Molecular biologists have been discovering DNA regions that control the formation of body parts during development. Some of the most important of these DNA regions are known as Hox genes.

Humans and all other mammals have 39 Hox genes. Individual Hox genes control the function of other types of genes, and the same Hox gene can control different sets of genes in different parts of the body.

Hox genes are also involved in the development of many different anatomical features, including limbs, the spine, the digestive system, and the reproductive tract in diverse species of both invertebrate and vertebrate animals. For example, as illustrated in the figure (right side of page), the same Hox genes that control the development of body parts in the fruit fly Drosophila also control the development of body parts in mice and other mammals. Colors indicate the activity of the same Hox gene in both kinds of organisms.

Hox genes also direct the formation of fins in fish and limbs in land-dwelling vertebrates. They are expressed in different patterns in limbed animals, resulting in the formation of fingers and toes. Changes in the expression of these genes were likely involved in the evolution of the early tetrapods, such as Tiktaalik.