Evolution happens.

Darwin wasn't the first to suggest the theory of evolution—that all living things originate from ancestral forms and that their distinguishable differences are due to modifications in successive generations. But no one gave it more thought, or provided more evidence for it, or more deeply ingrained the theory into our collective consciousness than Charles Darwin. Today, researchers using the genetic techniques of "evo devo" can trace the evolution of the various kinds of organisms that Darwin fit into his "tree of life" (here, from an 1837 notebook, his first such tree). As the geneticist Theodosius Dobzhansky once wrote, "Nothing in biology makes sense except in the light of evolution."

Evolution happens through natural selection.

Darwin was not content to say that organisms evolve. The theory of evolution, he wrote, "even if well founded, would be unsatisfactory until it could be shown how the innumerable species inhabiting this world have been modified so as to acquire that perfection of structure and coadaptation that most justly excites our admiration" [italics added]. In what has been called "the greatest idea anyone ever had," Darwin theorized how those modifications occurred—through a process he called natural selection, which holds that individuals within a species that are best suited to their environment will survive and reproduce more than those less suited, thereby passing on their more useful traits and genetic qualities to successive generations. "Nothing that we have learned in the intervening 175 years," writes paleontologist Niles Eldredge of the time since Darwin first started formulating his theory, "has contravened Darwin's basic description of how natural selection works."

Evolution happens through natural selection. (continued)

Suitably, one of the most striking examples of natural selection in action concerns the very Galapagos finches that Darwin made famous. Since 1973, biologists Peter and Rosemary Grant, working on the tiny island of Daphne Major in the Galapagos, have studied a species of finch called Geospiza fortis (upper right in illustration, which appeared in Darwin's 1839 book about his five-year journey aboard the Beagle). After a drought in 1977 devastated plants bearing small seeds, more than 1,000 of the 1,200 G. fortis finches on the island died. The Grants discovered that larger G. fortis, which could break open larger seeds than smaller G. fortis could, survived better. The survivors mated in 1978, and, on average, their offspring had beaks 4 percent larger than those of the previous generation. Following another drought in 2003, G. fortis with smaller beaks survived better, in part because of stiff competition for bigger seeds after a larger finch species, G. magnirostris, settled the island. Between 2003 and 2005, the Grants found, G. fortis beaks shrank by 5 percent.

Evolution by natural selection must have a mechanism.

Darwin knew in his heart that evolution by natural selection happened, but he didn't know how. In fact, he proposed a mechanism of heredity that proved to be dead wrong, something he called pangenesis. Ironically, the actual mechanism of heredity—genetics—was revealed in Darwin's lifetime, though he never knew it. Working with pea plants, the Austrian priest Gregor Mendel discovered the fundamental laws of genetics in 1865. But Mendel's work was neglected until 1900, and it wasn't until the 1940s that scientists identified DNA as the genetic material (here, Francis Crick's first sketch of DNA's double helix structure). Today, the clear focus of the revolutions now under way in genetics, medicine, and other fields is on the genomes of living organisms.

The mechanism—whatever it is—is natural, not supernatural.

Darwin held off on publishing his theory for 20 years, until forced to by fellow naturalist Alfred Russell Wallace, who had independently come up with the same idea. One of the reasons for Darwin's delay was he knew what a shock his theory would represent to Christians, including his beloved wife Emma, who believed that God had created all living things, whole and independent of all others. Darwin didn't hold this view. His friend, the biologist T. H. Huxley, agreed. "As a natural process of the same character as the development of a tree from its seed, or of a fowl from its egg," Huxley wrote, "evolution excludes creation and all other kinds of supernatural intervention." Of course, many people around the world still believe in a divine instrument of creation.

Embryology is "the strongest single class of facts in favor of change of forms."

Darwin considered the process that a fertilized egg goes through on the way to becoming a multibillion-celled adult as crucial to understanding how basic anatomical parts of both animals and plants change over evolutionary time. The new science of evolutionary developmental biology, "evo devo," is revealing the truth behind Darwin's assertion quoted above. Until recently, scientists could say that forms do change, but they couldn't say exactly how, because they didn't know which genes were essential for the development of any organism. They now know that, remarkable as it may seem, all organisms on Earth, from microbes to man (a human embryo is seen here), share a common "tool kit" of so-called master genes, which govern how bodies and body parts form. When and which of these genes are turned "on" or "off" during the embryonic stage determines how each organism is built.

Sexual selection also drives evolution.

The peacock posed an enigma to Darwin. How could its extravagant tail feathers have come about? How do they aid in the "survival of the fittest" (a phrase coined by the philosopher Herbert Spencer)? Darwin proposed an answer in his book Descent of Man, in which he introduced a novel concept—sexual selection, the critical role that the opposite sex plays in the evolution of traits. Over time, Darwin believed, peahen choices about which peacocks constituted the finest mates could have led to the males' absurdly ostentatious tails. In the 1990s, biologist Marion Petrie of the University of Newcastle-upon-Tyne showed Darwin's hunch to be true. On average, peacocks have 150 "eyes" on their tails. By clipping off just a few of those eyes, Petrie found he could significantly reduce a male's chances of mating; males with fewer than 130 eyes rarely mated at all. Petrie reasoned that the healthier a peacock was, the more he could invest in the production of eyes—something that peahens clearly saw for themselves.

All animals, including humans, descend from a common ancestor.

Darwin knew the idea that man was just another animal would go over like a lead balloon in Victorian society. But in the Origin of Species, he was unequivocal on this notion, writing that "I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from one primordial form, into which life was first breathed." Evidence supporting this view has accumulated ever since, with arguably the strongest support coming from evo devo. Scientists comparing the genomes of members of the three domains of life—archaea, bacteria, and the eukaryotes (animals, plants, fungi, and protists)—have discovered about 500 genes that all living things share. These "immortal" genes have survived essentially unchanged for over two billion years (including the portion of a protein's sequence seen here, with shared amino acids indicated by shading). Says Brown's Ken Miller: "Despite the extraordinary diversity of life, all living organisms share a nearly identical set of essential genes, reflecting their evolutionary development from a common ancestor."

Humans evolved from an ape-like ancestor.

Fearing that many of his contemporaries would be deeply offended if he suggested that humans and apes share a common ancestor, Darwin barely touched on human evolution in the 1859 Origin of Species. But by the 1871 Descent of Man, he was ready, stating baldly that "man is descended from a hairy, tailed quadruped, probably arboreal in its habits." He also maintained that the difference between the mind of man and that of a chimpanzee or gorilla is a matter of degree, not of kind. Summing up, Darwin wrote, "We must … acknowledge, as it seems to me, that man with all his noble qualities … still bears in his bodily frame the indelible stamp of his lowly origin." Today, many a schoolchild can cite the figure perhaps most often called forth in support of this view—namely, that we share almost 99 percent of our DNA with our closest living relative, the chimpanzee. (The illustration shown here appeared in T. H. Huxley's 1863 book Man's Place in Nature. All skeletons are to scale except the gibbon, which is shown twice natural size.)

Modern humans arose in Africa.

In Darwin's day, virtually no fossils of early humans were known—one of the few was Neanderthal Man, discovered in Germany's Neander Valley in 1856—nor could anyone date them reliably. But noticing our close anatomical relationship to chimps and gorillas, Darwin surmised that Homo sapiens had evolved in Africa. In his customary careful way, he wrote, "It is somewhat more probable that our early progenitors lived on the African continent than elsewhere." Today, the fossil known as Lucy is only the most famous of numerous fossils of human and human-like species whose age and other particulars confirm the notion that modern humans first came into existence in Africa. Here, a 1999 evolutionary tree based on DNA shows how the five great apes, including us, arose from a common ancestor. Branch length indicates how far the genes of each population have diverged from those of near relatives.

The Earth is at least several hundred million years old.

In 1658, based on his reading of the Bible and on historical records, the Irish clergyman James Ussher declared that God had created the Earth on October 22, 4004 B.C. Geologists of Darwin's day begged to differ, arguing for a far older Earth. Darwin, knowing that evolution required vastly more time than just six millennia, concurred. In the first edition of the Origin of Species, he estimated our planet to be at least several hundred million years old. In fact, as is well known today, it is far more primeval. In the 1900s, after Marie and Pierre Curie discovered radioactivity, physicists learned how to use the half-life of uranium and other radioactive elements to estimate the age of rocks. The oldest minerals found on Earth are about 4.1 to 4.2 billion years old, indicating our planet is at least that old. But meteorites, which scientists believe formed at the same time as the planets and which, unlike the Earth, are not geologically active and therefore retain evidence of their formation, date even further back. Using several different dating methods on dozens of different kinds of meteorites, scientists have, in instance after instance, come up with an age of roughly 4.5 billion years—the current widely accepted estimate of our planet's age.

Gaps in the fossil record will be filled in with key transitional fossils.

In his notebooks, Darwin worried about the many gaps in the fossil record that then existed, "like pages torn from a book." Perhaps the most glaring was the complete lack of fossils before the Cambrian Period (544-510 million years ago). "To the question why do we not find rich fossiliferous deposits belonging to these assumed earliest periods before the Cambrian system, I can give no satisfactory answer," he wrote. If his theory of evolution was right, he knew such gaps would eventually be filled in. Only in the past half century have paleontologists unearthed fossils from the Precambrian, and it's a huge and ever-growing trove, with the oldest fossils dating to about 3.5 billion years ago. Moreover, they continue to unearth key fossils that bridge those gaps Darwin bemoaned, including Archaeopteryx, shown here. To see a range of such transitional fossils, go to Fossil Evidence.

An insect with a foot-long tongue must exist to pollinate this orchid.

Not all of Darwin's conjectures were so broad in scope or so earth-shaking; some were simple predictions. Take the case of this species of orchid, Angraecum sesquipedale, from Madagascar. When Darwin saw this orchid in 1862, he, like anyone who saw it, was astonished by the length of its spur, which can reach over a foot in length. (See long, slim tube in photo.) "Astounding," he wrote. "What insect could suck it?" For some as-yet unknown insect must, he insisted, and it had to have a foot-long tongue to get at the plant's nectar, which pools at the very base of the spur. Entomologists of his day were skeptical, for no such creature had ever turned up. But more than 40 years after Darwin's death in 1882, scientists discovered a giant hawk moth in Madagascar, and it lapped the orchid's nectar with, yes, a foot-long tongue. The moth was named Xanthopan morganii praedicta in honor of his prediction. Once again, Darwin was right.