Introducing Darwin and natural selection

Keenly observing nature in all its forms—from fossil sloths to mockingbirds, primroses to children—Darwin saw that we all are related. Every living thing shares an ancestry, he concluded, and the vast diversity of life on Earth results from processes at work over millions of years and still at work today. Darwin's explanation for this great unfolding of life through time—the theory of evolution by natural selection—transformed our understanding of the living world, much as the ideas of Galileo, Newton, and Einstein revolutionized our understanding of the physical universe.

Darwin's theory of evolution by natural selection underlies all modern biology. It enables us to decipher our genes and fight viruses, as well as to understand Earth's fossil record and rich biodiversity. Simple and at times controversial, misunderstood and misused for social goals, the theory remains unchallenged as the central concept of biology. Charles Darwin, reluctant revolutionary, profoundly altered our view of the natural world and our place in it.

Charles Darwin looked closely at life. The vast and marvelous diversity of life on Earth, from barnacles to butterflies, ostriches to orchids, made him curious. Countless species, living and extinct—why so many? Some were only slightly different from one another—what could explain that? Each organism so beautifully adapted to its environment—how could it happen?

With persistence and passion, Darwin set out to find answers to these questions. He conducted experiments. He read widely and corresponded with fellow naturalists around the world. And he studied the evidence using simple tools, at times merely a microscope or a magnifying glass. Darwin looked closely—and we saw the world in a new way.

Before Darwin was born, most people in England accepted certain ideas about the natural world as given. Species were not linked in a single "family tree." They were unconnected, unrelated, and unchanged since the moment of their creation. And Earth itself was thought to be so young—perhaps only 6,000 years old—that there would not have been time for species to change. In any case, people were not part of the natural world; they were above and outside it.

These attitudes reflected a broader view of the world as stable and unchanging. There was a natural order to things. Most English people lived in farming communities and did not travel far from where they were born. Their lives were much like the lives of their parents. Soon the Industrial Revolution and democratic reforms would remake society—but before Darwin, it was still possible to see the world as timeless and unchanging.

Could life on Earth have evolved? Before 1800, only a handful of naturalists in England and France had given this idea serious consideration. And even they couldn't see how there could have been enough time for evolution to occur.

Relying on interpretations of the Bible, most people in England believed that Earth was only about 6,000 years old—not nearly old enough for countless species to have evolved.

Today, we know from radiometric dating that Earth is about 4.5 billion years old. Had naturalists in the 1700s and 1800s known Earth's true age, early ideas about evolution might have been taken more seriously.

Before Darwin's time, humans were not considered part of the natural world. People saw that we resembled other animals, especially other primates like the orangutan and the chimpanzee. Still, despite the undeniable similarities between "us" and "them," only a handful of early naturalists classified humans as animals.

In England during the 1700s and early 1800s, few questioned the Biblical story of creation. The prevailing view was that people were created to rule over animals, "over the fish of the sea and over the birds of the sky." Naturalists could see that, particularly beneath the skin, a chimpanzee looks a lot like a human—but the idea that we might somehow be related to apes was almost unthinkable.

By 1800, European naturalists knew a great deal about plants and animals. They collected specimens, carefully studied them and even classified similar species in groups. But only a few bold thinkers, including Jean-Baptiste Lamarck and Erasmus Darwin, Charles's grandfather, speculated that species had evolved—that is, that all life shared a common ancestor.

Why didn't more people grasp that similarities in skeletal structures—so clearly visible—were a clue that species are related? In part, no one could convincingly explain how evolution worked. How could distinctive features, like the anteater's long nose, have taken shape over time? How could new species arise? Few naturalists, however, were even asking such questions. Most were comfortable with the prevailing view that each species resulted from an act of special creation by a Creator.

Early evolutionists like Jean-Baptiste Lamarck had difficulty explaining how new species arose. Lamarck saw that many animals seemed to have acquired useful adaptations. The giraffe's long neck, for instance, was perfect for feeding on high leaves. But how did the giraffe get its neck? Lamarck thought it resulted from the constant effort of reaching for food. Constant use of a body part, he argued, made it larger and stronger.

But there was one key problem. Can changes produced during an animal's lifetime be passed on to its offspring? Does a parent lifting weights produce a muscular baby? Lamarck argued that such acquired traits could be inherited, but few others were persuaded. A convincing mechanism for evolution had yet to be discovered.