The Human Evolution Blog

It is readily apparent that humans are, by far, the most intelligent species on the planet. How this came to be, however, is anything but clear.

Our substantial cognitive abilities are made possible by our enormous brains. When it comes to brain size (relative to body size), humans have the largest brains of any vertebrate. Even further, the neurons of our brain are more interconnected than those of other animals. The real difference in our brains is not that we have more gray matter (cell bodies), but more white matter (axons, which connect neurons to each other).

This begs the question, why haven’t other animal lineages steadily become more intelligent as well? Why do humans stand alone?

For example, the goblin shark has been swimming the oceans for over 100 million years in more or less its current form. In all that time, they haven’t evolved larger or smarter brains. Why not? Assuming that intelligence and other advanced cognitive abilities surely bring a substantial survival advantage, one would think that the goblin shark would have gradually evolved to become more and more intelligent. But there is no evidence that this has happened. Why not?

The answer is that the evolution of a big and interconnected brain is a very improbable event. Big brains are costly in a variety of ways, which brings a whole host of drawbacks.

The cost of a big brain

For example, the human brain consumes a whopping 20% of the body’s energy resources. The constantly pumping human heart, by comparison, consumes only around 5% of the body’s energy. Feeding our big brains placed severe dietary demands on early humans. This is not a minor consideration. As I’ve written previously, most animal species are teetering on the brink of starvation pretty much all of the time. The human brain needs 3-4 times more calories (as a function of total body needs) than the brains of other mammals. This is a great cost.

Anatomical constraints are another possible reason why more animals haven’t evolved very large brains. Evolution can only work with the available anatomy. Brains are confined to the cranium. One can’t grow a larger brain without also growing a larger skull. This means that we would need simultaneous random mutations to facilitate growth of both the cranium and the brain. This would be a quite rare event.

Moreover, having a larger skull comes at great cost as well, at least for mammals. For primates especially, the size of the birth canal places strict limits on how large the head can be at birth. The price that humans pay for having such a large head is considerable. Despite human infants being roughly the same size as chimpanzee infants and having roughly the same gestational interval (8-9 months), maternal mortality is much higher in humans than it is chimps.

Before the dawn of modern medicine, mothers died in childbirth at disturbingly high rates. Even today, the maternal mortality rate is 2-3% in countries like Somalia and Afghanistan. Given the high birth rate in these same countries, the cumulative lifetime risk of dying in childbirth is about 1 in 12. This means that everyone in Somalia knows many women that died in childbirth. While Somalia is underdeveloped to be sure, they still have at least some modern advantages, such as soap, sterile bandages, and at least some access to clean or boiled water. How much higher would the maternal mortality rates have been in the pre-historical era, let alone the distant ancestral environment?

In contrast, maternal mortality in chimps, orangutans, and gorillas is vanishingly small. It’s simply unheard of for an ape mother to die giving birth. This brings the cost of a big skull into focus.

If mutations leading to bigger brains and bigger skulls bring higher rates of maternal and/or infant death, they are unlikely to be favored by natural selection. The question is not why haven’t more animals evolved big brains. The question is how did we evolve them despite the disadvantages?

Cooperative competition

The big conundrum here is that the “ancestral environment” in which humans evolved is not at all unique. We diverged from chimpanzees six or seven million years ago but our lifestyle was not very different from theirs for most of that time. We lived in the same climate, ate similar foods, and formed small family-based groups of around the same size. What was different about how we lived that favored intelligence?

Professor Sergey Gavrilets of the University of Tennessee recently sought to answer this question by considering what kind of evolutionary forces would have favored the development of big brains despite their many drawbacks.

One feature stuck out: competitive cooperation. This might seem like an oxymoron, but what is meant by this is the banding together of one group cooperatively and cohesively in order to compete with a different group. It is within-group cooperation and between-groups competition.

Humans engage in between-group competition perhaps more than any other species. It’s easy to find examples of this from the modern world: states, nations, cultures, and religions show great cohesiveness internally, but fierce competition with each other. The same is true for universities, clubs, unions, and professions. We’re always banding together (cooperating) in order to compete.

Think about team sports. From the World Series to neighborhood pick-up games, team sports involve the close cooperation of some individuals pitted fiercely against others. Players get traded; teams get shuffled; and yet, the cohesion is easily recreated when the next game begins. Humans seem especially built for forming bonds of cooperation in order to fight battles of competition.

This feature of human behavior seems to be unique to our species. Many other species cooperate, of course, and every species competes in one way or another. But humans are oddly flexible regarding with whom we’ll cooperate and with whom we’ll compete. Also, humans take both  cooperation and competition to the extreme.

Altruism, kin selection, and group selection

One of the biggest conundrums in evolutionary biology has been the evolution of altruism – helping others at a cost to yourself. How could behaviors ever emerge and persist if they incurred a cost to one’s self? Natural selection would quickly eliminate such tendencies because the selfless never prosper and pass on their genes.

The first attempt to explain altruism is called the theory of group selection, which states that features can be favored by evolution due to their benefits for a group, rather than individuals. This idea initially failed to gather strong support from biologists because it failed in most experiments and computer simulations. Selfishness always seems to win over altruism long before altruism could spread through a population and exert its group benefits. In the natural world, thieves, cheats, and jerks seem to do well.

Group selection, however, has seen a recent resurgence because of the increasing discovery of the phenomenon of reciprocal altruism. Reciprocal altruism is the idea that some animals will help other animals in need, even at a cost to themselves, but this generosity is expected to be returned when the tables are turned. Most famously documented in vampire bats, reciprocal altruism is being discovered in hundreds of species, most especially mammals.

In addition, the power of kin selection has never been in dispute and seems to have played a larger role in primate and human evolution than previously appreciated. Kin selection is the notion that genes and behaviors can be favored by evolution not only because they help one’s self, but because they help one’s close family members. Because close family members have a high likelihood of sharing genes, helping one’s relatives is very nearly like helping one’s self, at least in terms of aiding the success of one’s genes.

The social dynamics of wolf packs are an extreme case of kin selection. A typical wolf pack includes an alpha male and an alpha female together with several of their siblings and of course the children. Only the alphas breed. The other adults forego reproduction and instead assist the pack and promote the survival of their nieces and nephews in various ways. They don’t need to breed because the majority of their genes are present in their nieces and nephews and will get passed on that way.

What does this have to do with human intelligence?

Biologists and anthropologists now see a way to explain why humans evolved to be so social, so altruistic, and yet, so fiercely competitive at the same time, and intelligence is key to the equation.

For reciprocal altruism to work, animals must have good enough memories to remember who is naughty and who is nice. Otherwise, some individuals could simply mooch off of everyone else’s generosity and never give back in return. These cheaters would prosper from the efforts of others and the whole system of reciprocity would fall apart. Because of this, reciprocal altruism and good memories reinforce each other and could be increasingly favored over time.

Furthermore, kin selection requires that animals recognize who is family and who is not. Wolf packs accomplish this by strictly maintaining packs that are small, tight-knit, and family-based. This has drawbacks because it leads to a great deal of inbreeding.

In primates, however, the social groups are more dynamic. In chimpanzees, members can switch from one band to another. In gorillas, young male interlopers periodically challenge an alpha male (the silverback) for the right to dominate a harem. The idea of “family” changes over time.

It is in this environment that humans evolved. Small bands of mostly related individuals competing against other such bands. However, the membership of the band was subject to change. For example, when two bands clashed, the triumphant band could subsume the surviving members of the vanquished band.

For this system to work, humans had to have an adaptable and expandable sense of “family.” Family isn’t just genetic relatives, but anyone with whom we closely associate.

With this in mind, we have a stage that was perfectly set for intelligence to be favored in the ancestral lineage leading to humans. Early hominids were living in small family-based bands. Reciprocal altruism was common, as was kin selection, close social interactions, and dominance hierarchies, all of which require good memories and social instincts.

Add to this, that hominids were beginning to walk upright, thus freeing their forelimbs to carry and utilize hand tools, even while on the move. Armed with tools in hand, close relationships with pack mates, and good memories, these hominids naturally turned to organized hunting. Meat provides much more calories for the increasingly demanding hominid brain than does the vegetarian diet of the other apes.

The behavior of organized group hunting would strongly favor mental abilities such as recall of past events, pattern recognition, prediction of future events, calculation, and communication. The hunt made us smart.

From hunting to violence

It probably wasn’t long after the evolution of group-organized hunting that our forebears turned to intra-species violence. The process of hunting down big game in the African savannah is not that different from tribal warfare. It involves methodical reconnaissance, careful stalking, and communicating with bandmates in order to coordinate an attack. One must observe, draw conclusions, and formulate intricate plans. This is where the social features of hominids may have led to the strong directional evolution of increasing intelligence.

Our ability to expand our sense of self to include family and other members of our band promoted our cooperative instincts. Doing battle with our tribes promoted our competitive side. In both cases, contemplation of past and future events would be strongly favored. To win a fight with a neighboring tribe, it’s not enough to overpower them; you must also outsmart them.

But you must outsmart them collectively. A truly “smart group” doesn’t just have smart members; they work together smartly. Social cohesion and intelligence would thus reinforce each other in a case of runaway co-evolution. Indeed, the brain size of our lineage nearly tripled over the last 2.5 million years.

In the ancestral environment that shaped the human form and our behaviors, organized hunting and violent conflict may have been key phenomena in promoting both our intelligence and our social dynamics. The prosocial and antisocial sides of human nature – our good side and dark side – may have been shaped through that conflict. Warfare isn’t just about aggression against enemies; it’s about cooperating well with your allies.

We humans are uniquely able to identify with and feel connected to our fellow humans. We are also uniquely able to de-humanize and do harm to them. We all have a Dr. Jekyl and Mr. Hyde within us. Thus, the trick to peace is viewing our fellow humans as part of “us” instead of “them.”

-NHL     (@nathanlents)