It wasn't new, but evolution certainly was heretical - it challenged the Church of England's teachings that God had created Adam and Eve and every plant and animal species on Earth. Darwin slowly came to see that his specimens supported a colder, more rational view of nature that he felt compelled to probe despite the risk to his reputation. He wanted to learn more, to discover how the process worked, to find proof for his ideas.
In his 2006 book The Reluctant Mr. Darwin, American science writer David Quammen says the great naturalist produced his theory in much the same way that a mother kiwi labours to lay an egg that weighs a fifth of what she does. First, Darwin seized on the idea that all animals vary slightly - other than identical twins, for example, no two humans are quite the same - and concluded that sexual reproduction was essential for producing random variations. Over time, these small differences can accumulate into adaptations to meet particular circumstances, such as a drought or a flood.
Other writers and thinkers offered inspiration. One of his "aha!" moments came in 1838 when he read the sixth edition of Essay on the Principles of Population by Thomas Malthus. The English political economist describes how species tend to reproduce beyond sustainable levels but runaway population growth is checked by starvation. This showed that death can be a creative force, says Harvard University science historian and Darwin authority Janet Browne - the weak died and the strongest, or best adapted, survived.
Four years later, Darwin finished a summary of his ideas, using the "natural means of selection" to describe how the culling of weaker individuals shapes a species over time.
Natural selection, the mechanism for evolution, was Darwin's big breakthrough, biologist Brian Hall says, along with the idea that evolution changes organisms over time - or "descent with modification" - and that all organisms are related in a tree of life.
"The ultimate genius was the way he put all that together," says the Dalhousie University evolution expert. "Nobody else had done that before him in a detailed way, although Wallace came pretty close."
Alfred Russel Wallace was a cash-strapped but brilliant naturalist who shocked Darwin into finally going public in 1858 - almost two decades after coming up with his theory - by sending him an essay that laid out a nearly identical approach. Had the essay gone to another scientist, Dr. Hall says, Wallace would be remembered as the father of evolution. Instead, Darwin and his friends arranged to have it read aloud at a scientific meeting, along with excerpts from his work.
As a result, they officially share the credit, but Darwin got all the attention.
Why did he sit on his theory for so long? There were many reasons for his procrastination, including a desire to amass as much evidence as possible and an obsession with classifying barnacles that lasted almost a decade. And his ill health, by his own account, cost him years of productive work. He suffered from regular but mysterious bouts of retching and vomiting, heart palpitations, headaches, painful boils and flatulence so turbulent that he needed to be alone after eating a meal.
Then there was Emma, the cousin he married in 1839. A devout Anglican, she fretted over his growing skepticism about God. The loss of their beloved daughter, Anne, just 10 when she died in 1851, badly weakened by a bout of scarlet fever, destroyed the remnants of his Christian faith.
But he also was troubled by the knowledge that his theory would challenge the religion that offered his wife so much solace: If he was right, then Genesis was wrong. Even when he finally wrote The Origin of Species, he avoided the question of how humans evolved, leaving that to later books.
Like many couples, Darwin and his wife had trouble discussing their most troubling disagreement, so early in their marriage she wrote him a letter about her concerns about his work and his immortal soul. He reread it many times, and at the bottom he wrote: "When I am dead, know that many times I have kissed and cried over this."
THE CELEBRITY DARWIN: FAME MADE A PUBLIC FIGURE (AND A MONKEY) OF HIM
The Origin of Species appeared on Nov. 24, 1859. Written for a general audience, it quickly sold out, making Darwin famous and controversial - a very public figure whose face peered out from cartoons of monkeys and apes.
Yet he remained removed from the storm. His fragile health gave him an excuse to continue his reclusive country life at the house he had bought south of London. From there, he shrewdly managed a masterful public-relations campaign through a constant flow of letters to the friends and colleagues who vigorously defended his work in scientific journals and public forums. While he experimented with orchids, they took on his many attackers, both in print and in person.