Marriage, we’re told by the president and a lot of other people, can only be between one man and one woman. Anything else would go against thousands of years of tradition and nature itself. If the president’s DNA could talk, I think it might disagree.

In the 1980s, geneticists began to study variations in human DNA to learn about the origin of our species. They paid particular attention to the genes carried by mitochondria, fuel-producing factories of the cell. Each mitochondrion carries its own small set of genes, a peculiarity that has its origins over two billion years ago, when our single-celled ancestors engulfed oxygen-breathing bacteria. When a sperm fertilize an egg, it injects its nuclear DNA, but almost never