Charles Darwin was just 28 years old when, in 1837, he scribbled in a notebook "one species does change into another"—one of the first hints of his great theory. He'd recently returned to England after his five-year journey as a naturalist aboard the HMS Beagle. In South America, Oceania and most memorably the Galápagos Islands, he had seen signs that plant and animal species were not fixed and permanent, as had long been held true. And it was as if he had an inkling of the upheavals to come as he pored over specimens he had collected and others had sent him: finches, barnacles, beetles and much more. "Cuidado," he wrote in another notebook around that time, using the Spanish word for "careful." Evolution was a radical, even dangerous idea, and he didn't yet know enough to take it public.

For another 20 years he would amass data—20 years!—before having his idea presented publicly to a small audience of scientists and then, a year later, to a wide, astonished popular readership in his majestic On the Origin of Species, first published in 1859. Today, Origin ranks among the most important books ever published, and perhaps alone among scientific works, it remains scientifically relevant 150 years after its debut. It also survives as a model of logical thought, and a vibrant and engaging work of literature.

Perhaps because of that remarkable success, "evolution," or "Darwinism," can sometimes seem like a done deal, and the man himself something of an alabaster monument to wisdom and the dispassionate pursuit of scientific truth. But Darwin recognized that his work was just the beginning. "In the distant future I see open fields for far more important researches," he wrote in Origin.

Since then, even the most unanticipated discoveries in the life sciences have supported or extended Darwin's central ideas—all life is related, species change over time in response to natural selection, and new forms replace those that came before. "Nothing in Biology Makes Sense Except in the Light of Evolution," the pioneering geneticist Theodosius Dobzhansky titled a famous essay in 1973. He could not have been more right—evolution is quite simply the way biology works, the central organizing principle of life on earth.

In the 150 years since Darwin published Origin, those "important researches" have produced results he could never have anticipated. Three fields in particular—geology, genetics and paleoanthropology—illustrate both the gaps in Darwin's own knowledge and the power of his ideas to make sense of what came after him. Darwin would have been amazed, for example, to learn that the continents are in constant, crawling motion. The term "genetics" wasn't even coined until 1905, long after Darwin's death in 1882. And though the first fossil recognized as an ancient human—dubbed Neanderthal Man—was discovered in Germany just before Origin was published, he could not have known about the broad and varied family tree of ancestral humans. Yet his original theory has encompassed all these surprises and more.

Around the world, people will celebrate Darwin's 200th birthday with lectures, exhibits and festivities. In England, where Darwin already graces the ten-pound note, a special two-pound coin will be struck. Cambridge University is hosting a five-day festival in July. In North America, Darwin events are scheduled in Chicago, Houston and Denver, among many other places. Smithsonian's National Museum of Natural History established an "Evolution Trail" that highlights concepts from Darwin's work throughout the museum, and a special exhibit shows how orchids have evolved and adapted according to Darwin's theory.

As towering historical figures go, Charles Darwin does not provide much by way of posthumous scandals. The liberty-extolling Thomas Jefferson was slave master to his longtime mistress, Sally Hemings; Albert Einstein had his adulterous affairs and shockingly remote parenting style; James Watson and Francis Crick minimized their debt to colleague Rosalind Franklin's crucial DNA data. But Darwin, who wrote more than a dozen scientific books, an autobiography and thousands of letters, notebooks, logs and other informal writings, seems to have loved his ten children (three of whom did not survive childhood), been faithful to his wife, done his own work and given fair, if not exuberant, credit to his competitors.

He was born in Shrewsbury, England, on February 12, 1809, into a well-off family of doctors and industrialists. But his up-bringing wasn't entirely conventional. His family was active in progressive causes, including the antislavery movement. Indeed, an illuminating new book by Adrian Desmond and James Moore, Darwin's Sacred Cause, concludes that Darwin's interest in evolution can be traced to his, and his family's, hatred of slavery: Darwin's work proved the error of the idea that the human races were fundamentally different. Both of his grandfathers were famous for unorthodox thinking, and Darwin's mother and physician father followed in those footsteps. Darwin's paternal grandfather, Erasmus Darwin, was a physician and natural philosopher of vast appetites—and correspondingly corpulent physique—who developed his own early theory of evolution. (It was more purely conceptual than Charles' and missed the idea of natural selection.) On his mother's side, Darwin's grandfather was the wealthy Josiah Wedgwood, founder of the eponymous pottery concern and a prominent abolitionist.

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