by Mark Pagel
But our brains must do even more: rather than merely playing a defensive game as our immune systems do, our brains attempt to stay one step ahead of their rivals, and this means getting inside their minds to try to anticipate what they might do next. It becomes necessary to develop what have been called “theories of mind”—a sense of knowing what you think another animal knows, and being aware that it is having similar thoughts about you. Our brains can effortlessly think about situations like “I know that she wants to buy that work of art.” Psychologists call this first-order social intelligence. With a little more effort we can think, “I know that she wants to buy that work of art, and that she knows I am thinking I want to buy it.” This second-order intelligence can easily extend to a third level in the form of “I know that she wants to buy the art, and that she knows I am thinking I want to buy it, and that she is aware that I am aware of her desire to buy it.” Some psychologists say that some of us can routinely deal with even higher-level orders of social intelligence! But what is most revealing about stating them in a sentence is that it makes the mental calculations sound torturous and lengthy but this is something we do almost without thinking.
This pressure to be able to think about and imagine what is going on in the mind of your competitor may have been the force that gave rise in humans to what we now recognize as our conscious minds. With consciousness, an animal can “bring to mind” the things it ought to be thinking about, consider alternatives, and devise plans. In The Selfish Gene, Richard Dawkins suggested that consciousness might have been the final stage of animals becoming ever better at simulating in their own minds what must be going on in someone else’s mind, and then sifting among the alternatives that the simulation throws out. Ultimately to make the simulation complete it must include a model of itself interacting in the world it is attempting to simulate, and poof, this is self-awareness. (Dawkins cautioned not to take this idea too seriously because he thought it might lead to an infinite regress. The difficulty is that if consciousness arises from some virtual observer who reads and interprets the simulation, how do we account for that observer? The answer might be that a simulation of oneself needs another to “see” it, and so on.)
The “persistence hunting” style of the San Bushmen of the Kalahari Desert might be a case of humans using mental simulation to great advantage over a less intelligent adversary. San hunters use a combination of running and tracking to pursue their prey across the South African veld, and this can require them to run many miles for extended periods of time. Endurance running of this sort is only seen in humans, and is thought to have been one of the earliest forms of human hunting, having evolved perhaps as early as 2 million years ago. If true, it could go some way toward explaining why humans seem to be so good at running long distances in marathon foot races. Among the Sans’ favorite large prey are the kudu and eland, both of which are large grazing animals that can stand over five feet at the shoulder. Like most large grazing animals, kudu and eland have evolved to escape from predators that can put on bursts of speed, but not run long distances. This makes them vulnerable to the San who, remarkably, seem capable of keeping up their dogged pursuit in the hot sun in hunts that can last sometimes eight hours, eventually running their prey to exhaustion.
Now, the link to mental simulation is this. Like any hunter, the San creep up on their prey in an attempt to surprise them. Normally they are spotted and the animals move off by trotting away to a safe distance. The San continue to pursue them this way, forcing the animals repeatedly to move, tiring them out. Sometimes the animals move far enough away that the San lose sight of where they have gone. When this happens, the San are forced to follow the animals’ tracks. On some occasions they even lose the tracks and it is at these times they seem to rely on a mental simulation to work out where the kudu have gone. San hunters report that they try to think like a kudu (or eland) would think. They will get down on all fours and try to put themselves into the mind of a kudu, even reenacting how it might behave in an attempt to work out which direction it has gone. If this works, and their simulation leads them to the animal, it might move off again. The hunt then becomes a test of who will collapse first, the man or the animal. But the man always has the one-sided advantage of his mind simulation. Just imagine if the eland or kudu could think like him.
Of course, other humans do have the ability to simulate what is going on in their adversaries’ minds. When competition among brains is fierce, as is suggested by the idea of an arms race, there are many losers, and so it would not have been sufficient merely to keep up: one had to stay among the leaders to survive. If, as we have imagined, the competition was mainly centered on this newly emerging consciousness and sophistication in social and psychological traits, the sieving or filtering out of survivors from non-survivors each generation would have sorted people most strongly by their brains, rather than sheer brawn. Among the evolving human lineage, a social and psychological arms race provides a plausible mechanism, then, not just for the enlargement of our brains but also for the rapid pace of that enlargement over time. Increasingly, the survivors would have been those able to deploy sophisticated psychological strategies emboldened by strongly perceived emotions, and able to engage in strategic and fluid coalitions. These would have to be met by yet more sophisticated strategies in others.
Once a species starts evolving along this trajectory, it might be difficult or impossible for other similar species to keep up. This might then also provide the answer to a question that has long puzzled anthropologists: why we alone emerged as the sole survivor of our lineage, and why no other species acquired intelligence like ours. The other Homo species that had been spawned along the way, including H. habilis, H. erectus, H. ergaster, H. heidelbergensis, and H. neanderthalensis, would each have had to compete with the next large-brained and shrewd species that was about to emerge; perhaps that next one just got one step ahead and the others never recovered. It has even been suggested that our dominance of our particular niche in life is what kept chimpanzees from moving on from theirs. As for the rest of the animal kingdom, few have entered the social corridor the primates did which set our social competition into overdrive.
THE DNA THAT MAKES US HUMAN
IF THIS story about our emerging brains is broadly correct, then we might expect to find genes that cause our brains to enlarge but also change the nature or structure of our brains, not just add more brain material. A computer with more processors is not necessarily cleverer than one with fewer processors, just faster. The publication of the entire sequence of the human genome in 2001 made available a list of our genes. This alone was of limited value for understanding our evolution. But as similar lists became available for other species, it became possible to ask questions about which of our genes differed most from them. The answers display the exquisite precision with which natural selection can sculpt our genes, even though it only gets to see their actions via our behavior.
Darwin’s great idea of “descent with modification” teaches us that these comparisons can be used to answer questions about what has happened along an evolving lineage of species over vast stretches of time. Darwin realized that species evolve and give rise to one or more daughter species that then go on to do the same. This means that any pair of species we care to examine will have, at some time in their past, shared a common ancestor from which they both descend. If we wind the clock of evolution back around 300 million years, a species existed that, although no one would have known it at the time, would become the common ancestor to both present-day mammals and birds. A comparison of the same gene in a contemporary bird and a contemporary mammal then records the evolutionary changes that have occurred in one of these lineages, plus those in the other, or 600 million years altogether.
If we wind that clock back just 6 million years or so, we find the species that was the common ancestor to ourselves and chimpanzees, and this is why comparisons between these two species are so informative about the question of what makes us ge
netically human. But the task of finding differences between humans and chimpanzees is made daunting by two factors. One is that both species have about 3 billion of the chemicals called bases or nucleotides that make up our DNA genetic code. These 3 billion bases are strung out along the structures we call chromosomes. Worse, we are about 90–95 percent identical to chimpanzees in the sequence of these bases along the chromosomes, and more like 98–99 percent in the sequences that we call genes—sequences that carry the codes or instructions for making proteins. What this means is that in the 12 or so million years that separate the two living species (6 million years each since our common ancestor), only about 1 percent of our protein coding DNA has changed.
Still, powerful computers make it possible to check these billions of bases, and when they were checked for humans, forty-nine regions of our genome emerged in which the pace of evolution had dramatically accelerated in our lineage compared to chimpanzees and to other animals. The regions were called Human Accelerated Regions, or HARs, and they should be the Holy Grail genes that make us human, the parts of our genome that really distinguish us from chimpanzees. This is because accelerated change is an indicator that natural selection has been acting particularly strongly on a gene. It means that each time some new helpful variant has arisen, it has quickly spread through the population until everyone has it, and then this process has been repeated as new and different variants arise.
The HARs were ranked from HAR1 to HAR49, with HAR1 being the most rapidly evolving of all the segments. Comparisons between chickens and mice showed that HAR1 hardly evolved at all in the 300 million years from the common ancestor of birds and mammals, changing in just two of its 118 bases. But then there was an abrupt acceleration. In just the 6 million years from the common ancestor of chimpanzees and humans to modern humans, HAR1 managed eighteen changes. This translates to a 450-fold increase in this bit of DNA’s rate of evolution. But the best part of the story is that this most rapidly evolving segment of our DNA is active in human brain cells. HAR1’s high rate of evolution tells us that it must have granted substantial benefits. Those lucky enough to carry copies of it that had one or more of the beneficial mutations must have enjoyed clear advantages over their less lucky and somewhat dim friends.
What makes HAR1 even more extraordinary is that at just 118 bases long, it is about one tenth the length of a typical gene. Genes, as we said, are segments of DNA that contain the code to make proteins, which in turn are the building blocks of bodies. Our hair and fingernails, muscles and eyes, hearts and kidneys, and our skin are all made of proteins. HAR1 does not make a protein; in fact, it is not even a gene. Instead, HAR1 turns out to be a segment of our DNA that influences or regulates how other genes are expressed, and this might be why it can make such a difference to our brains. In HAR1’s particular case, it influences the ways that neurons in our brain develop and project into new areas. Rather than simply making our brains bigger, it changes the structure, density, and complexity of our brains and the connections their neurons make with each other. The changes that HAR1 brings are just such as we might have hoped of a species that supposedly finds it easier to think than the Neanderthals did. Merely adding processors to a computer is not the same as getting these processors to talk to each other and share their information more effectively.
Many of the genes that influence the size of our brain size are located in our genomes near small regulatory segments like HAR1. So, the real wonder of our evolutionary changes since the chimpanzee is how few changes have led to such profound differences and over such a short period of time. Natural selection hit upon just those changes that could make a big difference to our behavior. Many more of these segments and the genes they affect will come to be implicated in our brain growth and development, but one stands out for its possible link to language in humans. FOXP2 is a segment of DNA that, like HAR1, regulates the expression of other genes. All mammals have it, and it is expressed throughout the body, including in brains. Unlike HAR1, it has not changed so dramatically since we split from our ape ancestors, but it has recently acquired what appear to be two critical changes that affect the control of facial muscles that are involved in producing speech. Even mice fitted with a copy of the human form of FOXP2 are said to squeak differently from those with the normal mouse form!
Very recent evidence shows that the Neanderthals had this same variant of FOXP2, leading many people to conclude that they also had language. But this is premature. They might have had language, but simply having this variant is not proof. FOXP2 affects our brains by altering the expression of at least one hundred genes: it is thought to cause about fifty of them to be expressed more and another fifty to be expressed less. So, for FOXP2 to work similarly in the Neanderthal brain, we would have to find the same one hundred other genes, and presume that they worked the same way in both brains. But this seems unlikely. My car has an engine and so does a Ferrari. But my car is no Ferrari. We know our brains differed from the Neanderthals’ in having a more fully developed and highly interconnected cortex, the uppermost layer of our brains. But we also have reason to suspect that the social arms race we have suggested is responsible for our unusual brain might not have been so pronounced in the Neanderthals. The archaeological record points to a species with far fewer artifacts, hinting at little social learning of the sort that is so prevalent in our species. And, as we saw in Chapter 2, it is the presence of social learning that established the need for systems of exchange and cooperation; in short, that established the need for a social brain.
DOMESTICATION BY BRAIN GENES
ONE OF the most surprising effects of our big socially charged brains was to preside over their own diminution. Having steadily enlarged for roughly 2 million years, they have shrunk by around 10 percent in the last 30,000. We also became less robust or more gracile—thin-boned—during this time, so it might just be that our brains were adjusting to a reduced stature. But one of the most reliable differences between domesticated animals and their wild ancestors is that the domesticated ones have smaller brains: as a rule, domesticated animals are just a bit dim, or less “street smart.” Could our brains have domesticated us as well?
Domestication is like taking up residence in a protective bubble, and right across the history of evolution it is linked to things becoming simpler. Single-celled organisms that have taken up residence inside the cells of other organisms normally have many fewer genes than their wild ancestors. They jettison genes they no longer need, genes that served functions in their wild state but that are now provided by their host. The structures called mitochondria that exist inside each of our cells and that produce energy are thought to be ancient bacteria that took up residence inside cells like ours over 1.5 billion years ago. They probably had around 3,000 genes when they moved in; now they have 16.
The same sort of protective bubble is erected when an animal is domesticated. Now your shepherd looks after you, sees off predators, finds or steers you to food and water, and keeps you warm. The animal’s response seems to be to jettison features it no longer needs, and this includes shedding some of its costly and expensive brain. And why not? It no longer needs so much of it now that the shepherd is doing the thinking. Domesticated rats, mice, mink, cats, dogs, pigs, goats, sheep, llamas, and horses all have smaller brains than their wild ancestors. Wolves outperform dogs on searching tasks. Domestication also tames animals directly as their human handlers preferentially breed the less aggressive ones. Dogs become less packlike; sheep and cows become calm and relaxed around humans, and more dependent on them. They also become less tuned in, or less switched on, because their human masters are doing all the hard work.
The word “bovine” technically just means cowlike but is used as an adjective to mean stolid, slow-moving, and dim-witted. Maybe our big brains have made us more bovine by cosseting in us technologies built from social learning. Remember it was this social learning that we thought removed much of the need for us to be inventive in the first place. Who among us
is good at tracking game, lighting a fire without matches, or finding edible plants in the local wood? Technology and mastery of the environment are great levelers of people, and this acts as a further boost to domestication. The anthropologist Robert Lee reported that among the San Bushmen of the Kalahari, when disputes reach a point where there might be open conflict, someone will often declare, “we are none of us big, and others small; we are all men and we can fight; I am going to get my arrows.”
Our brains might also have domesticated our outward appearance, making us one of the more peculiar-looking of the mammals. Mammals are really only distinguished from the other animals by two key traits. Mammals have fur—no other kind of animal does—and we evolved the ability to lactate or nurse our young, again something no other animal does. But nearly all of our other features we share with other animals. Now, fur, like feathers in birds, is a valuable invention for its combination of being breathable, its ability to shed water and snow, and for being an excellent insulator, and an insulator that works even when wet. These are just the qualities you might expect of a material natural selection devised to cover warm-blooded animals that sweat, get rained on, and often live in cold climates.
