Recent studies have identified important genes that direct embryonic development. Specific developmental regulators control the formation of heads and tails, backs and bellies, forelimbs and hindlimbs, and the left and right sides of the body. Many key developmental genes are conserved among animals that look very different. A diversity of body forms can emerge from changing where and when these shared developmental regulators are expressed. For example, fins and limbs have been extensively modified in many different animals. Major changes in the fins of stickleback fish occur by altering the expression pattern of a major developmental control gene involved in hindlimb development. Intriguingly, fish evolving independently in widely separated waters have alterations in the same basic genetic and developmental elements. Fossils suggest that similar developmental mechanisms were used in animals that evolved millions of years ago. The great extent of shared developmental machinery reveals a deep common ancestry for living forms and makes it possible to discover general rules of evolution from highly detailed studies of select organisms.