The Seeds of Life

by Edward Dolnick

  Swammerdam delighted in the glistening appearance of the fat in the larva of the soldier fly, which gleamed like new-fallen snow. He likened the penis of a bee to a piece of crystal stemware. He probed hornet excrement and thrilled at the sight of fragments of newly devoured flies that now shone like gold. Truly God was the “Artist of all artists.”

  In June 1669 Cosimo de Medici, on a grand tour of Europe, visited Amsterdam. The prince and a companion stopped at the Swammerdam home and examined the renowned collection of wonders. Perhaps the most wondrous sight of all was a silkworm caterpillar, not part of the collection but a favorite prop of young Jan Swammerdam. For several years Swammerdam had delighted in dazzling scientifically minded observers with a tour de force of dissection.

  On this June day Swammerdam picked up the caterpillar and asked his distinguished visitors to examine it closely. Did they see wings? Did they see antennae? Did they detect any signs whatever of the future moth? They did not. Swammerdam made a deft slit with his scalpel and gently peeled the caterpillar’s skin away. Slowly, carefully, he pried at the soft body. There was no doubt—he had found wings, and antennae, and legs. They had not taken on their final form, but there they were!

  This was a revelation. For starters, it explained the age-old mystery of caterpillar and moth—the two looked so different that it had always been assumed they were two animals, not one. The classical picture was not that a caterpillar transforms into a moth (or a butterfly) but that the caterpillar dies and then the moth rises to life from the carcass. In this devout age, when believers felt that every bit of good or bad fortune and every twig on every tree carried a divine message, this story seemed especially compelling. Just as it was “clearly evident,” one Dutch writer observed, “that from dead caterpillars emerge living animals; so it is equally true and miraculous, that our dead and rotten corpses will rise from the grave.”

  Despite his own deep faith, Swammerdam despised such talk. To say that life could spontaneously arise from dead matter was to believe in chance and randomness. That was atheism, and it was sinful. Nothing could be further from true religion.

  But for Swammerdam, and for generations of scientists to come, his silkworm demonstration did far more than resolve an ancient mystery. Biologists drew two key conclusions from his bravura performance. First, life proceeded not in sudden shifts but in a slow and gradual unfolding. That was true of insects, as Swammerdam had shown, and since God was a universal ruler, it was true for all creatures. God made no local laws.

  “The analogy between the silkworm in its cocoon and the fetus in the womb was a compelling one,” in the words of one modern historian, because creatures throughout the animal kingdom begin life as tiny, wormlike forms nestled inside protective coverings. “It was natural to suppose that the same process was occurring inside one as in the other—the emergence of visible form from subvisible form.”

  The second conclusion was more sweeping. Here was more proof—compelling proof—for the doctrine that developing organisms did not sprout new bits but simply revealed premade structures that had been there all along. “Legs, wings, and the rest are folded up, and as it were packed in a most intricate manner,” Swammerdam explained, and he went on to emphasize that what he had observed in insects applied to life generally, and certainly to human beings.

  The adult had been there from the start, preformed within the embryo and waiting for the proper moment to make its debut. Next came an enormous leap that Swammerdam and his peers regarded as but a small, logical step: what was true of each individual organism—that it was preformed—held for that organism’s entire lineage, back to the beginning and on into the future. (Otherwise, the mystery of how a growing organism knew to take on its proper form had only been pushed back a generation.) Those ancestors and descendants had all been preformed and in waiting ever since the original moment when God had created life.

  A few diehards rejected the whole elaborate theory, but they were on the defensive. William Harvey, the skeptics’ great hero, had felt sure that, somehow, a tiny embryo acquired brand-new organs and structures as it grew—it did not simply unfurl ready-made bits that had been cunningly packed into a compact package. Harvey and his allies had studied chicken embryos carefully, dissecting them shortly after conception and then studying how they changed day by day. The whole intricate process played out according to a detailed timetable, as change followed change and one bit of tissue gave rise to another.

  That view is remarkably close to what modern-day biologists believe. Today the development process is called “epigenesis,” and legions of scientists strive to understand just how an organism’s DNA directs its growth. But with no inkling of DNA or any idea that living bodies are made of cells, Harvey’s “explanation” struck his contemporaries as vague and unconvincing. What were these mysterious forces that shaped each bone and sinew of the growing animal? Where was the plan laid down? Who was in charge?

  Harvey’s allies faced long odds because they could not explain the indisputable fact that living creatures developed and grew in an orderly way. Perhaps some patterns in nature could arise on their own; it did not seem necessary to imagine God cutting out individual snowflakes with scissors. But snowflakes all embody the same straightforward geometry. Hands and brains are not only infinitely varied but linked so that they work in perfect harmony. Could anything so intricate happen by itself? If new organisms really do start out as structureless blobs, one French scientist scolded, then they could grow up only “by a miracle which would surpass every other phenomenon in the world.”

  To seventeenth-century thinkers, any theory that did not highlight God’s role opened the door to chance and randomness. Anything could happen. Unless embryos were ready-made, explained John Ray, one of the great naturalists of the age, there was not “the least reason why an Animal of one Species might not be formed out of the Seed of another.”

  EVEN PUTTING GOD TO ONE SIDE FOR A MOMENT (WHICH NO ONE at the time had any inclination to do), the picture of gradual development made no sense. Twenty centuries before Harvey, the Greek philosopher Anaxagoras had asked indignantly, “How could hair come from what is not hair and flesh from what is not flesh?”

  No one had a good answer. Swammerdam and his fellow scientists added further objections of their own. The notion in the 1600s and 1700s was that the most basic units in the body were organs—heart, lungs, eyes, and so on—along with nerves, blood vessels, and various other connectors. All those structures were essential for life. If a new embryo did not have them from the get-go, how could it live and grow? (And, as Anaxagoras had asked, if it had not had them always, at least in rudimentary form, how could they suddenly appear?)

  Nor was that all. If the parts that formed a living creature appeared one by one, as Harvey had proposed, which came first? What good would a heart do, without arteries and veins to carry the blood it pumped? And what use would those vessels be, without a heart to send the blood around the body?

  Swammerdam had won the day for his fellow preformationists, but his triumph tasted of ashes. Pious and racked by guilt for not devoting all his time to prayer, he confessed in anguish that he had chased the false gods of honor and praise. His discoveries had let him “see, touch, and feel God on all sides,” but that did not console him. “I have not loved Him purely and exclusively,” Swammerdam wept, “but only incidentally and in so far as it gave me pleasure” and won the respect of his peers. In his despair he damned his scientific investigations as “foolish pursuits.”

  In 1675, at the age of thirty-eight, Swammerdam announced his withdrawal from science. He followed a French religious mystic to her island retreat off the coast of Denmark. There he devoted himself to “heavenly reflections,” as his new mentor advised, but after nine months he returned to Amsterdam and his insect studies. He lived only five more years. Swammerdam died in 1680, felled by fever and entrapped, in the words of one scientific colleague, by “melancholic madness.”

  FOURTEEN

&nb
sp; SEA OF TROUBLES

  ALL THROUGH THE 1600S AND INTO THE 1700S, THE WAR between ovists and spermists dragged on. Ovists held a far stronger position but could not quite dispatch their foes. No one from either side challenged the merits of preformation theory itself. That doctrine loomed over the battlefield, as solid and formidable as a fortress.

  The ovists had a variety of arguments to draw on. They invoked tradition: for ages past, everyone had known that life comes from eggs. They cited analogy: look at all the eggs in the animal kingdom. They appealed to science: the eminent Harvey had stood up for the egg, and, more recently, so had Steno, Swammerdam, and de Graaf. They enlisted common sense: the large, stable egg seemed a far safer place to encase countless generations of ever smaller, ever more fragile embryos than did a tiny, thrashing sperm cell.

  All this made a strong case. What could be more reassuring than new findings that confirmed ancient beliefs? The spermists, in the meantime, found themselves reeling in the face of this all-fronts assault. They did have Leeuwenhoek on their side, and the newness and sweep of his discoveries made for a dash of glamour. But the ovists’ findings fit together and strengthened one another, as streams merge to form a river. Here is one creature with eggs, and there is another. Here is an animal with ovaries and oviducts, and here is a woman with structures that look just like them. In utter contrast, Leeuwenhoek’s discovery of tiny animals in semen came out of nowhere. It was new, which was noteworthy, but it stood apart, bewilderingly isolated from the rest of the emerging scientific picture. Leeuwenhoek had plainly found something. But what, precisely? Were these “animalcules” humans in miniature form? Or proto-humans that would transform into humans? Or some other sort of animal altogether?

  On the other hand, the spermist side did have some points in its favor. For one, spermist theory put the spotlight back on males, which struck many scientists as altogether fitting. They argued happily that men dominated every creative field, whether it was painting or sculpture or poetry or architecture. Didn’t it stand to reason that when it came to the most important creative act of all—shaping a new life—that, too, should be a male domain?

  And the image of a squadron of lively, rambunctious sperm cells jolting the egg into action made an appealingly mechanistic picture that was completely in keeping with the intellectual mood of the age. Explanations that invoked levers and pumps and pushes and twists were in vogue. Explanations, like the ovists’, that relied on mystical breaths and “seminal auras” met with sneers and smirks.

  Even so, Leeuwenhoek was slow to win allies to the spermist camp. He had first seen spermatozoa back in 1677, but for decades many of his fellow scientists responded to his findings with skepticism rather than deference. In those early years, that made sense. Sperm cells were hard to see, even for Leeuwenhoek, who had both endless stamina and excellent vision. (Excellent, at least, for close-up work. Some scholars believe that Leeuwenhoek was nearsighted, which might have helped him focus on tiny specimens only a few inches from his face.)

  His microscopes were not like the ones we picture today, with eyepieces and specimens mounted on glass slides. They were tiny, handheld devices that consisted essentially of two metal plates that held a tiny lens between them. He made them himself, hundreds in all.* Using them was finicky, maddening work.

  FIGURE 14.1. One of Leeuwenhoek’s original microscopes.

  Leeuwenhoek fixed his specimen to the point of a needle (or, if he was looking at living creatures, put his drop of water or blood or semen inside a tiny glass tube). Then, using screws attached to the mounting pin, he manipulated the sample near to the lens. Students in high school biology classes today do things the other way around: they move the lens, and the sample sits still.

  The whole apparatus was only about the size of a business card, and the lens was a miniature glass bead a fraction of an inch in diameter. (A drop of water that falls on a phone or an iPad magnifies the screen in the same way that the bead did.) Leeuwenhoek found that the smaller the lens, the sharper the view. For hour upon hour, he stared blinking and watery-eyed at his tiny samples, trying for a clear and steady view.

  Even his enemies conceded that Leeuwenhoek had “the patience of an angel.” To see properly he had to hold the microscope so close to his face that often his eyelashes hit against it. “On the close inspection of three or four drops,” he wrote, “I may indeed expend so much labor that the sweat breaks out on me.”

  Other microscopists lacked Leeuwenhoek’s skill, and they lacked his microscopes, which were far better than anyone else’s. Beyond granting an occasional peek by way of demonstration, he would not let others use them. Nor would he divulge his viewing techniques or his manufacturing secrets. The result was that no one else could match his achievements, but no one else could verify them, either.

  Finally, around the year 1700, Leeuwenhoek gained some important allies. The spermist case took off, at least briefly. Nicolas Andry was a French doctor with a fine reputation and a much clearer prose style than Leeuwenhoek. His observations of “Spermatic Worms” confirmed Leeuwenhoek’s and, in places, went beyond them. “These Worms are not found before the Age proper for Generation. They are found dead or dying in old Men, and in those who have Gonorrhea or Venereal Distempers. What must we infer from those Circumstances? Does not the thing seem to speak of itself, and tell us plainly that Man, and all other Animals, come of a Worm?”

  Leeuwenhoek’s most eminent ally was Gottfried Leibniz, Newton’s great enemy. Leibniz was a philosopher and mathematician renowned for his astonishing intellectual range and power. One rival confessed that even the prospect of challenging Leibniz seemed pointless. Far better, he wrote, “to throw away one’s books and go die peacefully in the depths of some dark corner.” That judgment fit nicely with Leibniz’s opinion. A man of formidable vanity, his favorite gift to couples getting married was a collection of his own sayings. But he knew everyone, as well as everything, and he wrote endlessly. This was a good man to have on your side.

  Still, spermists faced a sea of troubles. Their run of good fortune lasted only from around 1700 to 1720. Otherwise, ovists dominated almost the whole of the 1700s. Strangely, much of this ascendance was not of their own doing. Though ovists happily pointed out weaknesses in the spermist case, the damage to the spermist side came mostly from unexpected directions.

  From first impressions, for one thing. Eggs conjured up cheery thoughts. Sperm conjured up worms. Nicholas Andry’s description of spermatozoa appeared not in a book on sex and conception but in a long volume on parasites entitled An Account of the Breeding of Worms in Human Bodies. By the time sperm cells made their appearance, readers had suffered through countless descriptions of a “Patient almost dead with Pain” who recovered after “a Worm came out of his right nostril, above a Span long” and “a Person, who voided Thirteen long worms alive and Woolly like a caterpillar, from his Nostrils, Ears, and Mouth.”

  And Andry was doing his earnest best to make the case for spermism. But even he admitted to shuddering a bit when he looked at sperm cells under a microscope. Dissect a testicle and take a close look at it, Andry advised, as he had done. “You shall discover in it such a hideous number of little worms that you shall hardly be able to believe your own eyes.”

  It was hard to imagine that so grand a subject as new life could have its origin in anything as vile as a sea of writhing worms. Man was made in the image of God, after all. From God forming a man of the dust of the ground to humankind wrapped up inside a worm was a mighty fall. It was bad enough that, in Shakespeare’s phrase, we end as “food for worms.” To say that we begin within a worm as well was too grim to contemplate. The notion seemed to make our lifetimes little more than a bizarre interlude. Hail humankind, the once and future worm!

  For Andry and his contemporaries, microscopic life horrified in two different ways at once. Blindly twisting mini-creatures were revolting in their own right, first of all, and they never appeared on their own but always in vast numbers, whic
h made matters even worse. To the naked eye, a dewdrop shimmering in the sunlight was an emblem of fragility and elegant design. But peek through the microscope. Thousands of living creatures jostled for space in every drop of water, in every fleck of blood, and, in particular, in every jot of semen. This was not exhilarating, a vision of life where there had been emptiness and sterility, but a glimpse of chaos and oppressive crowding where there had been pattern and order. Under the microscope’s lens, the world was transformed from a luxuriant park to a jammed and sweltering subway car.

  Making matters worse for the spermist camp, they could never quite decide what Leeuwenhoek’s tiny animals were. Their purposeful swimming, which was the very clue that made it seem likely they had something to do with generating new life, raised all sorts of perplexing questions. Leeuwenhoek and the other spermists took for granted that these “tiny animals” were indeed animals, but what kind? If they made their first appearance in a teenage boy, where had they been for the previous dozen years? Did these strange animals mate and reproduce? Most important, exactly what did it mean that each spermatozoa contained a human embryo inside it? Were sperm cells animals that became miniature human beings? Or animals that contained human beings? (For the ovists, matters were a bit simpler, because everyone knew that a living creature could develop inside an egg.)

  Leeuwenhoek believed he could see two different sorts of spermatozoa, which he took to be male and female. The two sorts did not mate with one another, he suggested, but the males gave rise to male babies and the females to female babies. Precisely how that happened was, for spermists, a profoundly puzzling question. The most notorious suggestion came in 1694 from a Dutch microscopist and mathematician named Nicolaas Hartsoeker. “Each little animal actually encloses and hides an even smaller being under a tender and delicate skin,” he wrote. Sex between a man and a woman serves as a kind of transport system for bringing that little animal together with an egg, which would nurture it. “Perhaps if we could see the small animal through the skin that hides it, we would see something as this figure represents.”