Scientists previously thought that evolution slowly changed the genes that create specific proteins. As the proteins changed, so did the creatures that owned them. The current research shows that opossum and human protein-coding genes have changed little since their ancestors parted ways, 180 million years ago. It has been the regulation of their genes - when they turn on and off - that has changed dramatically.

“Evolution is tinkering much more with the controls than it is with the genes themselves,” said Broad Institute director Eric Lander. “Almost all of the new innovation … is in the regulatory controls. In fact, marsupial mammals and placental mammals have largely the same set of protein-coding genes. But by contrast, 20 percent of the regulatory instructions in the human genome were invented after we parted ways with the marsupial.”
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It had been initially thought that most of a creature’s DNA was made up of protein-coding genes and that a relatively small part of the DNA was made up of regulatory portions that tell the rest when to turn on and off. As studies of mammalian genomes advanced, however, it became apparent that that view was incorrect. The regulatory part of the genome was two to three times larger than the portion that actually held the instructions for individual proteins.