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Francis Crick winged into the Eagle, a pub popular with researchers at Cambridge University's nearby Cavendish Laboratory, boasting to one and all, "We have found the secret of life." It was early in 1953, and the "we" referred to thirty-six-year-old British biophysicist Crick and twenty-four-year-old American biochemist James D. Watson, then working at the Cavendish on a postdoctoral fellowship. Watson reported feeling "slightly queasy" at Crick's boast, but over the next few decades, many biologists came to see it as fully justified. In one of the great "eureka" experiences of modern science, Watson and Crick had discovered the gene's double-helical structure by brilliantly and rapidly combining the findings of others with insights of their own. This surprisingly simple, highly elegant structure shed new light on the mechanics of evolution by suggesting how genetic reproduction, inheritance, and variation operated at the molecular level.Although the gene stood at the heart of the modern synthesis, it was a black box prior to 1950. Until then, many scientists envisioned the gene as a complex assemblage of proteins that would take decades to decipher. Yet a growing body of evidence suggested that a much simpler macromolecule, deoxyribonucleic acid (or DNA), carries hereditary information. Watson and Crick followed the latter trail, and it led them to glory. They found that DNA is structured somewhat like a twisted railroad track with sturdy rails along its outer edges and a sequence of connecting ties, each composed of one of two different pairings of four base molecules commonly identified by their initials: A, T, G, and C. If DNA splits lengthwise, then each half replicates the whole by attracting new pairs for its remaining bases from the cell's organic soup, A to T and G to C. The macromolecule carries genetic information in the sequence of its base molecules, which serve as a template for forming ribonucleic acid (or RNA) and, in turn, proteins. Information flows only one way in this mechanism—from the DNA to the proteins that construct the organism, never from the organism back to the DNA. The result nicely matches the neo-Darwinian principle that inborn hereditary information guides individual development without any gene-altering feedback from the environment. In these and other respects, DNA structure provides a serviceable molecular foundation for evolution to proceed in a manner fitting the modern synthesis. Both concepts are starkly materialistic and functionally reductionist. Still, tensions developed between molecular biologists and neo-Darwinian evolutionists.
