But now here was Darwin, without any evidence, insisting that the earlier seasmust have had abundant life and that we just hadn’t found it yet because, for whatever reason, it hadn’t been preserved. It simply could not be otherwise, Darwin maintained. “The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained,” he allowed most candidly, but he refused to entertain an alternative possibility. By way of explanation he speculated—inventively but incorrectly—that perhaps the Precambrian seas had been too clear to lay down sediments and thus had preserved no fossils.

Even Darwin’s closest friends were troubled by the blitheness of some of his assertions. Adam Sedgwick, who had taught Darwin at Cambridge and taken him on a geological tour of Wales in 1831, said the book gave him “more pain than pleasure.” Louis Agassiz dismissed it as poor conjecture. Even Lyell concluded gloomily: “Darwin goes too far.”

T. H. Huxley disliked Darwin’s insistence on huge amounts of geological time because he was a saltationist, which is to say a believer in the idea that evolutionary changes happen not gradually but suddenly. Saltationists (the word comes from the Latin for “leap”) couldn’t accept that complicated organs could ever emerge in slow stages. What good, after all, is one-tenth of a wing or half an eye? Such organs, they thought, only made sense if they appeared in a finished state.

The belief was surprising in as radical a spirit as Huxley because it closely recalled a very conservative religious notion first put forward by the English theologian William Paley in 1802 and known as argument from design. Paley contended that if you found a pocket watch on the ground, even if you had never seen such a thing before, you would instantly perceive that it had been made by an intelligent entity. So it was, he believed, with nature: its complexity was proof of its design. The notion was a powerful one in the nineteenth century, and it gave Darwin trouble too. “The eye to this day gives me a cold shudder,” he acknowledged in a letter to a friend. In theOrigin he conceded that it “seems, I freely confess, absurd in the highest possible degree” that natural selection could produce such an instrument in gradual steps.

Even so, and to the unending exasperation of his supporters, Darwin not only insisted that all change was gradual, but in nearly every edition ofOrigin he stepped up the amount of time he supposed necessary to allow evolution to progress, which pushed his ideas increasingly out of favor. “Eventually,” according to the scientist and historian Jeffrey Schwartz, “Darwin lost virtually all the support that still remained among the ranks of fellow natural historians and geologists.”

Ironically, considering that Darwin called his bookOn the Origin of Species , the one thing he couldn’t explain was how species originated. Darwin’s theory suggested a mechanism for how a species might become stronger or better or faster—in a word, fitter—but gave no indication of how it might throw up a new species. A Scottish engineer, Fleeming Jenkin, considered the problem and noted an important flaw in Darwin’s argument. Darwin believed that any beneficial trait that arose in one generation would be passed on to subsequent generations, thus strengthening the species.

Jenkin pointed out that a favorable trait in one parent wouldn’t become dominant in succeeding generations, but in fact would be diluted through blending. If you pour whiskey into a tumbler of water, you don’t make the whiskey stronger, you make it weaker. And if you pour that dilute solution into another glass of water, it becomes weaker still. In the same way, any favorable trait introduced by one parent would be successively watered down by subsequent matings until it ceased to be apparent at all. Thus Darwin’s theory was not a recipe for change, but for constancy. Lucky flukes might arise from time to time, but they would soon vanish under the general impulse to bring everything back to a stable mediocrity. If natural selection were to work, some alternative, unconsidered mechanism was required.

Unknown to Darwin and everyone else, eight hundred miles away in a tranquil corner of Middle Europe a retiring monk named Gregor Mendel was coming up with the solution.

Mendel was born in 1822 to a humble farming family in a backwater of the Austrian empire in what is now the Czech Republic. Schoolbooks once portrayed him as a simple but observant provincial monk whose discoveries were largely serendipitous—the result of noticing some interesting traits of inheritance while pottering about with pea plants in the monastery’s kitchen garden. In fact, Mendel was a trained scientist—he had studied physics and mathematics at the Olmütz Philosophical Institute and the University of Vienna—and he brought scientific discipline to all he did. Moreover, the monastery at Brno where he lived from 1843 was known as a learned institution. It had a library of twenty thousand books and a tradition of careful scientific investigation.

Before embarking on his experiments, Mendel spent two years preparing his control specimens, seven varieties of pea, to make sure they bred true. Then, helped by two full-time assistants, he repeatedly bred and crossbred hybrids from thirty thousand pea plants. It was delicate work, requiring them to take the most exacting pains to avoid accidental cross-fertilization and to note every slight variation in the growth and appearance of seeds, pods, leaves, stems, and flowers. Mendel knew what he was doing.

He never used the wordgene —it wasn’t coined until 1913, in an English medical dictionary—though he did invent the termsdominant andrecessive . What he established was that every seed contained two “factors” or “elemente,” as he called them—a dominant one and a recessive one—and these factors, when combined, produced predictable patterns of inheritance.

The results he converted into precise mathematical formulae. Altogether Mendel spent eight years on the experiments, then confirmed his results with similar experiments on flowers, corn, and other plants. If anything, Mendel wastoo scientific in his approach, for when he presented his findings at the February and March meetings of the Natural History Society of Brno in 1865, the audience of about forty listened politely but was conspicuously unmoved, even though the breeding of plants was a matter of great practical interest to many of the members.

When Mendel’s report was published, he eagerly sent a copy to the great Swiss botanist Karl-Wilhelm von Nägeli, whose support was more or less vital for the theory’s prospects. Unfortunately, Nägeli failed to perceive the importance of what Mendel had found. He suggested that Mendel try breeding hawkweed. Mendel obediently did as Nägeli suggested, but quickly realized that hawkweed had none of the requisite features for studying heritability. It was evident to him that Nägeli had not read the paper closely, or possibly at all. Frustrated, Mendel retired from investigating heritability and spent the rest of his life growing outstanding vegetables and studying bees, mice, and sunspots, among much else. Eventually he was made abbot.

Mendel’s findings weren’t quite as widely ignored as is sometimes suggested. His study received a glowing entry in theEncyclopaedia Britannica —then a more leading record of scientific thought than now—and was cited repeatedly in an important paper by the German Wilhelm Olbers Focke. Indeed, it was because Mendel’s ideas never entirely sank below the waterline of scientific thought that they were so easily recovered when the world was ready for them.

Together, without realizing it, Darwin and Mendel laid the groundwork for all of life sciences in the twentieth century. Darwin saw that all living things are connected, that ultimately they “trace their ancestry to a single, common source,” while Mendel’s work provided the mechanism to explain how that could happen. The two men could easily have helped each other. Mendel owned a German edition of theOrigin of Species , which he is known to have read, so he must have realized the applicability of his work to Darwin’s, yet he appears to have made no effort to get in touch. And Darwin for his part is known to have studied Focke’s influential paper with its repeated references to Mendel’s work, but didn’t connect them to his own studies.

The one thing everyone thinks featured in Darwin’s argument, that humans are descended from apes, didn’t feature at all except as one passing allusion. Even so, it took no great leap of imagination to see the implications for human development in Darwin’s theories, and it became an immediate talking point.

The showdown came on Saturday, June 30, 1860, at a meeting of the British Association for the Advancement of Science in Oxford. Huxley had been urged to attend by Robert Chambers, author ofVestiges of the Natural History of Creation , though he was still unaware of Chambers’s connection to that contentious tome. Darwin, as ever, was absent. The meeting was held at the Oxford Zoological Museum. More than a thousand people crowded into the chamber; hundreds more were turned away. People knew that something big was going to happen, though they had first to wait while a slumber-inducing speaker named John William Draper of New York University bravely slogged his way through two hours of introductory remarks on “The Intellectual Development of Europe Considered with Reference to the Views of Mr. Darwin.”

Finally, the Bishop of Oxford, Samuel Wilberforce, rose to speak. Wilberforce had been briefed (or so it is generally assumed) by the ardent anti-Darwinian Richard Owen, who had been a guest in his home the night before. As nearly always with events that end in uproar, accounts vary widely on what exactly transpired. In the most popular version, Wilberforce, when properly in flow, turned to Huxley with a dry smile and demanded of him whether he claimed attachment to the apes by way of his grandmother or grandfather. The remark was doubtless intended as a quip, but it came across as an icy challenge. According to his own account, Huxley turned to his neighbor and whispered, “The Lord hath delivered him into my hands,” then rose with a certain relish.

Others, however, recalled a Huxley trembling with fury and indignation. At all events, Huxley declared that he would rather claim kinship to an ape than to someone who used his eminence to propound uninformed twaddle in what was supposed to be a serious scientific forum. Such a riposte was a scandalous impertinence, as well as an insult to Wilberforce’s office, and the proceedings instantly collapsed in tumult. A Lady Brewster fainted. Robert FitzRoy, Darwin’s companion on theBeagle twenty-five years before, wandered through the hall with a Bible held aloft, shouting, “The Book, the Book.” (He was at the conference to present a paper on storms in his capacity as head of the newly created Meteorological Department.) Interestingly, each side afterward claimed to have routed the other.

Darwin did eventually make his belief in our kinship with the apes explicit inThe Descent of Man in 1871. The conclusion was a bold one since nothing in the fossil record supported such a notion. The only known early human remains of that time were the famous Neandertal bones from Germany and a few uncertain fragments of jawbones, and many respected authorities refused to believe even in their antiquity.The Descent of Man was altogether a more controversial book, but by the time of its appearance the world had grown less excitable and its arguments caused much less of a stir.

For the most part, however, Darwin passed his twilight years with other projects, most of which touched only tangentially on questions of natural selection. He spent amazingly long periods picking through bird droppings, scrutinizing the contents in an attempt to understand how seeds spread between continents, and spent years more studying the behavior of worms. One of his experiments was to play the piano to them, not to amuse them but to study the effects on them of sound and vibration. He was the first to realize how vitally important worms are to soil fertility. “It may be doubted whether there are many other animals which have played so important a part in the history of the world,” he wrote in his masterwork on the subject,The Formation of Vegetable Mould Through the Action of Worms (1881), which was actually more popular thanOn the Origin of Specieshad ever been. Among his other books wereOn the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects (1862),Expressions of the Emotions in Man and Animals (1872), which sold almost 5,300 copies on its first day,The Effects of Cross and Self Fertilization in the Vegetable Kingdom (1876)—a subject that came improbably close to Mendel’s own work, without attaining anything like the same insights—and his last book,The Power of Movement in Plants . Finally, but not least, he devoted much effort to studying the consequences of inbreeding—a matter of private interest to him. Having married his own cousin, Darwin glumly suspected that certain physical and mental frailties among his children arose from a lack of diversity in his family tree.

Darwin was often honored in his lifetime, but never forOn the Origin of Species orDescent of Man. When the Royal Society bestowed on him the prestigious Copley Medal it was for his geology, zoology, and botany, not evolutionary theories, and the Linnaean Society was similarly pleased to honor Darwin without embracing his radical notions. He was never knighted, though he was buried in Westminster Abbey—next to Newton. He died at Down in April 1882. Mendel died two years later.

Darwin’s theory didn’t really gain widespread acceptance until the 1930s and 1940s, with the advance of a refined theory called, with a certain hauteur, the Modern Synthesis, combining Darwin’s ideas with those of Mendel and others. For Mendel, appreciation was also posthumous, though it came somewhat sooner. In 1900, three scientists working separately in Europe rediscovered Mendel’s work more or less simultaneously. It was only because one of them, a Dutchman named Hugo de Vries, seemed set to claim Mendel’s insights as his own that a rival made it noisily clear that the credit really lay with the forgotten monk.

The world was almost ready, but not quite, to begin to understand how we got here—how we made each other. It is fairly amazing to reflect that at the beginning of the twentieth century, and for some years beyond, the best scientific minds in the world couldn’t actually tell you where babies came from.

And these, you may recall, were men who thought science was nearly at an end.