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Figure 2. Molecular cagesClick to Enlarge Image

Hemoglobin—the oxygen-transport protein that gives blood its red color—got its start at about the time life originated on earth, nearly four billion years ago. Now it is almost ubiquitous, appearing in the cells of plants, animals and even bacteria, and a study of this protein affords scientists a rare glimpse back as well as forward in time. A look at the ancestral hemoglobins indicates that newly arising proteins co-opt the chemistry of older ones and gain new functions through structural alterations. But these studies have revealed an additional way to modify function. Scientists are coming to the realization that changes in a protein's regulation—the when and how of its expression—can also give rise to functional differences. The surprise, says the author, is that these regulatory changes outpace structural ones—an important lesson for students of molecular evolution and a possible indicator of where protein evolution will go in the future.

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