Around 66 million years ago, at the end of the Cretaceous period, an asteroid the size of Mount Everest smote the Earth. It landed in Mexico’s Yucatan peninsula, punching a 20-mile deep crater into the ground. That impact, and the climatic upheaval that happened afterwards, ended the long reign of the dinosaurs. Of this dynasty of ruling reptiles, only the birds—a specialized group of feathered dinosaurs—survived.
But the birds didn’t escape unscathed.
Birds first appeared around 150 million years ago, during the late Jurassic period. They evolved from small predatory dinosaurs that were similar to Velociraptor. By the end of the Cretaceous, they were flourishing. But the same catastrophe that finished off their dinosaur cousins also killed most of them off. Even incredibly diverse and widespread groups, like the enantiornithines (eh-NAN-tee-OR-nih-theens), died out. The surviving birds were forced to re-evolve much of the diversity that once existed, and most groups of modern birds arose from those survivors, in the aftermath of the asteroid strike.
But which lineages survived, and why?
“A lot of people have focused quite intensively on trying to understand what went extinct [at the end of the Cretaceous],” says Daniel Field, from the University of Bath. “But we know very little about how or why birds managed to sneak across.” In a new study, Field and his colleagues have shown that the species that made it through the extinction event mostly lived on the ground, as modern chickens do today. They walked and strutted into the future, while their relatives that perched in branches and flew through trees largely died out—because many of those branches and trees were on fire.