Then a professor told him about the internship of a lifetime: a 5-year, around-the-world trip as a naturalist aboard a British surveying ship. The ship’s captain, Robert FitzRoy, wanted a travel companion who would also collect specimens along the way. Unsure what he wanted to do with his life but eager to see the world, the 22-year-old Darwin jumped at the chance. He later said of the trip, “The voyage of the Beagle has been by far the most important event in my life and has determined my whole career.”

Yet he almost didn’t go. Darwin’s father, Robert Darwin, thought his son should buckle down and prepare to enter the clergy. A trip around the world seemed to him a useless distraction—a “wild scheme,” he called it—and he refused at first to let his son go. But eventually, at the cajoling of his family, he relented. Charles packed his bags, said goodbye to his girlfriend, Emma, and set sail for South America. It was December 1831.

The passage aboard the 90-foot vessel was frequently harrowing, and Darwin suffered debilitating bouts of seasickness, but his journey aboard the Beagle set in motion one of the great revolutions in science. What he saw on that trip planted the seeds of ideas that have completely changed the way we view the world and our place in it. As the evolutionary biologist Stephen Jay Gould put it, “The world has been different ever since Darwin.”