Scientists' "just so story" about dogs is that their first evolutionary step in our direction came when their ancestors began raiding the village trash heap. Proto-dogs that were less afraid of humans were more successful at the dump, and the inexorable pressures of natural selection took hold.

Every time I take something from the kitchen out to our compost pile, Sadie follows me and checks it out. She has never once found something to eat amid the coffee grounds and avocado skins and corn husks we throw in among the leaves and yard clippings, but she still checks it out with great discipline, a discipline borne of her evolutionary roots.