... The tree-of-life concept was absolutely central to Darwin's thinking, equal in importance to natural selection...Without it the theory of evolution would never have happened. The tree also helped carry the day for evolution. ... For much of the past 150 years, biology has largely concerned itself with filling in the details of the tree. "For a long time the holy grail was to build a tree of life," says Eric Bapteste, an evolutionary biologist at the Pierre and Marie Curie University in Paris, France. A few years ago it looked as though the grail was within reach. But today...