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The vindication came not from fossils, or from specimens of living creatures, or from dissection of their organs. It came from a book. What Watson and Crick found was that every organism carries a chemical code for its own creation inside its cells, a text written in a language common to all life: the simple, four-letter code of DNA. "All the organic beings which have ever lived on this earth have descended from some one primordial form," wrote Darwin. He was, frankly, guessing. To understand the story of evolution—both its narrative and its mechanism—modern Darwins don't have to guess. They consult genetic scripture.

Consider, for instance, the famous finches of the Galápagos. Darwin could see that their beaks were variously shaped—some broad and deep, others elongated, still others small and short. He surmised (somewhat belatedly) that in spite of these differences, all the Galápagos finches were close cousins. "Seeing this gradation and diversity of structure in one small, intimately related group of birds," he wrote in The Voyage of the Beagle, "one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends."

This, too, was inspired guesswork. But by analyzing the close similarity of their genetic codes, scientists today can confirm that the Galápagos finches did indeed descend from a single ancestral species (a bird whose closest living relative is the dull-colored grassquit).

DNA not only confirms the reality of evolution, it also shows, at the most basic level, how it reshapes living things. Recently, Arhat Abzhanov of Harvard University and Cliff Tabin of Harvard Medical School pinned down the very genes responsible for some of those beak shapes. Genes are sequences of DNA letters that when activated by the cell make a particular protein. Abzhanov and Tabin found that when the gene for a protein called BMP4 is activated (scientists use the word "expressed") in the growing jaw of a finch embryo, it makes the beak deeper and wider. This gene is most strongly expressed in the large ground finch (Geospiza magnirostris), which uses its robust beak to crack open large seeds and nuts. In other finches, a gene expresses a protein called calmodulin, which makes a beak long and thin. This gene is most active in the large cactus finch G. conirostris, which uses its elongated beak to probe for seeds in cactus fruit.

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