What, exactly, did Darwin's theory say?
In his 1859 book On the Origin of Species, Charles Darwin laid out an explanation for the diversity of life, the relationships between plant and animal groups, and the way living things seem so perfectly adapted to their environments. He based his idea of natural selection on a number of influences including the observations he made in South America and the Galapagos Islands on his famous voyage of the Beagle.
He also spent time thinking about domestic animals, especially dogs and the special pigeons that the English were fond of breeding.
Darwin could see that offspring inherit traits that are similar to their parents but not exactly the same. He recognized there must be a source of heredity and a source of variation, although without genetics, he didn't understand that they could be one in the same.
Genetics wasn't well understood at the time, but Darwin realized there was some as-yet-unknown process of heredity by which traits were passed down - but not perfectly - so new variation was always arising.
That led to diversity on which natural selection could act.
Darwin's idea for natural selection was inspired by Thomas Malthus, who showed that creatures usually produce many more progeny than will survive. Darwin realized that nature would cull from each generation, leaving only those most well-adapted to environmental conditions. Over time, that could change the size or color or behavior of a species, eventually allowing new species to branch off from older ones.
Darwin first started to confide his idea to other scientists in the early 1840s but he didn't go public with it for nearly 20 years. He was reportedly worried about getting everything right, and also knew there would be scandalous implications about religion and the origin of human beings. His own wife was reportedly very devout and offended by any idea that seemed to contradict humanity's creation in the image of God.
What finally lit a fire under Darwin was a letter he received in 1858 from a lesser-known naturalist named Alfred Russell Wallace. The letter had a manuscript of Wallace's new theory, and it was identical to Darwin's natural selection. Wallace had come up with it independently. A year later, Darwin abandoned the scientific tome he had been working on, in favor of a shorter, more popularized version he called On the Origin of Species.
What did people believe before?
Darwin did not come in and stop people from believing in a literal interpretation of the bible. More than 200 years earlier, Galileo punched a hole in such dogmatic thinking by showing that the Earth was not the center of things. Some people even believed in evolution before Darwin's theory came along though they tended to call it "transmutation." What those pre-Darwinian evolutionists lacked was an understanding of how evolution worked.
The most prominent evolutionary theory to predate Darwin's was formulated by a French naturalist named Jean-Baptiste Lamarck. He proposed that living things progressed from "lower" to "higher" through what he called a "great chain of being." In Larmarck's version of evolution, living things could pass down traits they'd acquired during their lifetimes-most famously the giraffe that would stretch its neck to reach leaves, thus giving birth to yet longer-necked offspring. Lamarck also believed that evolution stretched toward the goal of producing mankind. Darwin argued against any directionality or goal in evolution.
In arguing for lack of directionality, Darwin was influenced by an earth scientist named Charles Lyell. Lyell proposed that the Earth existed for millions of years and that it changes over time without progressing in any way. The earth changes with no ultimate goal, and life adapts with those changes.
Darwin also challenged the idea that there was some wall between species. He saw a more continuous variation as different varieties of a species could gradually branch off to become a species of their own.
How did the world react to Darwin's idea when he published it in 1859?
Origin sold out its first printing. Many people claimed they'd already thought of the idea themselves.
Educated people were pretty quick to embrace the idea that life evolved, said Penn philosopher of science Michael Weisberg. But many didn't grasp Darwin's concept of natural selection as the mechanism underlying evolution. People also clung to the old Lamarckian idea of evolution as progress. Some still do.
There was also some controversy over where humans came from and how we fit into the tree of life. Soon after the publication of Origin, a famous took place in Oxford, England, at a meeting of the British Association for the Advancement of Science. Arguing for Darwin's idea was biologist Thomas Huxley, and against it was Samuel Wilberforce, who famously asked Huxley whether he was descended from an Ape on his grandmother's or grandfather's side.
What's Darwin's evidence?
Much of the evidence for Darwin's theory was already out there, waiting to be tied together with one unifying idea. In Origin, Darwin documents extensive observations of plants and animals he made in his Native England and on his voyage. He noticed patterns that backed his idea-he noticed that successful species often had many closely related "varieties" just as larger categories that were well-adapted had many closely related species. He proposed that varieties were incipient species. He also made many observations of domestic animals. Breeding pigeons was popular in 19th century England and so Darwin made many observations of these birds. He also conducted experiments among the plants in his own garden to back some of his ideas.
Some people found Darwin's idea obvious, according to historian William Bynum. Others slapped their heads and wondered why they didn't think of it first. Natural selection helped explain the pattern of similarities across living things. It helped explain the way the turtles and finches of the Galapagos looked different from island to island.
And it explained the fossils people had been collecting for several hundreds years and the way those fossils varied depending on the layer of strata they came from.
Has further evidence been found subsequently?
Darwin's theory did hit a couple of snags in the early years. The first one came, ironically, from genetics, according to Penn's Weisberg. Unbeknownst to Darwin, while he was working on evolution, a monk named Gregor Mendel was working out the laws of inheritance using pea plants. But unlike Darwin, who was a prominent scientist, Mendel was an obscure figure in his own time and wasn't discovered by the scientific community until the early 20th century.
Darwin had no idea how living things managed to inherit traits from their ancestors, but he did insist that changes were very small and gradual. Mendel figured out a set of laws through which traits could be inherited - genetics - but it wasn't always as gradual as Darwin thought. In Mendel's famous pea plants, for example, seeds could go from smooth to wrinkly in a generation.
For a decade or so, scientists were actually divided into Mendelians and Darwinists, the two seeming to be incompatible, said Penn's Weisberg.
In the 1920s, scientists began to put the two together into a synthesis that became neo Darwinism. Since then, science has uncovered DNA as the material that makes up the genes. They've also compared DNA from different creatures to show that Darwin was correct to connect them all into one big family tree.
Where do humans fit into all this? Did we really evolve from apes?
Yes. We did not evolve from any of the existing apes, but from a line of now-extinct apes. On that lineage are all the common ancestors we shared with chimps and other existing apes. That doesn't conflict with the fact that we're very different from our fellow apes in many way; we're mostly hairless, walk upright, can talk, have different dietary needs and a different mating system. A lot can happen in a few million years.