Wired for Culture: Origins of the Human Social Mind by Mark Pagel Page B
Authors: Mark Pagel
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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 reduce 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,
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 reduce 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,
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