Editor's Note: This article originally appeared in Spektrum, and has been translated from German. We are publishing it as part of our tribute to Charles Darwin on his 200th birthday.

Before marriage, Charles Darwin had confessed everything to her. That he was in the process of rewriting the history of life. That, according to his convictions, all living things descended from a common ancestor. And that species were not to be attributed to God's endless creativity, but were the product of a blind, mechanical process that altered them over the course of millions of years. This alone was pure heresy. Darwin even nursed doubts about the very survival of human beings.

And this man, who had gone around the world once, and was going to marry Emma Wedgwood, did not believe a single word of the biblical story of creation. "Reason tells me that honest and conscientious doubts cannot be a sin," wrote the deeply religious Emma to her betrothed in a cautioning letter in November 1838. "But I felt that it would be a painful rift between us." Charles was supposed to find his way back to the right faith by reading the Bible: "I implore you to read the parting words of our Savior to his apostles, beginning at the end of the 13th chapter of the Gospel according to John," she wrote.

But for Charles Darwin there was no turning back. He definitely assured Emma in his reply that he would take her concern seriously. But in fact he was experimenting at that time with all kinds of heretic theories. "Love of godhood is a result of intellectual organization, oh you materialist!" he confided to himself in revolutionary words in his secret notebook. And although his theories were not yet mature, he was completely aware of their explosive nature: By dissociating intellect and morality from god's power of creation, and attributing them instead to self-evolving forces, Darwin undermined the very foundations of a society shaped by the Anglican Church, with its hopes of eternal life and the omnipresent threat of punishment.

"As soon you realize that one species could evolve into another, the whole structure wobbles and collapses," he remarked. And if man were nothing but a superior animal, where would that leave his spiritual dignity? And if he himself is the product of evolution, then what about his moral accountability to God?

Believe only what is proved
"Charles' confession was a big shock for Emma," explains the science historian John van Wyhe from the University of Cambridge. "On the other hand, he impressed her with his openness and honesty. Nothing would have hurt her more badly than the feeling that her future husband was keeping secrets from her." But Emma's worries over the well-being of Charles's soul could not come in the way of her wedding at the end of January 1839. His habit of "never believing anything till it is proven" had apparently prevented him from "taking into account other things that cannot be proved in the same manner, and which, if they are true, would probably go far beyond our power of imagination," she complained in another letter. Emma's worst fear was that Charles was forfeiting his salvation through his religious disbelief, and this threatened to separate them in death.

Her letter was to go unanswered. "Charles respected Emma's faith and probably kept his religious doubts to himself," van Wyhe says. The man from the English town of Shrewsbury, northwest of Birmingham, had drawn his theories from it. His wife's reactions had shown him how difficult it was to convince other people of his ideas: The criticism would be devastating were he to publish his theories without adequate proof; and his scientific career would be ruined.

If he wanted his theories to be accepted, he would have to leave the tricky "issue of apes" on the periphery and write only about how oranges or animals changed gradually. And he would need collaborators, respected naturalists who would stand by his side—scholars like Charles Lyell, whose Principles of Geology had given him important intellectual stimuli.

He had collected enough facts. Charles Darwin had spent five long years in the remotest corners of the Earth and observed, described and dissected with inexhaustible meticulousness. But his proud booty was to reveal its secrets only gradually; small bits of a puzzle that fell into place slowly, forming a larger, overall picture, and gave shape gradually to his theory of the evolution of species. "The journey on the Beagle was, by far, the most important event in my life, and shaped my whole career," Darwin once said, looking back on his time  on board the ship.

In August 1831 the British Admiralty was looking for a young gentleman to provide company for Capt. Robert Fitzroy, a small, dark haired man with fine features and an aristocratic arrogance during his long mission. The HMS Beagle was supposed to chart the South American coast on its survey journey. "You are exactly the person they are looking for," the old botany professor John Stevens Henslow wrote to his former student, the 22-year-old Darwin. Fitzroy wanted a natural scientist as companion, because this would mean unprecedented opportunities for him to engage in research on the extended stopovers on land. The ship was equipped for scientific research; a man of "commitment and intelligence could do wonders," Henslow gushed.

Darwin was indeed not a full-fledged natural scientist, but he could still make up for this deficit by taking along some books. The young man plunged into the preparations as if electrified, and in all haste: the journey was to begin soon. It was the chance of a lifetime. But sober reality didn't dawn on him until after the Beagle set out from Plymouth, shortly after Christmas in 1831.

If only he had listened to his father! Robert Waring Darwin had been against Charles's feverishly embraced the adventure, right from the start. Another of these useless ideas that had gotten into this fickle son's head—further proof of his aimlessness. The aspiring natural scientist had abandoned his study of medicine, and the homeless years in the company of rough seamen were going to ruin him completely. It was only the appeal of an uncle that persuaded the elder Darwin to consent. But after just three days on rough seas, the aspiring natural scientist was already yearning for the soft meadows of his Shropshire home, on the Wales border. Even a lonely parish in the country would have been an utterly welcome prospect to him now: firm land beneath the feet, above all!

All day he could not hold down anything but rusks (breads) and raisins or, if even these repulsed him, glögg (spiced wine), offset with sago. And when he tried to stand up in his tiny cabin, he almost knocked himself out cold. From the deck, he could hear the shrill voices of four crewmen, who were being punished by Fitzroy with a total of 134 lashes for their Christmas escapades.

"Before the journey, I used to say often that I was certainly going to thoroughly regret the whole enterprise," he wrote in his dairy that day. "But I had never thought how vehemently I was going to do so."

Fitzroy, who swore by physiognomy (judging character based on outward appearance), had also known it all along: Darwin's nose pointed to a deficiency in energy and resoluteness; his knowledge of people indicated that this young gentleman was never going to make it till the end of the journey.

Chaos of rapture
And how wrong he was! Once the Beagle reached the South American coast on the 28th of February 1832, Darwin became enraptured by what seemed to him to be paradise. While the crew chartered and plotted the harbor of Salvador in the Baía de Todos os Santos (the Bay of All Saints), Darwin wandered in astonishment through the Brazilian rainforest, caught in the "chaos of rapture," completely bewitched by its wealth of vegetation. "The landscape in Brazil is like looking into a Thousand and One Nights, with the advantage that this is real," he wrote in his diary.

Resting in a shaded spot, he listened to the humming, squeaking and pulsating life around him. He heard parakeets screeching, saw hitherto unimagined varieties of orchids and anthills standing more than 10 feet (three meters) tall. Not a single bizarre detail seemed to escape the young researcher: He once discovered a spider that preyed on alien webs and played dead and fell down if it sensed danger. Then he came across a wasp that stung caterpillars and used them as food for its own larvae in its nest of loam. "And imagine," he cried out after having shot a particularly magnificent lizard, "calling a pleasure such as this duty!" but the journey had some more surprises waiting.

In the bay of Punta Alta in Argentina, Darwin chiseled out the fossilized bones of a colossal protozoan from the cliffside. Beside himself with joy, he lugged the valuable find on board the Beagle. The booty, scorned by Captain Fitzroy as "a box full of useless stuff," was to make him famous later. There was only one comparable specimen in England at that time. When he returned to the site a few months later, he was able to free from the cliff almost the complete skeleton of a bizarre mammal, the size of a horse, with an enormous pelvis and a narrow, long face, resembling that of an anteater. "Formerly, this place must have been teeming with large monsters", he recorded later in his travel log. But why did they die out? And why were the extinct giants so similar to the animals now found in South America, except for their size?

Darwin began to ask questions. The gauchos told him about a unique variety of a South American bird called a rhea, smaller and darker than usual in form. Very few had seen one, but their nests had been found, and everyone confirmed that it was found more frequently farther south. After a long search, he found the unique creature: on his plate for dinner! Incredible: he had finally found the unique bird and almost eaten it inadvertently! Fortunately, it was still possible to save the "head, neck, legs and one wing," and some large feathers; they were conserved promptly and stacked away in the hold. Why is a type of ostrich found only in North Patagonia, and the others are found only in the south? Why did the Almighty have to create two such closely related species, whose environments hardly differed?

In the beginning of the year 1835 the Beagle reached the coast of Chile. After a morning ramble through nature, Darwin was stretched on the ground when it began to shake. "The Earth—the epitome of firmness," the natural scientist wrote, trembled under his feet "like the crust on a liquid."

It was only in the following days that the terrifying dimension of the catastrophe that lasted about two minutes, became evident to Darwin as the Beagle sailed up the long Chilean coast. "The entire coast was strewn with balconies and household objects, as if a thousand ships had been stranded", he reported. The city of Concepción at the foot of the Andes offered a terrible scene: "The ruins were so scattered and the whole scene had so little of anything akin to a habitable place, that it was barely possible to still imagine the earlier state." The inhabitants spoke of the worst earthquake mankind had ever known. The shock waves had reached Concepción, "rumbling like distant thunder"; fires had broken out everywhere. Those who had managed to salvage their material possessions were living in fear of plunderers. And then the wave came: a tsunami, taller than 20 feet (six meters), broke over the city. Innumerable people drowned or were washed away.

Once he had recovered from the initial shock, the young researcher went looking for the cause of the quake. The local people along the coast told him about a shallow edged by cliffs, which was earlier covered completely by water, but had become exposed after the earthquake. And on the island of Santa Maria, barely 30 miles (50 kilometers) or so away, he came across fresh banks of mollusks just above the flood line, which had already begun to rot away. The land must have heaved just a few meters away! That was the unequivocal evidence for the hypotheses postulated by Charles Lyell in his "Principles of Geology": mountains such as the Andes had not been formed in one colossal upheaval, but grew, barely perceptibly, over the course of millions of years, as the result of countless small quakes, to which Darwin had just been a witness. But had not the Archbishop of Armagh, James Ussher, calculated in 1658 the Earth's age accurate to the day? Accordingly, god must have created our planet on the night preceding October 23, 4004 B.C. on the Julian calendar.

Cultivated part of hell
Toward the middle of the year the Beagle left behind the South American continent and set sail for the Galápagos islands, where the crew came upon a cheerless scenario: "A jagged field of [irregular or wavelike layers of] black, basaltic lava pockmarked with huge fissures, and covered everywhere with stunted, sun-burnt brushwood," Darwin complained in his report. The land, overheated by the midday sun, lent the sweltering air a closed and oppressive feeling, like an oven; and it smelled very unpleasant. Countless crabs and iguanas ran helter-skelter in all directions as the new arrivals clambered from cliff to cliff, "like one might imagine the cultivated part of hell," Darwin wrote. The birds were not afraid of human beings and were very tame; where was the joy of hunting, then?

Conscious of duty, he added the animals to his collection. He thought he had collected wrens, finches, black and spotted thrushes. But the forms of the beaks puzzled him: some were thick and strong, like those of the grosbeaks, others, on the contrary, were thin like those of songbirds. But he did not stop to figure out which bird came from which island. It was too late when Darwin realised that he had missed an opportunity. Shortly after departure, the vice governor of the English penal colony at the Galápagos Islands told him that each of the colossal turtles that were native to these islands could be assigned to its respective island of origin, based on the appearance of its shell. In other words, the turtles of those islands were unique variants, perhaps even separate species; Darwin had already suspected something similar for the plants. Could it possibly be true for the birds as well? It was no longer possible to discover the truth, because his specimens were not adequately labeled and the Beagle was already on its way home across the Pacific.

On October 1836 the ship reached England. Barely had he touched shore when Darwin handed over the birds from the Galápagos to the renowned ornithologist John Gould. The latter was not too bothered about how the bills had evolved on the birds. In the case of the spotted thrushes, Darwin had suspected that they were distinct varieties (ranks below that of a species). Gould, however, found that these were in fact three new species, closely related to the species that are native to the South American continent. Darwin had made another mistake: Gould recognised that what were supposed to be black thrushes and wrens were also types of finches. They were so unique that he later put them under a new group of finches that consisted of 14 species, each of which had its own ecological niche on the Galápagos. Was it possible that something similar applied to the species Darwin had originally classified as finches as well? Darwin contacted Captain Fitzroy, whose crew members had put together their own collections, and had been more conscientious in labeling them. And indeed, like in the case of the thrushes, every island had its own species of finch! Had God created separate kinds of birds for each island? Darwin had his doubts.

In his notebook, he speculated on the uniqueness of animals: Darwin's finches now no longer lived in the 6,000-year-old world created by God in seven days, but on an archipelago that must have risen, not too long ago, at least in geologic terms, from the Pacific.

Once they had appeared, birds from the South American continent could have reached the group of islands. Over generations, the animals changed and adapted themselves to their respective environments, finding their way into as yet unoccupied ecological niches.

In his notebook, he drew a branched genealogical tree showing how old species gradually evolve into new ones, or else they would die out, like the large mammals that Darwin had chiseled out of the stone in Patagonia. In his thoughts, he slowly came closer to the question of the origin of humans. At the London Zoo he studied the latest attraction, a female orangutan called Jenny. In her face he recognised traits that babies also have. "Man from monkey?" he asked himself in his notes.

Idea of natural selection
Now the young researcher stood on the threshold of heresy. While he made preparations for his wedding, Darwin also looked for the mechanisms through which species underwent change. One evening he came across the bleak book An Essay on the Principle of Population written by the British economist Thomas Robert Malthus (1766 to 1834). In it, Malthus showed why the population was destined to explode in the course of a few years unless checked by hunger catastrophes or epidemics. His calculations were simple: Whereas the sources of food followed an arithmetic progression (1, 2, 3, …), the rate of propagation followed a geometric one (1, 2, 4, 8, 16, 32, …). "Hence, it can be claimed with certainty that the population will double every 25 years unless controlled", Malthus concluded. Darwin immediately drew parallels in the natural world: "Every species must have the same number killed year [after] year by hawks and cold and other reasons, even one species of hawk decreasing in number must affect instantaneously all of the rest…. One may say there is a force like a hundred thousand wedges trying [to] force every kind of adapted structure into the gaps in the economy of nature, or rather forming gaps by thrusting out weaker ones."

The idea of natural selection as the driving force in evolution was thus born. Accordingly, there is a relentless competition for survival going on in nature. Some individuals have an advantage because of certain characteristics they possess, which improve their chances of survival in the environment they inhabit. Hence, their chance of bearing offspring is disproportionately higher so that these characteristics can be passed on from generation to generation. The changes are no doubt too small to be observable from one generation to the immediate next one, but as a passionate geologist, Darwin was thinking in terms of entirely different timeframes. "I now had a theory, finally, with which I could work," he wrote later. However, it was to take several years until it was published.

Life's work shattered
One day in June 1858 Darwin received mail from overseas. The sender, Alfred Russel Wallace, a young and enthusiastic natural scientist who had traveled around the world at his own expense, and earned his livelihood by exporting exotic animals. Darwin had requested two years earlier the bellows (lungs) of pigeons and poultry breeds from the Malayan archipelago; since then, Wallace had been in touch with the already well known private scholar.

The package, which was collected from the Moluccan island of Ternate, however, did not contain information about Malayan bird species that Darwin had requested, but a scientific manuscript of about 20 pages. In an accompanying letter, Wallace requested that Darwin to forward the essay to Lyell for publication, if he felt it was significant enough. He hoped his ideas would contribute to filling the "missing link" in the evolution of species. As Darwin read the article, he saw his life's work "shattered": someone else had pulled ahead of him. "Wallace could not have prepared a better resume if he had my handwritten draft of 1842", he finally wrote, in an embittered missive to Lyell. Even the vocabulary was the same: Wallace, too, wrote of "variants" that had been eliminated through a "fight for survival" from their original species. Darwin's comment in response was simple and to the point: "This has destroyed all my originality."

Charles Lyell was not surprised. He had urged Darwin time and again in the past to speed up his work, having read an article by a hitherto unheard of researcher that had appeared in a scientific journal that encompassed the essential arguments of the theory advanced by Darwin, and later even by Lyell himself. But Darwin had ignored the dangers, informing his old teacher that only an extensive tome with appropriate footnotes would be capable of convincing the public of his theory. Hesitatingly, he had revealed to a few other natural scientists his godless theory, over a period of nearly two decades: "It is as if one were confessing to a murder," he wrote to his closest confidante, the botanist Joseph Dalton Hooker.

And Wallace had even read Malthus's work. While he was confined to bed following a serious attack of malaria in Ternate, he applied the overpopulation theory of the British economist to the natural world, around 20 years after Darwin had done the same. Now, was a rank outsider going to steal the well deserved laurels from the famous natural scientist Charles Darwin?

Together with Hooker, Lyell hatched a plan that was to go down in the history of biology as a "delicate arrangement". Yes, they would publish Wallace's manuscript, but only along with two extracts from Darwin's work which would precede the article, so that their priority would be recognizable. Charles Darwin, who was mourning the death of a son, consented. "I will do everything I am told to do." And even Wallace consented to it after his return. "Wallace never criticized this arrangement and acknowledged Darwin's priority," according to science historian John van Wyhe. "He acknowledged without envy that he could never have documented the evidence of the mechanisms of evolution so well."

At a meeting of the Linnaean Society of London on July 1, 1858, both works were read without receiving much attention. The society's annual report noted that the year 1858 had drawn to an end "without any discoveries that could revolutionize the research disciplines". Now out in the open, Darwin did not want to lose any time.  He completed his work in haste. The day on which the work On the Origin of Species by Means of Natural Selection or the Preservation of Favored Races in the Struggle for Life was published, November 24, 1859, started a new epoch in biology. This time, the response was overwhelming: all 1,250 copies of the book were sold out on the very first day of its appearance.

Under the chairmanship of Henslow, there was confrontation between supporters and opponents on June 30, 1860, at the meeting of the British Association for the Advancement of Science in Oxford. Darwin himself was ill and could not attend. Nevertheless, the proceedings were heated. When Bishop Samuel Wilberforce asked if Darwin's close friend Thomas Henry Huxley had descended from the apes on his grandfather's side or his grandmother's side, he replied: "Had the question been addressed to me, whether I would rather have a miserable ape for a grandfather or a man highly talented by nature and with great influence and importance, but who uses his skills and influence merely for the purpose of bringing in ludicrousness into a serious scientific discussion, then I would without hesitation confirm my preference for the ape."

Captain Fitzroy burst in on the commotion: Clad in military uniform and holding up a Bible, the former commander of the Beagle swore in the presence of all that he believed more in God than in human beings. The book published by his travel companion of yore had apparently caused him a lot of pain.

It was not until 1871 that Darwin commented on The Descent of Man, on the origins of our own species. Eleven years later, he died in his country home near London. Until the very end, his wife Emma, with whom he had been married happily for 43 years, had watched by his bedside. Darwin's ideas were to survive, his much quoted prophecy, which was the only place in the On the Origin of Species to give any insight into his own view on whether the "ape question," was to become true. It is said there: "Light will fall on the origin of man and his history."