Woese developed a new method to compare a type of genetic material called rRNA across bacterial species. In 1977, he revealed that he had successfully used this method to trace the path of bacterial evolution, finding an entirely new domain in the process — a whole branch of the tree of life that others had missed altogether.

At the time, scientists grouped all life into two main categories: the bacteria and the non-bacteria, or eukaryotes. Woese exploded that view. His research showed that there is a third group of organisms, the archaea, which are genetically distinct from other life-forms.

Previously, scientists had lumped the archaea in with the bacteria. Woese proved that archaea and bacteria had sufficiently divergent evolutionary histories to be considered different from one another.

Woese’s discovery was controversial at the time, though it is no longer. And his work laid the foundations for a new field of science called molecular phylogeny that has delivered astounding insights about evolutionary history.

Quammen details how scientists have now revealed genetic evidence that the branches of the tree of life are not so isolated from one another as Darwin thought. Organisms from different branches can transfer genes to each other — and often do.

Bacteria, plants and animals can swap genes across species lines, and this activity seems to have shaped evolution in profound ways. Scientists call this swapping “horizontal gene transfer,” to distinguish it from the “vertical gene transfer” that occurs between parents and offspring. Horizontal gene transfer explains why bacteria acquire resistance to antibiotics so quickly, for instance: They can transfer antibiotic resistance genes back and forth among species relatively easily.

But horizontal gene transfer also poses a major challenge to the Darwinian concept of evolution, in which species evolve over time into separate lineages. Darwin thought that evolutionary change happened very slowly, as incremental genetic changes were passed down, over hundreds of thousands of generations, from parents to children. And while this slow change does happen, gene-swapping via horizontal gene transfer happens much faster, driving major changes relatively quickly.