around 24,000 years ago. By the time the European explorers arrived tens of thousands of years later, the Tasmanians had no bone tools, no fishhooks, no hafted tools or spears, and no spear throwers. They had even lost their appetite for fish! Having lost bone tools they could not sew clothes. They had resorted—as did the Tierra del Fuegians of South America—to smearing their bodies with seal or other marine mammal body fat to preserve their own body heat.
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
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
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