If the success of a species might be measured by its numbers, humankind seemingly has much to celebrate. Remarkably, two millennia ago, there were just 300 million of us. Within your grandparents’ lifetime, that figure exploded to 2.5 billion. Now we have just passed another milestone. As the turn of the new year beckons, there are now eight billion humans on Earth. Only insect species outnumber us.
That's a product of our relatively prodigious brains and complex social structures, which have allowed us to circumvent the usual rules of evolution—species size is limited by the resources that are available to sustain it. It's because of advances in medicine, agriculture and nutrition, so we live longer.
But that, some suggest, is also a big problem. They say that with the United Nations estimating we will number 10 billion by mid-century, we're outpacing the ability of the planet to provide sustenance, fuelling conflicts over water supply, driving up living costs and unemployment and therefore, crime. Our multitudes—everywhere, all at once, such that those of us alive today represent seven percent of all the humans who have ever existed—are squeezing out the Earth's biodiversity too, and driving climate change. Somehow, they say, we need to get those numbers down.
This isn't a new idea. In the late 18th century Thomas Malthus first pondered the issue, proposing that in time the growth of the population will inevitably slow down, either by a decrease in the birth rate or, less positively, an increase in the death rate. Excessive population growth he said, could only lead to poverty and famine. In more modern times, the demographer Paul Ehrlich—sponsor of the campaign group Population Matters, and a founding father of population studies—rebooted the disaster movie mood back in 1968 with his book The Population Bomb.
“The battle to feed all of humanity is over,” it intones. “Hundreds of millions of people are going to starve to death.” In 1970, he said “the end will come” in the next 15 years. It didn't. Ehrlich had since argued that
