The diversity of life we see today is the result of evolutionary processes playing out over geologic time. Evolutionary process can be studied by experiment, both in the field and in the laboratory, but evolutionary history is another matter. There is no experiment we can do to determine why the dinosaurs became extinct—we cannot rerun the events of 66 million years ago, this time without the meteorite impact. The history of life must be reconstructed from evolution’s two great patterns: the nested similarity observed in the forms and molecular sequences of living organisms, and the direct historical archive of the fossil record.