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The scaly, green Tyrannosaurus rex of monster movies is history. The real T. rex was probably covered in a fine feathery fuzz, as were most of the dinosaurs in its family, known as the theropods, which later gave rise to birds. Rich fossil beds in northeastern China have yielded specimens confirming that a wide variety of strictly earthbound dinosaurs sported feathers during the Cretaceous period, some 125 million years ago.
Studying those fossils along with feather development in modern birds has allowed researchers to reconstruct the likely steps in feather evolution. The earliest protofeathers were little more than hollow barbs of keratin, the tough protein that makes up scales, hooves and hair. At some point the barbs developed horizontal ridges that separated into filaments, then split open vertically, resulting in a tassel-like feather. Long, filamentous tail feathers were recently found in a fossil belonging to a dinosaur lineage known as the ornithischians, which diverged from the dinos that would become theropods 70 million years before the Cretaceous—suggesting that feathers could be a very ancient and widespread feature.
The original purpose of plumage might have been simply to provide lightweight warmth, but the vivid hues and patterns seen in modern birds also play a critical role in mating display. Not all feather colors are produced by pigment, however. Nanoscale keratin structures within the feathers trap air and scatter light of certain wavelengths, depending on their shapes—the dark blues of the Eastern bluebird, for instance, result from twisted air channels and keratin bars. Further studies of how these nanostructures self-assemble could yield new techniques for making colored and light-emitting materials.