In 2004, the rebel geneticist Craig Venter took a sailing trip to Bermuda and, unable to resist doing a little research on the side, hauled up 50 gallons of the Sargasso Sea and began trawling it for DNA. It looked for all the world like cold, sterile saltwater, but Mr. Venter had landed a whopper. He found 1.2 million distinct genes in his sample, all new to science. Based on previous research, he knew that none of this DNA came from fish or plants or any other visible life-form. It was all microbial. For perspective, human beings have 23,000 genes; Mr. Venter had uncovered perhaps thousands of new microbes without even...