by Mark Pagel
If you think these examples say more about Fuegians or Tasmanians than they do about you, ask yourself if you could make fire without matches, or tell the difference between the edible and inedible plants in your local wood or forest. Dependence of knowledge and learning on the size of the group is surprising to us because we enjoy the benefits of societies that write things down, or draw pictures, or take photographs. Even so, we still see the value of groups today as when we get together to play card or other games. Often no one individual knows all the rules, but by pooling everyone’s slightly different portions of the knowledge the game can usually be reconstructed.
DEMOGRAPHY AND THE “RULE OF TWO”
IT IS not enough that our species could use social learning to acquire the skills to move into most of the environments on the planet. And neither is it enough to say that we acquired the psychological dispositions to protect and keep intact our cultural survival vehicles. For our species to occupy the entire globe, our populations would have had to be expanding. Had they not been, we would either have stayed put, or when we moved to new lands we would have vacated the territory we left behind. But this is not what happened: the combination of human culture and social learning has meant we have repeatedly produced excess numbers of people, enough in fact to occupy the entire world. Indeed, human beings are distinguished in the biological world as having broken a hallowed rule of demography that we can call the “rule of two,” and to have done so over long periods of time.
The significance of this achievement is appreciated when we recognize that any proper history of life on Earth is a history of death, and the reason is the rule of two. The rule gets its name from the fact that throughout history, females—of plant and animal species—have left, on average, just two offspring that will survive long enough to do the same. Some have left more, others fewer, but the average is roughly two. It is a surprising statistic because, for example, a female rat has a prodigious ability to make more rats: she reaches maturity at about thirty-five days old, she can produce a litter of up to twelve pups, wean them in a month, and then start all over again, breeding year round until she dies, typically at two to three years. This high output is true of most small animals. Indeed, the rabbits’ impressive reproductive potential is immortalized in the phrase “breed like rabbits.” But even then, a typical female rabbit leaves just two surviving offspring.
A larger animal like a female elephant takes longer to reach maturity—around ten years—and when she does reproduce it is one at a time, and it takes her far longer to rear her offspring before she can reproduce again. But female elephants reproduce into their sixties, and so they also tend to leave about two surviving offspring. The same is true even of those wildly fecund organisms the trees. An oak or chestnut tree that lives for centuries and rains acorns and chestnuts down in our forests and on our lawns and streets could produce millions of offspring in its lifetime, but oaks and chestnuts on average leave just two surviving offspring trees. Go outside and stare at the vast trunk of one of these trees—some weighing hundreds of tons—and all its many branches. It is a sobering thought that all of that effort in making the wood, and in producing all of the tree’s bark, branches, and leaves over so many years, comes to so little.
This rule of two turns out to have a simple explanation that tells us it could not be any other way. For every offspring of a female, a male has been involved (the rule of two becomes the “rule of one” if we have in mind asexual species that reproduce on their own). If a male and female produce two surviving offspring before they die, those two replace this male and female. If this male and female were to leave behind even just three offspring, and each of these three in turn produced three that survived to reproduce, and so on, the numbers of this species would increase without end. Consider just a population of fifty males and fifty females in which each of these females produced three surviving offspring. In the first generation, the 50 females would produce 150 surviving offspring (three each), increasing the population size by 50 once both the parents had died. These 150 would in turn become 225 when each of the 75 females in this generation left behind three offspring. It is easy to see that the world would quickly become covered in layers of rats, or rabbits, and even only slightly less quickly in a layer of elephants if they could break the rule of two. If oaks and chestnuts could leave more than two, our world could become forests of these great trees. The capacity of common bacteria to reproduce is so great that if their growth went unchecked we would in a matter of days (or less time) all be standing up to our waists in a mat of bacteria that carpeted the entire world.
We learn three lessons from this. One is that for most species the difference between the numbers of offspring they produce and the numbers that survive is so large that it is not much of an exaggeration to say that all offspring ever born die before they get a chance to reproduce. Such startlingly high levels of mortality are the same as saying that competition for survival is fierce. It is this competition that lies behind the nineteenth-century philosopher Herbert Spencer’s summary of Darwin’s evolution by natural selection as “survival of the fittest.” Those few of us who have survived are well adapted: we are the rare descendants of a long line of other rare survivors who were our ancestors. The genes in our bodies and those of every other organism are those that have survived for millions or even in some cases billions of years because they were good at producing successful vehicles, while uncountably greater numbers have died trying. This means we can expect the genes we see today to be very good at promoting their interests, and they will do so by means of the ways they vary the bodies they produce. But even with all this fine-tuning, the average female still produces just two surviving offspring.
The second thing we learn is that different organisms have adopted different tactics for trying to break through the two barrier. Some—like oak trees and rabbits—go all out. Others, like elephants and whales, show more restraint, but put more effort into each offspring.
The third thing we learn is that all those different ways of producing offspring, some as rabbits, others as trees, are just different but approximately equally good ways of making vehicles for transporting genes into the next generation. All that time, bulk, energy, and trillions of individual cells required to make an adult elephant yield the same number of surviving offspring averaged over long periods of time as a rabbit, or even a single-celled yeast (of which there are, technically, not males and females, but two mating types called α and a). Nearly every cell that resides inside a complex organism like a tree or ourselves never sees the light of day, laboring away instead to propel a small number of others into the future. It is even starker than this. The egg of a female and the sperm of a male are single cells. We could say that the trillions of cells that make up our bodies spend a lifetime devoted to seeing just two of their kind escape into the next generation.
It is easy to read this as demonstrating that animals act for “the good of the species,” holding back so as not to overpopulate. But the truth is nothing of the kind. Occasionally, a species will break the rule of two for short periods of time. If a more fecund female came along who could on average leave three surviving offspring, or four for that matter, natural selection would favor her: her greater number of surviving offspring would gradually come to dominate the population in which she lived, and eventually all females would be of her kind, able to trace their ancestry ultimately back to her. But if this happened, this species’ overall numbers would rapidly increase and two things would follow. One is that at some point the species would reach what is called its “carrying capacity,” a number that attempts to describe how many individuals of a species the environment can support. If the population expands above the carrying capacity, some of the excess individuals will die of starvation. The other is that this species’ increased numbers would mean that its predators would come to enjoy a bounty of prey and their numbers would thereby increase. The combination of running out of food and the extra predators would re
duce the average number of surviving offspring from the superfemales back to two.
Some species can break the two barrier for short periods of time when they have just evolved or when they are introduced to a new area. A newly evolved species that consisted of just a single male and female would have to break the barrier ever to increase in numbers. So, the surviving species we see around us have broken the barrier at some point in their history. But these species will now be at their carrying capacity and leaving on average just two surviving offspring. When rabbits were introduced to Australia, they bred like rabbits. The Australian environment had not had rabbits before, and it is likely that the diseases that kill other small Australian animals did not affect them. But the growth of rabbits was soon contained by introducing a virus that controlled their numbers by killing some of them and making others weak or ill.
Now another newly introduced creature—the cane toad—is eating its way across Australia. It seems unstoppable because its poisonous skin either kills or repels the native Australian predators. These cane toads will eventually reach their carrying capacity, and other animals are already discovering how to avoid their poisons. Some field biologists report that the kookaburra has learned how to flip the cane toad over onto its back before eating it, to avoid the toxic skin. If this strategy succeeds, kookaburras will also probably leave more than two surviving offspring, at least for a while, and Australia will ring to the sound of kookaburras even more than it currently does.
Nature is never quite as tidy and predictable as these examples suggest, but the rule of two is what we often call the balance of nature, and it is how things have worked for billions of years. That is, until a species came along that discovered how to break this rule and do so over long periods of time. Once again, that species is human beings, and for at least the last 80,000 years or so we have carpeted the planet with our excess offspring, and continue to do so. Our discovery for breaking the rule of two was to build cultural survival vehicles. The Earth had not seen the likes of this before or since, and this is the sense in which we saw in the Introduction that culture became our species’ biological strategy. Here was a force that could not only deploy technologies such as fire, clothes, and shelter to adapt different environments to it but has been able throughout its history repeatedly to produce innovations that reset the world’s carrying capacity to hold more people in a given area. Plagues, wars, and droughts, and the occasional collapse of civilizations, have at times slowed our march but as yet not stopped it.
We didn’t break the rule of two only by altering the carrying capacity: modern human women achieve a higher birth rate than other large Great Apes. As a rough estimate a wild chimpanzee female might give birth once every four to six years. The comparable figures for gorillas and orang-utans are once every four to five and once every six to nine years, respectively. By comparison, human women living in hunter-gatherer groups might have had a baby about every three to four years, and a two-year gap is common in modern societies. Human women also maintain a longer reproductive lifespan than these Great Apes, reproducing for around thirty years of their lives. This is nearly double that of a gorilla, ten years more than a typical chimpanzee, and perhaps five years more than an orang-utan.
Our rapid rate of reproduction might owe something to a peculiar feature of our species. Human women have a long period late in their lives when they don’t reproduce, called the menopause. Many people simply assume that the menopause is a consequence of our living longer, and that in our “state of nature” women would not have lived long enough for it to occur. But this idea has in more recent years given way to an intriguing suggestion. It is that the menopause might have evolved as an act of nepotism or help directed at relatives. Natural selection might have favored women who ceased their own reproduction late in life to help their daughters or their daughters-in-law to reproduce, rather than compete with them. Those who advance this idea, known as “the grandmother hypothesis,” suggest that having an extra pair of hands around would have meant that the daughters or daughters-in-law could reproduce more quickly. Natural selection would have favored this period of menopause if by helping her daughter or daughter-in-law this grandmother eventually gained more grandchildren than she would have had she chosen to continue to reproduce herself.
Another possibility is that human women have been able to maintain a higher reproductive rate than other Great Apes and still provide for their young simply because modern human societies from early in our history have been more efficient at providing food, shelter, protection, and other resources for people. Whatever the reasons for our higher growth rate, reproduction is to the growth of populations what interest is to money. Populations grow by compounding themselves as the babies born now grow up themselves to have children. So, this higher total reproductive output of our species along with our ability to control our environments has meant that wherever human groups ventured, they would likely have filled up their space and found themselves in constant and intense competition with other human groups doing the same. This tells us that competition among cultural survival vehicles throughout our history has been intense, and just as is true of our genes, those that have survived will have acquired traits that make them good at promoting their inhabitants’ survival. For nearly all of our history up to sometime around 80,000 years ago, we were like other animals and we barely increased in numbers. The species that immediately preceded us, Homo erectus, the Neanderthals, and even so-called premodern or archaic Homo sapiens, struggled to replace themselves and then went or were driven extinct. As recently as 20,000 years ago, our numbers may have amounted to just a few millions, maybe fewer, in the world. But our growth has been rapid, and especially so in the last 10,000 years, with over 6 billion of us today. It is all down to social learning and our distinct cultural survival vehicles.
CHAPTER 2
Ultra-sociality and the
Cultural Survival Vehicle
That even a disposition to die for our cultures can be adaptive,
just as it can be to fight to the death for your own body
VISUAL THEFT
WE HAVE SEEN that by sometime around 160,000–200,000 years ago our species might have acquired the capability to learn new behaviors from watching and imitating others. This put us on a trajectory of cumulative cultural evolution as ideas successively built and improved on others. It is something no other species has achieved, and it continues today at ever-increasing rates because the sheer volume of cultural knowledge acts as a vast crucible for innovation. We need look no further than the chairs we sit in, the televisions we watch, the books we read, the cars we drive, the computers we work on, the spaceships and high-energy physics laboratories we produce, or even the food we eat, to see its effects. And so, to most commentators social learning is “job done,” “end of story”—our species could make things, so we have prospered in a way that other animals didn’t. But in fact our acquisition of social learning was just the beginning of our story as a species because it would create a social and evolutionary crisis, the resolution of which would lay the foundations of our psychology and social behaviors and determine the future course of the world.
Here is why. Social learning is visual theft. If I can learn from watching you I can steal your best ideas and without having to invest the time and energy that you did into developing them. If I watch which lure you are using to catch fish, or how you flake your hand ax to give it a sharp edge, or secretly follow you to your hidden mushroom patch, I am benefitting from your knowledge and ingenuity, and at your expense because now I might even catch the fish before you do. Social learning really is visual theft, and in a species that has it, it would become positively advantageous for you to hide your best ideas from others, lest they steal them. This not only would bring cumulative cultural adaptation to a halt, but our societies might have collapsed as we strained under the weight of suspicion and rancor.
So, beginning about 200,000 years ago, our fledgling species newly equipp
ed with the capacity for social learning had to confront two options for managing the conflicts of interest social learning would bring. One is that these new human societies could have fragmented into small family groups so that the benefits of any knowledge would flow only to one’s relatives. Had we adopted this solution we might still be living like the Neanderthals, and the world might not be so different from the way it was 40,000 years ago when our species first entered Europe. This is because these smaller family groups would have produced fewer new ideas to copy and they would have been more vulnerable to chance and bad luck.
The other option was for our species to acquire systems of cooperation that could make our knowledge available to other members of our tribe or society even though they might be people we were not closely related to—in short, to work out the rules that made it possible for us to share goods and ideas cooperatively. Taking this option would mean that a vastly greater fund of accumulated wisdom and talent would become available than any one individual or even family could ever hope to produce. That is the option we followed, and our cultural survival vehicles that we travelled around the world in were the result.