Until 1875, no one in the world knew where babies come from. Ordinary people didn’t know, and neither did the scientists who helped shape the modern world. Leonardo da Vinci didn’t know. Galileo didn’t know. Isaac Newton didn’t know.

They knew, that is, that men and women have sex and as a result, sometimes, babies, but they did not know how those babies were created. They did not know that women produce eggs, and when they finally discovered sperm cells, they did not know that those wriggly tadpoles had anything to do with babies and pregnancy. (The leading theory was that they were parasites, perhaps related to the newly discovered mini-creatures that swam in drops of pond water. This was Newton’s view.)

Why? Why did it take the greatest minds of the scientific revolution—the same individuals who successfully calculated the weight of the Earth, and traced the paths of comets that cut the sky only once in a lifetime—more than two centuries to resolve a mystery that every fourth-grader today could explain?

A hemisection of a man and a woman in coitus, Leonardo da Vinci., c. 1492. Public Domain

Because everything to do with anatomy was difficult and uncertain, for starters. Studying the human body required buying corpses from grave-robbers, or bribing hangmen to turn over bodies fresh from the gallows. “You might be stopped by your disgust,” Leonardo da Vinci wrote, no matter how strong your curiosity, “and if that did not hinder you, then perhaps by the fear of spending the night hours in the company of those dead bodies, quartered and flayed and terrifying to behold.”

Da Vinci made the cutaway drawing shown above, of a couple having sex, in about 1492. The drawing has a host of peculiar features. He drew two distinct channels within the penis, though in fact there is only one. In da Vinci’s depiction, the lower channel carries urine while the upper carries semen and connects with the spinal column and brain. (The role of the testicles in all this was not quite clear.) The spinal connection reflected a Greek belief that, in the words of one ancient writer, “sperm is a drop of brain.”

Da Vinci’s transparent woman, pictured below, has design oddities of her own. For a start, she lacks ovaries. As if to make up for that oversight, she has a mysterious tube running from uterus to nipple. That pathway does not exist, except in da Vinci’s imagination, but the idea was that mother’s milk was made from refined, transformed menstrual blood. (This theory, dreamed up by the Greeks, was an attempt to explain why pregnant women and new mothers do not menstruate.)

Studies of the fetus in the womb, by Leonardo da Vinci, c. 1510-13. Public Domain

Especially in anatomy’s early years, before microscopes, sexual riddles were almost beyond reach. Sperm and egg, even if you had known to look for them, were hidden and elusive. The human egg, though it is the largest cell in the body, is only the size of the period at the end of this sentence. Sperm cells, by contrast, are the smallest, far too little to see with the naked eye. (A human egg outweighs the sperm cell that fertilizes it by a million to one, the difference between a Thanksgiving turkey and a housefly.)

Religious faith made matters all the more perplexing. In the early years of the modern age, science and religion were not rivals but allies. All the titans of the scientific revolution were devout. All of them took for granted that, by studying God’s works, they were exalting his creation. But then came trouble.

They waited, like a series of ever-smaller Russian nesting dolls, one inside the other, in Adam’s testicles or in Eve’s ovaries.

For God was not simply the Creator who had shaped the stars and planets and made man in his own image. He was the only being with the power to create life. How could it be, then, that an ordinary couple huffing and puffing in the dark could create a new being?

Thus was born the now-bizarre seeming doctrine that eminent scientists espoused for more than a century. The idea was that parents do not create their children. God created every living being, and he had done so in one swoop, at the beginning of time.

That meant He must have stashed away every person who would ever live, all those destined to be born in the year 100, or in the 1200s, or 1500s, or some century still to come. They waited, like a series of ever-smaller Russian nesting dolls, one inside the other, in Adam’s testicles or in Eve’s ovaries. When the time came, each one would have its turn on stage.

Jacob Jordaens’ painting Adam and Eve, c. 1640. Public Domain

All through the late 1600s, the 1700s, and well into the 1800s, this strange theory of conception prevailed. The theory’s very strangeness, in fact, counted in its favor, much as we today pay homage to the grandeur and reach of the “theory of everything” so beloved of modern physicists. In the 18th century, the scientific debate turned not on whether the theory made sense, but on a battle between spermists, as they were called, and ovists.

The spermists focused on Adam. Within his body, they explained, were testicles; in those testicles were sperm cells; in those sperm cells were miniature proto-humans; in their testicles were micro-miniature proto-humans, who had testicles of their own, within which … and so on, forever. The ovists endorsed the same hallucinatory picture, except that they placed the endless sequence of nesting dolls inside Eve’s ovaries.

In 1694, a scientist named Nicolaas Hartsoeker drew a picture destined for notoriety. It showed a big-headed person inside a sperm cell, hands clutching knees as if he has been told to brace for a crash. But, contrary to legend, Hartsoeker did not claim he had seen this tiny figure, only that someone might see such a thing when microscopes grew more powerful.

Nicolas Hartsoeker’s illustration of a tiny proto-person inside a sperm cell, 1694. Public Domain

Microscopes would indeed reveal new worlds, but for more than a century they served only to send scientists racing off down blind alleys. The greatest of all microscopic investigators was a Dutch cloth merchant named Antony van Leeuwenhoek. Beginning in 1674, he had spotted tiny, living creatures in drops of pond water, in blood, in scrapings from his teeth, indeed, everywhere he looked. No one had ever suspected such micro-worlds. The idea made no sense, since it implied that God had lavished endless care on creatures destined never to be seen.

On an autumn night in 1677, Leeuwenhoek and his wife made love. He leapt up “immediately after ejaculation before six beats of the pulse had intervened,” and ran to his microscope with a sample of semen. There Leeuwenhoek saw “so great a number of living animalcules that sometimes more than a thousand were moving about in an amount of material the size of a grain of sand.” Thrilled, he dashed off a letter to the Royal Society. He did not say whether Mrs. Leeuwenhoek shared his delight.

Anton van Leeuwenhoek. Public Domain

But Leeuwenhoek, who had been trying to divine the secret of life, threw away this smokiest-of-all-smoking-gun clues. He decided, on second thought, that he had made a mistake. These tiny swimmers looked as if they were hurrying to some important destination, but in fact they had nothing to do with procreation.

Instead, Leeuwenhoek decided, he had found micro-animals that happened to live in semen. After all, hordes of microscopic creatures seemed to cavort everywhere he looked — in water, in tree sap, on his teeth, between his toes. Why shouldn’t semen have creatures of its own?

Until well into the 1800s, this parasite theory remained the conventional view. One picture from a medical text published in 1840 shows various parasites, including a sperm cell, alongside a tapeworm and other unappealing creatures.

Sperm cells had yet another strike against them. Why, if they were important, had God made hundreds of millions of them, when one would have sufficed? Surely the best of all possible designers would not have been so ludicrously wasteful.

But the true danger for the spermist view was not a scientific objection but a moral and medical one. A wave of anti-masturbation hysteria hit Europe in the 1700s and endured well into the next century. One acclaimed physician produced a best-selling tome warning of the ravages of masturbation. He described one of his patients, a 17-year-old watchmaker. His self-indulgence had left him bedridden and almost unable to move: pale, emaciated, “more like a corpse than a human being.” The unfortunate young man had lost his memory almost completely, though he retained just enough strength to acknowledge the vile habit believed to have brought him to this pass. “A pale bloody discharge issued from his nose; he foamed at his mouth; was affected with diarrhea and voided his feces involuntarily; there was a constant discharge of seminal fluid.” Within a few more weeks, he was dead.

Drawing of a “habitual masturbator,” from the 1847 book The Silent Friend. Public Domain

Every medical authority hammered home the same message: every drop of semen was precious. This should not really have counted as an argument against the spermists—semen and sperm cells were not the same thing—but an epidemic of fear was no time for fine distinctions. The spermist doctrine that waste was part of God’s plan had little chance in an era that preached that waste was a physical and moral catastrophe.

It’s tempting to look at our intellectual forebears and smile patronizingly at them. How foolish of them to have chosen to live so long ago. But we should resist temptation. They had set out to explain where new life comes from and found themselves ensnared in a related but even harder question: what is life? A straightforward inquiry about sex and anatomy had transformed itself into a slippery philosophical riddle.

For us, it would be as if scientists trying to map the brain found themselves trying to explain, where does hope come from? Where do ideas come from? We still don’t know. We understand perfectly well that brain gives rise to mind; the problem is that we cannot sort out just what that means. The scientists struggling with the babies mystery understood perfectly well that certain bits of matter were alive and others weren’t; the problem was that they couldn’t sort out how that could be.

Today, every 10-year-old knows where babies come from. But for millennia, the deepest thinkers on earth could only guess. That’s progress, but we shouldn’t be too smug. Every generation makes the mistake of thinking that the escalator runs only as high as their floor. Not so. We can be sure that in centuries to come, our descendants will look back at us and quote our earnest beliefs and shake their heads in astonishment.

This piece was adapted from The Seeds of Life: From Aristotle to da Vinci, from Sharks’ Teeth to Frogs’ Pants, the Long and Strange Quest to Discover Where Babies Come From by Edward Dolnick, available from Basic Books, an imprint of Perseus Books, a division of PBG Publishing, LLC, a subsidiary of Hachette Book Group, Inc. 2017.