The jaw-dropping diversity of life begs for explanation, but at the same time it defies description, so much so that it has inspired hyperbole, even hyperbole of the finest variety.

It’s easy to think of a genome as a collection of genes, perhaps because so many of the metaphors used to explain genes and genomes (blueprint, book of life, Rosetta Stone) can give one the impression that everything in a genome is useful or functional. But genomes are, in fact, packed with debris.

What is the evidence for common descent? To even ask the question, it seems to me, is to suppose that common descent is a proposal, or a hypothesis, and that a certain body of evidence supports the proposal. And that, of course, is quite true: common descent is a scientific theory, and a certain body of evidence supports that theory.