A Long Time Ago, When Headaches Were Less Common…

The Evolution of Primate Brains

To truly grasp the role that biological nuances have played—and still play—in human history, it is wise to trace back to the very origins. At the core, humans are mammals, Homo sapiens, the most widespread of all primates, yet intimately connected to our relatives, sharing their essence with only a few unique distinctions.

At what point, then, did Homo sapiens finally break away from the myriad other species inhabiting the Earth? Modern science appears to have a fairly solid grasp on this. Over the past 2 to 2.5 million years, our ancestors began to increase brain size dramatically and progressively [1,2]. In today’s world, where children begin learning music and abacus skills before mastering the art of tying their shoes, this trend seems almost inevitable. Yet, if having a large, clever brain were truly an unconditional ticket to evolutionary success and prosperity, nothing would have prevented other animals—or at least some of them—from following the same path. However, dolphins, wolves, and countless others have remained unchanged along this route for millions of years and generations.

In reality, a large brain is an almost extravagant luxury, given the conditions our ancestors faced. For an average mammal weighing around 60 kg, brain volume is typically about 200 cm³, whereas in humans of the same weight, it reaches approximately 1500 cm³. This brain must be carried around—on a neck already somewhat imperfectly adapted to upright walking. Being a delicate, vulnerable, and singular organ, it requires a heavy skull for protection. The brain also consumes a significant share of metabolic energy. In a resting state, it uses up 25 percent of the body’s energy, compared to just under 10 percent for our closest relatives, like chimpanzees. This demands more food intake and results in less muscle strength.

As a result of the shift to upright walking and the birth of infants with large heads, it was, unsurprisingly, women and children who bore the brunt of the consequences. Babies born slightly premature were less likely to endanger their mothers' lives during birth and thus had better chances of survival themselves. Consequently, human infants became the most underdeveloped at birth compared to other young animals. From my own purely maternal perspective—yet to be backed by hard evidence—I believe this played a significant role in the development of humanity. These infants required prolonged care, leaving both mother and child practically defenseless without societal support. Raising each person thus demanded extensive effort and care, not only from the parents’ instincts but also from other community members.

Nonetheless, at some point, the human brain began to grow at a significant pace, likely indicating the emergence of a feedback loop. So far, scientists have not pinpointed the definitive factor that triggered this feedback, though numerous theories exist. These theories can roughly be divided into two categories. The first includes qualitative descriptions of the advantages conferred by a well-developed cerebral cortex, highlighting the evolutionary benefits of Machiavellian intelligence for our ancestors—not only in competing with other species but also in managing intergroup and intragroup competition. The second category consists of attempts to directly model the process itself, a particularly appealing approach in the scientific context.

Most scientists—likely without exaggeration, as most of them are men—believe that the key lies in the notion that males with larger brains somehow attracted more females, making this new trait successful over time. You can imagine what advice Freud might have offered them on that. Joking aside, it's unclear to me why the idea that women with larger brains bore children who survived to adulthood more often and were more successful doesn't get more attention. But for now, let’s leave the boys with their illusions that everything revolves around them.

There’s an intriguing theory that the appeal of a larger brain may stem from Fisherian runaway selection. In this concept, a trait becomes established simply because it is conventionally seen as beautiful. Everyone desires conventionally attractive offspring, thus creating a self-reinforcing cycle—akin to a peacock’s tail [3]. Another theory, the handicap principle, suggests that females sometimes select males with exaggerated traits that may actually hinder survival. Possessing such a trait seems to signal that the male is otherwise so fit and healthy that he can "afford" a large, inconvenient tail, for instance [4]. It’s like showing up to an exam sleep-deprived, with a hangover and unprepared, yet still acing it—a testament to resilience. This concept, proposed by Israeli scientist Amotz Zahavi, wasn’t initially accepted by the scientific community and was critiqued by Richard Dawkins, who later retracted his critique after simulation studies lent it credibility. 

While all of this is fascinating, there’s a fairly obvious flaw in applying these ideas to brain growth, as our brains expanded in both males and females. In contrast, the peacock’s tail is solely an issue for the male of the species. For this reason, I find it hard to take seriously theories that brain enlargement wasn’t adaptive. It seems likely that it was; now the question is just how.

This brings us directly to the cultural drive theory developed by Alexander Markov, which will be the focus of the next article. The main idea behind this theory is to use numerical modeling to understand the specific advantages conferred by an expanding brain. But first, let’s discuss an earlier article that also modeled brain growth in an attempt to determine its evolutionary value.

The 2006 article by Gavrilets and Vose describes a model of prehistoric society[5]. In this model, simulated males in a polygynous and promiscuous population compete for mates, occasionally inventing "Machiavellian memes"—behaviors that enhance competitive abilities. These memes spread among other males through social learning, with the success of learning depending on meme complexity and each male's memory and learning abilities. Both traits come at a cost (assumed to require larger brains, though brain volume isn't directly modeled) and reduce survival rates. Genes for memory and learning abilities mutate at a set rate, with meme size weakly correlating with its fitness effect.

Initially, all males have no memory or learning abilities, and evolution begins with a "dormant phase" where these traits stay low, with only new memes present. However, a self-accelerating "cognitive explosion" eventually emerges, where cognitive abilities, cultural richness (meme count), and individual Machiavellian fitness all rapidly increase. The results suggest that the mechanisms underlying the "Machiavellian intelligence" hypothesis can indeed drive the evolution of substantial cognitive abilities within a timeframe of 10 to 20 thousand generations. The study shows that cerebral capacity evolves faster and to a greater extent than learning ability. Additionally, the model indicates a possible trend toward reduced cognitive abilities (due to the high costs of maintaining a large brain) as the reproductive benefits of a large brain decrease and meme exposure increases in modern societies. 

What stands out is the rather bold assumption about the role of gender in this process, as well as the presumption that memes are created exclusively by males. But let’s set that aside as one of the study’s working assumptions. The idea of an initially zero potential for learning is also quite odd—most mammals clearly have a basic capacity for learning. Naturally, I’m tempted to make a joke about men and intellectual potential, but I’ll show some wisdom and resist. Moreover, there are clear questions about the linear relationship between brain volume and abilities like memory, though that, too, is a point for further clarification.

What I appreciate here is the quantitative model, which can be replicated and refined—something Alexander Markov indeed pursued. Stay tuned for details on his research in the next issue!

Sources:

1)         Sapiens: A Brief History of Humankind by Yuval Noah Harar

2)        https://antropogenez.ru/anthropology-engl/

3)        Miller, G. F. (2000). The mating mind: How sexual choice shaped the evolution of human nature. New York, NY: Doubleday.

4)         Zahavi, Amotz (1975). "Mate selection—A selection for a handicap". Journal of Theoretical Biology. 53 (1). Elsevier BV: 205–214. Bibcode:1975JThBi..53..205Z. doi:10.1016/0022-5193(75)90111-3. ISSN 0022-5193. PMID 1195756.

5)        S. Gavrilets, A. Vose, The dynamics of Machiavellian intelligence, Proc. Natl. Acad. Sci. U.S.A.103 (45) 16823-16828, https://doi.org/10.1073/pnas.0601428103 (2006).]