The study of an organism's total complement of genetic material, called its genome, has become indispensable for shedding light on its biochemistry, physiology, and patterns of inheritance. Even more can be gained by comparing the genomes of multiple organisms to discern how their DNA sequences have changed over evolutionary time. This technique has become increasingly valuable with the explosion of genome sequencing activity in recent years. Today, hundreds of complete or near-complete genome sequences, ranging from simple microbes to human, have been deposited in scientific databases around the world.

All life on Earth has a common history, reflected by its common biochemical basis in DNA. Different organisms vary in their DNA sequences, of course, but perhaps not so much as one might think. Some of the genes controlling very basic biological tasks, such as the mechanism by which DNA is transcribed into RNA to code the...