Comparison of the human and chimp DNA sequences for the gene that encodes the hormone leptin (which is involved in the metabolism of fats) reveals only five differences in 250 nucleotides. Where the human and chimpanzee sequences differ, the corresponding nucleotide in the gorilla (shaded bars) can be used to derive the nucleotide that likely existed in the common ancestor of humans, chimpanzees, and gorillas. In two cases, the gorilla and human nucleotides match, while in the other three cases, the gorilla and chimpanzee sequences are the same. The common ancestor of the gorilla, chimpanzee, and human is most likely to have had the nucleotide that is the same in two of the three modern-day organisms because this would require just one DNA change rather than two.

they had common ancestors. Even humans and bacteria share some similarity in DNA sequences in certain genes, and these similarities correspond to molecular systems with similar functions. Biological evolution thus explains why other organisms can be studied to understand biological processes critical to human life. Indeed, much of the biomedical research carried out today is based on the biological commonalities of all living things.

The study of biological molecules has done more than document the evolutionary relationships among organisms. It also can reveal how genetic changes produce new traits in organisms over the course of evolutionary history. For example, molecular biologists have been examining the function of regulatory proteins that cause other genes in a cell to turn on and off as an organism develops from a fertilized egg. Small changes in these proteins, in the DNA regions to which these proteins attach, or even, as recently discovered, in small RNA molecules can have dramatic effects on the anatomy and function of an organism. Such changes could be responsible for some of the major evolutionary innovations that have occurred over time, such as the development

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