The gene that, when mutated, causes cystic fibrosis in humans is very similar to the corresponding gene in chimpanzees but is less similar to the corresponding gene in organisms that are less closely related to humans. The height of the green bars shows the similarity of the gene in other organisms to the human gene over a span of 10,000 nucleotides.

of people, on average differ by just a few percent from those of chimpanzees, reflecting our relatively recent common ancestry. But human DNA sequences are increasingly different from those of the baboon, mouse, chicken, and puffer fish, reflecting our increasing evolutionary distance from each of those organisms. Even greater differences in DNA sequences are found when comparing humans to flies, worms, and plants. Yet similarities in DNA sequences can be seen across all living forms, despite the amount of time that has elapsed since

The Evolution of Limbs in Early Tetrapods

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.

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