This did not prove that a living animal and a black lump were the same, but it did show that, as different as they might be, they both obeyed the same scientific laws. If there was a “vital force” that set life apart from other things, it was measurable, not mystical. Lavoisier had shown, in effect, that if witches do fly through the night, they obey traffic signals as they go.
That had been back in the 1780s. Six decades later, in 1848, one of the nineteenth century’s most eminent scientists, Hermann von Helmholtz, tightened the shackles on the life force even further. In the years since Lavoisier, scientists had found that his measurements were off a bit. Animals seemed to produce about 10 percent more heat than they should. All those scientists who believed that life could not be reduced to chemistry and physics cheered that discrepancy—that unexpected 10 percent is the vital force at work! But Helmholtz’s new experiments showed where the error had crept in and dashed the vitalists’ hopes.
This made for an odd impasse. Every time you tried to reach out and grab the life force in your hands, it vanished. Did that prove that the whole notion was misguided, a relic of earlier times like black magic and the evil eye? This was the view of hard-core mechanists like Helmholtz. But in the first half of the 1800s most scientists (and nearly all laymen) disagreed. Plainly something accounted for the difference between a living creature and the ground under its feet.
F. Scott Fitzgerald famously suggested that the test of a first-rate mind is the ability to believe two contrary ideas at once and still carry on. For nineteenth-century biologists, the challenge was almost the reverse. They had to proceed onward without losing heart while holding only half of a clear idea in mind. Life has something distinctive about it, and we only wish we knew what that could be.
Throughout the years that the What does it mean to be alive? camp had occupied themselves studying powdered nose-of-sphinx and watching guinea pigs shiver in icy containers, the Where do babies come from? faction had stuck close to the mysteries of sperm and egg.
They had found something remarkable.
TWENTY-FOUR
“THE GAME IS AFOOT”
IN THE 1820S, WHILE ALL EUROPE WAS HAPPILY SETTLING IN WITH Frankenstein for a night’s spooky reading by the fire, two young biologists took on the riddle of life from a different angle. Fifty years had passed since Lazzaro Spallanzani had dressed his frogs in boxer shorts. In all those years scientists’ understanding of conception had scarcely advanced.
In particular, biologists still echoed Spallanzani’s two best-known claims. First, when it came to fertilization, semen and egg had to make physical contact. “Nearby” was not good enough, and “auras” and “emanations” were fictions. Second, spermatozoa were parasites that had nothing to do with fertilization. The first was correct, the second wildly wrong.
The explanation for the long lull was simple enough: the riddles of sex and development in particular, and the mystery of life in general, seemed so daunting that no one knew how to take them on. Across Europe, scientists had responded to that predicament in distinctive ways. In Germany, where the Romantic movement had taken hold, they veered toward the mystical, with grand-sounding theories about life’s relentless impulse to strive ever upward. In England, scientists had virtually abandoned biology, which had been left to amateurs who puttered away with plants and pigeons. In France more than anywhere else, the experimental tradition had endured.
But no one was hopeful. The English physician and scholar Peter Roget* summed up the prevailing view. The mystery of where babies come from, he wrote in 1834, “surpasses the utmost powers of the human comprehension.” Science could not offer “the least clue” toward unraveling “this dark and hopeless enigma.”
A decade before, two young colleagues had in fact unearthed some considerable clues, although neither Roget nor anyone else at the time paid much heed. In three papers in 1824, Jean-Louis Prévost and Jean-Baptiste Dumas looked again at Spallanzani’s work from so many years before. Dumas was French and still in his twenties, Prévost Swiss and a few years older. They began by studying semen from all sorts of animals. Whether they examined mammals, birds, or fish, they always found spermatozoa. And when they looked at infertile animals like mules and hinnies (the offspring of a male donkey and a female horse, or vice versa), they found no spermatozoa.
So far, this was simply endorsing Leeuwenhoek and Spallanzani. But now the two scientists began a series of experiments that moved beyond the work of their famous predecessors. First they put a batch of frog eggs into plain water and another batch into water with semen stirred into it. As expected, only the eggs in the water-with-semen developed. Then came the key. Spallanzani had dismissed spermatozoa as parasites. Was he correct that the spermatozoa played no role in fertilization?
Prévost and Dumas dried out a sample of semen, so that it contained no moving spermatozoa. Then they dissolved it in water. Would that semen still have the power to fertilize frog eggs? The answer was an emphatic no. Suppose you zapped a semen sample with electricity, thus killing the spermatozoa. Was that semen potent? Again, no. If you had no swimming sperm cells, you had no fertilized eggs.
Next the two redid Spallanzani’s filter paper experiments. After passing a semen sample through five layers of filter paper, it no longer contained any spermatozoa, and it could not fertilize frog eggs. But if you took the blob left on the paper and dissolved it in water, it did contain spermatozoa, and it did fertilize eggs.
This was not proof that spermatozoa were the male’s crucial contribution to fertilization, but it surely pointed that way. Where you had spermatozoa, you had fertilization. Where you had none, you didn’t.
Prévost and Dumas shifted their focus from sperm to egg, and from frogs to dogs and rabbits. Here, too, they came tantalizingly close to an enormous breakthrough. They almost, but not quite, identified a mammalian egg. No one had ever done so. Within what are now called the Graafian follicles in the ovaries, they found tiny, oval structures hiding under the surface of about-to-rupture follicles. They suspected, correctly, that these were eggs, but they could not demonstrate that this was so. (They would have had somehow to label these eggs and follow them on their journey to the womb.)
The world of science continued on its way, unperturbed. Spallanzani’s reputation was too high, and the notion that spermatozoa were parasites too entrenched, for these new findings to make a splash. Worse still, to focus on spermatozoa was to endorse a doctrine that had been cast aside decades and decades before. What modern thinker wanted to return to Leeuwenhoek and his “animalcules”?
The bigger problem was that biologists in this era had gathered a giant heap of facts, but they had not begun to create a framework that made sense of them. Instead, they labored away like magpies collecting keys and rings and other shiny objects. Outsiders had mocked these diligent, misguided efforts for nearly a century. Biologists might “take pleasure in boring us with all the wonders of nature,” one physician-turned-philosopher had complained in the mid-1700s, but they needed to move beyond “counting little bones in certain fishes” and “measuring how far a flea can jump.”
Skeptics delighted in pointing out that physicists had moved beyond lists and catalogs long before, with their discovery of a handful of laws that applied universally. Rocks fell, and arrows fell, and the moon fell, and they all fell in precisely the same way. The Earth spun and so did skaters and wooden tops, and the identical rules applied to all of them. One all-embracing force pulled the planets toward the sun and a baby’s rattle to the ground.
But biologists, and thinkers generally, had nearly given up hope of finding any such unifying laws for the living world. “Purpose” and “urges” and “drives” were the essence of life, and they could never be explained in purely physical terms. Physicists had a more manageable task. To ask, What are stars for? seemed silly. Stars weren’t for anything. Neither were rainbows or rocks or any of the inanimate features of the world. They happened to turn up when the conditions were right.
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br /> When it came to biology, the picture looked completely different. To ask What are eyes for? seemed not merely sensible but essential. Eyes are for seeing, for finding one’s way in a dangerous and complicated world. How could you possibly talk about eyes without starting there? Rocks just fall; they don’t plummet through the air because they’re in despair or trying to impress their friends. But nothing alive “just” happens. Even a dog scrounging through the trash or a cat chasing a mouse has a reason for its actions.
And the elaborate structures in the living world surely did not just happen to arise. How could you possibly talk about brains and bones and roots and flowers without asking how they had come to be and what they were for? Such questions were unavoidable, but they seemed doomed to go unsolved.
Biologists had set out eagerly in pursuit of an irresistible question—How do living creatures move and reproduce?—and they had found themselves stymied and helpless. In the pursuit of life, science could see only so far, even in the simplest cases. “There will never be a Newton for a blade of grass,” Immanuel Kant declared, in 1790.
For decades, Kant seemed to have it right. Then the world shifted.
PRÉVOST AND DUMAS DESERVE A MEASURE OF THE CREDIT. Though largely passed over, they had spurred a few scientists to look anew at old questions. One was a young biologist named Karl Ernst von Baer, born in Estonia but working at the University of Königsberg in Germany. (By happenstance, this had been Kant’s university.) In 1827, Baer became the first person to see a mammalian egg.
Baer was no Newton, but he did share the English scientist’s faith that straightforward experiments trumped high-flown talk. To ask, What is light? was to invite endless prattle. The way forward, Newton had demonstrated, was to hold a prism up to a beam of sunlight and unweave the rainbow. Baer’s question was, What is life? He tackled it by searching inside a dog’s ovaries, as Prévost and Dumas had.
“When I observed the ovary,” he wrote, “… I discovered a small yellow spot in a little sac. Then I saw these same spots in several others, and indeed in most of them—always in just one little spot. How strange, I thought, what could it be?”
“I opened one of these little sacs,” Baer continued, “lifting it carefully with a knife onto a watchglass filled with water, and put it under the microscope. I shrank back as if struck by lightning, for I clearly saw a minuscule and well-developed yellow sphere of yolk. Before I found courage to look at it a second time, I had to recover, since I was afraid of having been deluded by a phantom. It seems odd that a sight expected and so much longed for could frighten one when it actually occurs.”
Baer went on to study eggs of all sorts, from crayfish, birds, frogs, lizards, snakes, and, notably, a host of mammals including rabbits, pigs, cows, hedgehogs, and, especially, human beings. He drew a simple conclusion: “Every animal which is begotten by a sexual union develops out of an egg.”
This was an insight that had been a long time coming. And this time—in contrast with Harvey’s pronouncement in 1651 that “everything comes from the egg”—it was a conclusion based not on argument and analogy but on evidence and observation. It had taken so long mainly because mammalian eggs were too tiny and well-hidden to find with the naked eye or with early microscopes.
But Baer had overcome psychological hurdles, too. Seventy-five years before Baer’s lightning bolt, back in 1752, the great Swiss anatomist Albrecht von Haller had not only given up the egg hunt after a long, futile search but effectively scared away everyone else from hunting, too.
Haller had dissected forty ewes soon after they had mated, much as Harvey had dissected deer. Like Harvey, too, Haller found no signs of an egg or anything else in his cut-open animals. Finally, two weeks after the sheep had mated, Haller found a tiny embryo in the womb. His conclusion—which became the conventional wisdom taught to generations of young students, including Baer—was that the supposed egg that emerged from the ovaries was in truth a fluid that “curdled” in the womb and formed the embryo.
Where Haller had lost his way, few others dared venture. Baer did. He announced his findings in a paper with a grand but awkward title, “On the Genesis of the Egg of Mammals and of Man.” The title was redundant, since humans were surely mammals, but Baer wanted to leave no room for doubt about what he had found.
His reward was meager, at least at first. The world responded to his breakthrough, Baer reported forlornly, with “deep silence.” At a meeting of the Society of Naturalists, in Berlin in September 1828, no one mentioned his paper. Unwilling to raise the topic himself, Baer let it go. On the last day of the meeting, one scientist finally asked Baer a casual question.
In any case, Baer had seen only half the puzzle. He continued to insist—despite the work of Prévost and Dumas—that spermatozoa were parasites and irrelevant to the mystery of fertilization. (It was Baer who came up with the name “spermatozoa,” meaning “animals of the semen.”)
The breakthrough came a decade later, with the realization that spermatozoa were not animals but something quite different.
TWENTY-FIVE
CAUGHT!
ON AN OCTOBER EVENING IN 1837, A BIOLOGIST NAMED THEODOR Schwann and a lawyer-turned-botanist named Matthias Schleiden met over dinner to discuss their work. Both men were brilliant and high-strung. Schleiden, who suffered from depression, had changed careers after a failed suicide attempt. Schwann would suffer a religious crisis in 1838 and abandon his research career soon after. Friends since their student days in Berlin a decade before, the two chatted away excitedly.
Schleiden reported striking news: after endless hours staring at plants under a microscope, he had found the secret of their structure. No matter what species you looked at, plants were formed of cells—countless distinct units arranged just so. Schleiden had been following up observations by the botanist Robert Brown—this was the Brown of “Brownian motion” and the Sphinx’s nose—who had noted that when he looked at orchids under the microscope, he found cell-like structures. More than that, each cell contained a round structure that Brown had dubbed a “nucleus.”
Schwann did a double take. He had been studying animals, not plants, but he had seen dark spots in different tissues. Could those spots be nuclei? The two men left their coffee half-finished and rushed off to Schwann’s laboratory to look at his slides.
Schleiden published first. All plants are formed of cells. Schwann was close behind. All animals are formed of cells. This was the cell theory in a nutshell, and at last biology had found its fundamental law. Material objects are built of atoms; plants and animals are built of cells. It seemed likely, and soon it would be proved, that each of those tiny cells was as complex and as busy as a crammed, whirring factory. The key to life was not a vital force, which had been pursued for millennia and never seen. Cells were the hallmark of life.
One last, crucial insight came two decades later. “All cells come from cells.” So declared a German physician named Rudolf Virchow, in 1858, and now all the building blocks of the theory of building blocks stood in place. In particular, the nature of sperm and egg finally came into focus. If spermatozoa were in fact sperm cells and the egg was a cell, too, then these two mysterious structures were finally on a par. The Hundred Years War over which one was truly important—and this war, like the real one, had actually lasted more than a hundred years—could end in a peace treaty that gave equal weight to both sides.
Better still, if Virchow was right that “all developed tissues can be traced back to a cell,” then conception finally made sense. It was not, as the ovists had insisted, that the embryo was hidden inside the egg, and semen merely spurred the egg to action. Nor was it, as the spermists claimed, that the embryo was hidden inside the spermatozoa, and the egg simply provided food to nourish it. Instead, the true picture must be that sperm cell and egg cell somehow merged into a single, new cell, which in turn divided and grew, as did its descendants and countless more generations of descendants, until a tiny embryo became a complex, multicellular, l
iving being.
THESE THREE YOUNG GERMANS WERE ONLY THE MOST PROMINENT among a host of colleagues and rivals, but the cell theory they helped shape in the mid-1800s is still taught in the opening days of every biology class today. Insight might have come far sooner, but happenstance and bad fortune conspired to trip up all the early investigators. The main problem was that microscopes weren’t up to the task. Robert Hooke had coined the term “cell” as far back as 1665, when he had examined a slice of cork and seen “a great many little Boxes” that reminded him of the cells in a beehive. Gazing at those empty boxes, Hooke saw why cork was so light in weight, but he was looking at a slice of dead wood, and he did not guess that a living cell is actually a frenetic workplace and not merely a geometric shape.
All through the 1700s, microscopes remained crude and difficult to use (and, as we have seen, unpopular). On top of that, scientists had focused their gaze in the wrong direction. Plant cells are easier to see than animal cells because plants have thick, sturdy cell walls, while animals have flimsier membranes; roughly speaking, the difference is that between a cardboard box and a plastic bag. But in the 1700s, almost no one was staring at plants under a microscope. Botanists spent their time on classification, not microscopy, and most scientists preferred working on animals in any case. Finally, in the early decades of the 1800s, microscopes improved, and plant biology came back into vogue.
The Seeds of Life Page 24