For three billion years, life on earth consisted of single-celled organisms like bacteria or algae. Only 600 million years ago did evolution hit on a system for making multicellular organisms like animals and plants.

The key to the system is to give the cells that make up an organism a variety of different identities so that they can perform many different roles.

So even though all the cells carry the same genome, each type of cell must be granted access to only a few of the genes in the genome, with all the others permanently denied to it.

People, for instance, have at least 260 different types of cells, each specialized for a different tissue or organ, but presumably each type can activate only some of the 22,500 genes in the human genome.