When Charles Darwin was thrashing out his theory of evolution, he would doodle sometimes in his notebooks. To explain how new species came into existence, he wrote down letters on a page and then connected them with branches. In the process, he created a simple tree. Across the top of the page, he wrote, “I think.”

That single tree has given rise to the thousands of trees that are published in scientific journals these days. A particular tree may show that humans are more closely related to chimpanzees than gorillas. It might show how the SARS virus in humans descends from viruses in other animals.

When you look at the picture of a tree in a scientific paper, it is