the whole population. If it’s not too costly, or, as in the case of humans, has unexpected evolutionary advantages, it can stick around.

This concept of a Runway process is reminiscent to chaos theory: you can have a common ancestor (shared initial condition), but by some act of fate (small mutation=small perturbation), the chaotic dynamics of the system guarantee that you can move arbitrarily far away on your Lorenz attractor. In other words: things could have turned out very differently if only for a tiny change due to the highly nonlinear dynamics of the evolutionary process (to get a historical application of that idea, read my article about the connection between genetics, chaos theory and Russia during the First World War), and maybe the history of our intelligence depends in large parts on some random mutations a long time ago that led us down the path of finding intelligence, whatever that might mean precisely, attractive.

There are some reasons for assuming that Runaway processes are not the full explanation for the growth of the human mind, and of course adaptive advantages should not be thrown out of the window. They sure enough had their share in the evolution of our intelligence.

But it’s a pretty safe bet to say that sexual selection played its part: and that smart might very well be the old sexy.