The first big clue to the origin of birds came in 1861, when quarry workers in Solnhofen, Germany, found a spectacular fossil of a 145-million-year-old bird that came to be called Archaeopteryx. It had feathered wings like living birds, but also had traits found in reptiles, such as teeth, claws and a long bony tail.

Charles Darwin, who had published “On the Origin of Species” two years earlier, was delighted. Archaeopteryx looked like what Darwin would have predicted if birds had evolved from reptilian ancestors. “It is a grand case for me,” he told a friend.

Grand as it might have been, Archaeopteryx did not close the case. It did not, for example, reveal which group of reptiles gave rise to birds, nor did it track how those ancestors evolved wings from nothing.

In the 1970s, John Ostrom, a paleontologist at Yale University, identified similarities in the skeletons of birds and ground-running dinosaurs called theropods, a group that includes the Velociraptor and the Tyrannosaurus rex. But no theropod fossils preserved wings, let alone feathers. Without more evidence, Dr. Ostrom and other paleontologists argued fiercely about the origin of birds for decades.