1859 Darwin and the 'Origin of Species': how one man redefined what it means to be human--and why his work is still a source of contention today.

• Author: Roberts, Sam
• Position: TIMES PAST

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It was the tortoises, finches, and mockingbirds he found on the Galapagos Islands off the west coast of Ecuador, beginning in 1831, that first got young Charles Darwin thinking. The animals varied from island to island, and also from similar species on the mainland. Why did they vary? And how did it happen?

Those questions would lead Darwin on a three-decade odyssey that culminated in 1859 in the publication of the British naturalist's most famous work, On the Origin of Species, which explained how animals, including humans, all evolved from a common ancestor. Darwin's theory of evolution by natural selection has survived for 150 years, though not without controversy that continues today.

Darwin was 22--and already infatuated with beetles and natural history--when one of his professors at Cambridge University recommended him as the resident botanist on a mapping expedition of South America. Darwin would spend five years on the H.M.S. Beagle on a voyage that circled the globe, mostly in the Southern Hemisphere, unlocking a new world of biology and geology along the way. His travels introduced him to previously undiscovered living species and to the fossilized remains of long-extinct ones that failed, it seemed, to adapt to a changing environment.

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In the mid-19th century, there were several schools of thought about how humans and animals as varied as worms and dinosaurs had come to exist. One argued that species had simply originated wherever and however they were currently found, echoing the Biblical view that God had created humans and all animal life. Another maintained that new species were formed as previous generations of related species were killed off by natural catastrophes, such as the Ice Age and asteroid and comet strikes. And one theory hinted that animals somehow adapted to their environments and passed 'along new traits to their offspring.

But Darwin's observations on his Beagle voyage suggested other possibilities.

"It is a grand spectacle to see all nature thus raging," he wrote in a letter in 1833.

Darwin's mind was raging too. He found slight variations in turtles and birds in the Galapagos from one island to the next; in Australia, where the Beagle arrived in 1836, he saw unique life forms that seemed to exist nowhere else.

"These facts seemed to me to throw some light on the origin of species," Darwin later wrote, "that mystery of mysteries, as it has been called."

NATURAL SELECTION

For Darwin, the voyage of the Beagle was only the beginning. Back in England, he began publishing journal articles (some of his letters were published even before he returned) with his painstakingly precise observations of flora and fauna. But the theory of evolution that would make him famous--which other scientists, too, were beginning to write about--itself evolved slowly.

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He would later recall that one of his keenest insights was that the struggle for survival by plants and animals meant that...