In 1837, Charles Darwin opened a notebook and drew a simple tree with a few branches. Each branch, which he labeled with a letter, represented a species. In that doodle, he captured his newfound realization that species were related, having evolved from a common ancestor. Across the top of the page he wrote, “I think.”

Two decades later Darwin presented a detailed account of the tree of life in “On the Origin of Species.” And much of evolutionary biology since then has been dedicated to illuminating parts of the tree. Using DNA, fossils and other clues, scientists have been able to work out the relationships of many groups of organisms, making rough sketches of the entire tree of life.

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