We still don’t know how animals evolved to fly

Feathered dinosaurs like Microraptor probably shot for the skies over and over. María Jesús Contreras for Popular Science

Dinosaur Mysteries digs into the secretive side of the “terrible lizards” and all the questions that keep paleontologists up at night.

WE STILL LIVE in an age of dinosaurs. Pigeons, penguins, and partridges are all members of the only lineage to survive the asteroid-driven disaster of 66 million years ago. The realization that at least some dinosaurs still flock among us has given a greater depth to paleontology than the field’s founders could have imagined. What we learn about living dinosaurs can help us better understand the species we can touch only as fossils. But even though we can trace the origins of birds from their Velociraptor-like ancestors, there’s one critical part of the story that we don’t fully understand. How on earth did dinosaurs such as Microraptor evolve the ability to fly?

The definition of flight can be a little tricky—it’s not simply about moving through the air. After all, there are marsupials, frogs, snakes, and other animals capable of gliding for impressive distances. Flight is something more specific, requiring the evolution of not only wings but a wing stroke. Watch a raven flap by and you’re watching a dinosaur demonstrate the exact mechanics of keeping itself aloft with one wingbeat after another. The question paleontologists face is how dinosaurs went from terrestrial reptiles scurrying over the ground to feathery, fluttering wonders.

Archaeopteryx lithographica, the earliest recognized bird at about 150 million years old, is of limited help. When the fossil was uncovered in the late 19th century, the splash of feathers found around the Jurassic dinosaur’s bones were quickly taken as an indication that its kind soared over the forests of prehistoric Bavaria. Over time, however, the genus Archaeopteryx started to look more awkward than aerodynamic. The avian ancestor had asymmetrical flight feathers with a shallow leading edge, a critical adaptation for powered flight—but its skeletal anatomy didn’t look capable of flight the way we see it in living birds. The contradiction led to a longstanding debate over whether Archaeopteryx actively flapped into the air, primarily glided, or perhaps even used a different flight stroke from its modern relatives. Whatever the answer, the solution to the mystery can’t be found in its bones alone. And as further feathery dinosaur species have been uncovered, the caper has only grown more complex.

Since the mid-1990s, paleontologists have uncovered dozens of feathery dinosaurs. Many of them are close relatives of Mesozoic birds or otherwise have adaptations related to flight, including the genus Microraptor, which had long feathers on its legs as well as its long arms. In fact, paleontologists think powered flight evolved at least three times among dinosaurs: once among birds and twice among their close dinosaur relatives such as Rahonavis ostromi. That’s not counting the number of feathery species whose anatomy made them more aerodynamically adept than others, but that still weren’t quite capable of keeping themselves aloft by flapping. Instead of a neat, orderly pattern of flight-related traits among birds and their ancestors, the emerging picture shows a tangled mess.

That changes the entire backstory of flying beasts. Up until recently, feathery dinosaurs were cast as representatives of stages in the evolution of flight. Now paleontologists have to figure out how they evolved flight independently multiple times among both birds and feathery nonavian dinosaurs. The path the ancestors of Archaeopteryx took might not be the same as the path taken by predecessors of Microraptor or Rahonavis.

Experts have tossed plenty of ideas about the origins of flight against the proverbial wall. These are broadly divided into “ground-up” and “trees-down” hypotheses, with most paleontologists favoring explanations that focus on how a ground-dwelling, Velociraptor-esque avian ancestor could evolve the ability to fly. Maybe feathery bird ancestors chased insects, leaping after them and trying to trap them with their arm feathers, which would favor dinosaurs able to stay in the air longer. Or maybe flight started with gliding and dinosaurs climbing trees to swoop through the forest, which would give an advantage to those that could flap their arms to soar just a little farther. The behavior of modern birds has provided some clues too, like the way chukar partridges flap their wings to better stabilize themselves while running up inclines.

Every hypothesis about how airborne dinosaurs evolved focuses on the behavior of animals we can’t observe in life. Experts have to draw out what clues they can from feathers, bones, the universal mechanics of flight, and how birds today manage to get into the air and stay there. While it’s possible to conduct wind-tunnel experiments based on skeletal mechanics and other inferred details to calculate how an Archaeopteryx would have fared while flying, there will always be a difference between what a prehistoric species could have done and how it actually behaved back in the Mesozoic. Evolution is not a tidy progression towards a particular outcome, but a story of constant change full of repeats, dead ends, and diversity.

There can’t be a single solution to the puzzle of how dinosaurs evolved to fly because scientists have more than one case to consider. Whether it consists of birds or nonavian dinosaurs, the history of each lineage has to be studied on its own terms. More than that, what seemed like a basic question about the first flying dinosaurs only creates more questions about what led different dinosaurs in different places and times, many miles and millions of years apart, to evolve similar abilities. Pterosaurs—fuzzy, flying reptiles that were related to dinosaurs—reigned over the skies more than 50 million years before Archaeopteryx, so it’s not as if Earth weren’t already full of fliers before dinosaurs caught on. The stories we now deduce of how flying dinosaurs gained their astonishing ability are far more complex than the ones we had even 20 years ago. When you see a house finch alight on a feeder or a turkey vulture slowly turn over a thermal, you’re catching a glimpse of one of the greatest secrets still cached in the fossil record.

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