One day in Delft in the fall of 1677, Antoni van Leeuwenhoek, a cloth merchant who is said to have been the long-haired model for two paintings by Johannes Vermeer—“The Astronomer” and “The Geographer”—abruptly stopped what he was doing with his wife and rushed to his worktable. Cloth was Leeuwenhoek’s business but microscopy his passion. He’d had five children already by his first wife (though four had died in infancy), and fatherhood was not on his mind. “Before six beats of the pulse had intervened,” as he later wrote to the Royal Society of London, Leeuwenhoek was examining his perishable sample through a tiny magnifying glass. Its lens, no bigger than a small raindrop, magnified objects hundreds of times. Leeuwenhoek had made it himself; nobody else had one so powerful. The learned men in London were still trying to verify Leeuwenhoek’s earlier claims that unseen “animalcules” lived by the millions in a single drop of lake water and even in French wine. Now he had something more delicate to report: Human semen contained animalcules too. “Sometimes more than a thousand,” he wrote, “in an amount of material the size of a grain of sand.” Pressing the glass to his eye like a jeweler, Leeuwenhoek watched his own animalcules swim about, lashing their long tails. One imagines sunlight falling through leaded windows on a face lost in contemplation, as in the Vermeers. One feels for his wife.
Leeuwenhoek became a bit obsessed after that. Though his tiny peephole gave him privileged access to a never-before-seen microscopic universe, he spent an enormous amount of time looking at spermatozoa, as they’re now called. Oddly enough, it was the milt he squeezed from a cod one day that inspired him to estimate, almost casually, just how many people might live on Earth.
Nobody then really had any idea; there were few censuses. Leeuwenhoek started with an estimate that around a million people lived in Holland. Using maps and a little spherical geometry, he calculated that the inhabited land area of the planet was 13,385 times as large as Holland. It was hard to imagine the whole planet being as densely peopled as Holland, which seemed crowded even then. Thus, Leeuwenhoek concluded triumphantly, there couldn’t be more than 13.385 billion people on Earth—a small number indeed compared with the 150 billion sperm cells of a single codfish! This cheerful little calculation, writes population biologist Joel Cohen in his book How Many People Can the Earth Support?, may have been the first attempt to give a quantitative answer to a question that has become far more pressing now than it was in the 17th century. Most answers these days are far from cheerful.