Credit: Katherine Taylor for Quanta Magazine

Quanta Magazine has an interesting article on the evolution of birds.  It’s pretty commonly known that birds are dinosaurs, theropod dinosaurs.  That group includes some huge monsters like Tyrannosaurus rex.  Even the closest relatives to birds were quite large, at 100 to 500 pounds.  With the greater understanding of genetics and evo-devo, the story of how a line of dinosaurs shrank and evolved into birds is becoming much clearer.

For decades, paleontologists’ only fossil link between birds and dinosaurs was archaeopteryx, a hybrid creature with feathered wings but with the teeth and long bony tail of a dinosaur. These animals appeared to have acquired their birdlike features — feathers, wings and flight — in just 10 million years, a mere flash in evolutionary time. “Archaeopteryx seemed to emerge fully fledged with the characteristics of modern birds,” said Michael Benton, a paleontologist at the University of Bristol in England.

To explain this miraculous metamorphosis, scientists evoked a theory often referred to as “hopeful monsters.” According to this idea, major evolutionary leaps require large-scale genetic changes that are qualitatively different from the routine modifications within a species. Only such substantial alterations on a short timescale, the story went, could account for the sudden transformation from a 300-pound theropod to the sparrow-size prehistoric bird Iberomesornis.

But it has become increasingly clear that the story of how dinosaurs begat birds is much more subtle. Discoveries have shown that bird-specific features like feathers began to emerge long before the evolution of birds, indicating that birds simply adapted a number of pre-existing features to a new use. And recent research suggests that a few simple change—among them the adoption of a more babylike skull shape into adulthood—likely played essential roles in the final push to bird-hood. Not only are birds much smaller than their dinosaur ancestors, they closely resemble dinosaur embryos. Adaptations such as these may have paved the way for modern birds’ distinguishing features, namely their ability to fly and their remarkably agile beaks. The work demonstrates how huge evolutionary changes can result from a series of small evolutionary steps.