Leafing through the mail at his home outside London one June day 150 years ago, Charles Darwin came across an envelope sent from an island in what is now part of Indonesia. The writer was a young acquaintance, Alfred Russel Wallace, who eked out a living as a biological collector, sending butterflies, bird skins and other specimens back to England. This time, Wallace had sent along a 20-page manuscript, requesting that Darwin show it to other members of the British scientific community.

As he read, Darwin saw with dawning horror that the author had arrived at the same evolutionary theory he had been working on, without publishing a word, for 20 years. "All my originality, whatever it may amount to, will be smashed," he lamented in a note to his friend the geologist Charles Lyell. Darwin ventured that he would be "extremely glad now" to publish a brief account of his own lengthy manuscript, but that "I would far rather burn my whole book than that [Wallace] or any man should think that I had behaved in a paltry spirit."

The threat to his life's work could hardly have come at a worse moment. Darwin's daughter Etty, 14, was frighteningly ill with diphtheria. His 18-month-old son, Charles, would soon lie dead of scarlet fever. Lyell and another Darwin friend, the botanist Joseph Hooker, cobbled together a compromise, rushing both Darwin's and Wallace's works before a meeting of the Linnean Society a few days later, on July 1, 1858. The reading took place in a narrow, stuffy ballroom at Burlington House, just off Piccadilly Circus, and neither author was present. (Darwin was at his son's funeral; Wallace was in New Guinea.) Nor was there any discussion. The society's president went home muttering about the lack of any "striking discoveries" that year. And so began the greatest revolution in the history of science.

We call it Darwinism, for short. But in truth, it didn't start with Darwin, or with Wallace either, for that matter. Great ideas seldom arise in the romantic way we like to imagine—the bolt from the blue, the lone genius running through the streets crying, "Eureka!" Like evolution itself, science more often advances by small steps, with different lines converging on the same solution.

"The only novelty in my work is the attempt to explain how species become modified," Darwin later wrote. He did not mean to belittle his achievement. The how, backed up by an abundance of evidence, was crucial: nature throws up endless biological variations, and they either flourish or fade away in the face of disease, hunger, predation and other factors. Darwin's term for it was "natural selection"; Wallace called it the "struggle for existence." But we often act today as if Darwin invented the idea of evolution itself, including the theory that human beings developed from an ape ancestor. And Wallace we forget altogether.

In fact, scientists had been talking about our primate origins at least since 1699, after the London physician Edward Tyson dissected a chimpanzee and documented a disturbing likeness to human anatomy. And the idea of evolution had been around for generations.

In the 1770s, Darwin's grandfather Erasmus Darwin, a physician and philosopher, publicly declared that different species had evolved from a common ancestor. He even had the motto "E conchis omnia" ("Everything from shells") painted on his carriage, prompting a local clergyman to lambaste him in verse:

Great wizard he! by magic spells
Can all things raise from cockle shells.

In the 1794 book of his two-volume Zoonomia, the elder Darwin ventured that over the course of "perhaps millions of ages...all warm-blooded animals have arisen from one living filament," acquiring new traits and passing down improvements from generation to generation.

His contemporary Samuel Taylor Coleridge mocked this sort of evolutionary theory as "darwinizing." But it was by no means a family monopoly. Evolutionary questions confronted almost all naturalists of that era as expeditions to distant lands discovered a bewildering variety of plants and animals. Fossils were also turning up in the backyard, threatening the biblical account of Creation with evidence that some species had gone extinct and been supplanted by new species. The only way to make sense of these discoveries was to put similar species side by side and sort out the subtle differences. These comparisons led "transmutationists" to wonder if species might gradually evolve over time, instead of having a fixed, God-given form.

In 1801, the French naturalist Jean-Baptiste Lamarck proposed that species could change in response to environmental conditions. Giraffes, for instance, had developed their fantastic necks to browse on the upper branches of trees. Lamarck mistakenly thought such traits could be acquired by one generation and passed on to the next. He is ridiculed, to this day, for suggesting that giraffes got their longer necks basically by wanting them (though the word he used, some scholars contend, is more accurately translated as "needing"). But his was the first real theory of evolution. If he had merely suggested that competition for treetop foliage could gradually put short-necked giraffes at a disadvantage, we might now be talking about Lamarckian, rather than Darwinian, evolution.

By the 1840s, evolutionary ideas had broken out of the scientific community and into heated public debate. The sensation of 1845 was the anonymous tract Vestiges of the Natural History of Creation, and it set both Darwin and Wallace on career paths that would converge in that fateful 1858 mail delivery. Vestiges deftly wove evolutionary ideas into a sweeping history of the cosmos, beginning in some primordial "fire-mist." The author, later revealed to be the Edinburgh journalist and publisher Robert Chambers, argued that humans had arisen from monkeys and apes, but he also appealed to ordinary readers with the uplifting message that evolution was about progress and improvement.

Vestiges quickly became a popular hit, a rose-tinted 2001: A Space Odyssey of its day. Prince Albert read it aloud to Queen Victoria at Buckingham Palace, and it was the talk of every gentlemen's club and social soiree, according to James A. Secord, author of Victorian Sensation. Jocular types greeted each other on the street with phrases like, "Well, son of a cabbage, whither art thou progressing?" Others took evolution more seriously. On a museum visit, Florence Nightingale noticed that small flightless birds of the modern genus Apteryx had vestigial wings like those of the giant moa, an extinct bird that had recently been discovered. One species ran into another, she remarked, much "as Vestiges would have it."

Clergymen railed from the pulpit against such thinking. But scientists, too, hated Vestiges for its loose speculation and careless use of facts. One indignant geologist set out to stamp "with an iron heel upon the head of the filthy abortion, and put an end to its crawlings." In Cambridge, at a meeting of the British Association for the Advancement of Science, an astronomer criticized the book's failure to explain how evolution might have occurred; Vestiges, in his view, was about as miraculous as the biblical account of Creation. (During this attack, the author, still anonymous, sat in the front row, probably trying not to squirm.) Even Darwin disliked what he called "that strange unphilosophical, but capitally-written book." He confided to a friend that the author's "geology strikes me as bad, & his zoology far worse."

Darwin had begun to develop his own theory of evolution seven years earlier, in 1838, while reading the demographer T. R. Malthus on factors limiting human population growth. It dawned on him that, among animals, hunger, predation and other "checks" on population could provide "a force like a hundred thousand wedges," thrusting out weaker individuals and creating gaps where better-adapted individuals could thrive. By 1844, he had expanded this idea into a manuscript of more than 200 pages.

But Vestiges heightened Darwin's characteristic caution. He hesitated to publish partly because radicals were taking up evolutionary theory as a way to undermine the idea of a divinely ordained social hierarchy. Darwin himself sat comfortably in the upper ranks of that hierarchy; he had inherited wealth, and his closest colleagues were other gentlemen naturalists, including the clergy. Admitting transmutationist beliefs in these circles, Darwin had written to his friend Hooker, would be like "confessing a murder." But beyond that, he also hesitated because the abuse being heaped onto Vestiges drove home the need for detailed evidence. Darwin, at age 37, backed away from theorizing and settled down to describing the minute differences within one invertebrate group: the barnacles. He would spend the next eight years at it, at some peril to his sanity.

Wallace was more receptive to Vestiges. He was just 22 when the controversy raged. He also came from a downwardly mobile family and had a penchant for progressive political causes. But Vestiges led him to the same conclusion about what needed to be done next. "I do not consider it as a hasty generalization," Wallace wrote to a friend, "but rather as an ingenious speculation" in need of more facts and further research. Later he added, "I begin to feel rather dissatisfied with a mere local collection.... I should like to take some one family to study thoroughly—principally with a view to the theory of the origin of species." In April 1848, having saved £100 from his wages as a railroad surveyor, he and a fellow collector sailed for the Amazon. From then on, Wallace and Darwin were asking the same fundamental questions.

Ideas that seem obvious in retrospect are anything but in real life. As Wallace collected on both sides of the Amazon, he began to think about the distribution of species and whether geographic barriers, such as a river, could be a key to their formation. Traveling on HMS Beagle as a young naturalist, Darwin had also wondered about species distribution in the Galápagos Islands. But pinning down the details was tedious work. As he sorted through the barnacles of the world in 1850, Darwin muttered darkly about "this confounded variation." Two years later, still tangled up in taxonomic minutiae, he exclaimed, "I hate a Barnacle as no man ever did before."

Wallace was returning from the Amazon in 1852, after four years of hard collecting, when his ship caught fire and sank, taking down drawings, notes, journals and what he told a friend were "hundreds of new and beautiful species." But Wallace was as optimistic as Darwin was cautious, and soon headed off on another collecting expedition, to the islands of Southeast Asia. In 1856, he published his first paper on evolution, focusing on the island distribution of closely related species—but leaving out the critical issue of how one species might have evolved from its neighbors. Alarmed, Darwin's friends urged him to get on with his book.

By now, the two men were corresponding. Wallace sent specimens; Darwin replied with encouragement. He also gently warned Wallace off: "This summer will make the 20th year (!) since I opened my first-note-book" on the species question, he wrote, adding that it might take two more years to go to press. Events threatened to bypass them both. In England, a furious debate erupted about whether there were significant structural differences between the brains of humans and gorillas, a species discovered by science only ten years earlier. Other researchers had lately found the fossil remains of brutal-looking humans, the Neanderthals, in Europe itself.

Eight thousand miles away, on an island called Gilolo, Wallace spent much of February 1858 wrapped in blankets against the alternating hot and cold fits of malaria. He passed the time mulling over the species question, and one day, the same book that had inspired Darwin came to mind—Malthus' Essay on the Principle of Population. "It occurred to me to ask the question, Why do some die and some live?" he later recalled. Thinking about how the healthiest individuals survive disease, and the strongest or swiftest escape from predators, "it suddenly flashed upon me...in every generation the inferior would inevitably be killed off and the superior would remain—that is, the fittest would survive." Over the next three days, literally in a fever, he wrote out the idea and posted it to Darwin.

Less than two years later, on November 22, 1859, Darwin published his great work On the Origin of Species by Means of Natural Selection, and the unthinkable—that man was descended from beasts—became more than thinkable. Darwin didn't just supply the how of evolution; his painstaking work on barnacles and other species made the idea plausible. Characteristically, Darwin gave credit to Wallace, and also to Malthus, Lamarck and even the anonymous "Mr. Vestiges." Reading the book, which Darwin sent to him in New Guinea, Wallace was plainly thrilled: "Mr. Darwin has given the world a new science, and his name should, in my opinion, stand above that of every philosopher of ancient or modern times."

Wallace seems to have felt no twinge of envy or possessiveness about the idea that would bring Darwin such renown. Alfred Russel Wallace had made the postman knock, and that was apparently enough.

Richard Conniff is a longtime contributor to Smithsonian and the author of The Ape in the Corner Office.