How Dinosaurs Started to Fly

When you see a tiny hummingbird flitting around a flower, you probably don’t say, “Wow, what a cute dinosaur!” But you should. Today’s birds are descended from a group of two-legged, meat-eating dinosaurs called theropods.

Probably the most famous theropod is the huge Tyrannosaurus rex, but birds evolved from smaller theropods — ones that were ostrich or pony-sized, not school bus-sized like a T. rex. Today, the connection is so well understood that scientists refer to birds as “avian dinosaurs.” (“Avian” means something related to birds.)

For many years, birds’ origins were a great mystery. But in the 1860s, a very curious fossil was discovered in Germany. It was of a strange creature, about the size of a large crow, and it had feathers along its arms and tail.

Many people didn’t know what to make of it. The fossil dated back about 150 million years to the Jurassic period, when dinosaurs ruled the earth. No birds were known to have lived back then. (Some people even thought the fossil might be of an angel!) But scientists who studied the fossil classified it as a bird — the earliest bird — and named it Archaeopteryx (ark-ee-OPT-er-ix), which in ancient Greek means “ancient feather or wing.”

Not all scientists were convinced that Archaeopteryx was a bird. Yes, it had feathers and wings. But it also had teeth, claws, and a long, bony tail like a dinosaur. It turns out that this half-dinosaur, half-bird fossil was a huge find — the first discovery of a transitional creature linking modern, living animals with distant ancestors that looked very different from them.

The discovery came at a perfect time.

Just a couple of years earlier, scientist Charles Darwin had explained how animals and other living things slowly change, or evolve, over time through a process called natural selection. It works like this: Living things pass along various traits — how they look, are built, and behave — from one generation to the next through their genes. As genes get copied, sometimes mistakes are made, creating variations in traits. Some changes don’t matter much, others are harmful, and some are helpful — like making an animal faster, so it can catch its next meal more easily or escape being eaten itself.

When a new trait gives an animal an advantage over others of its kind, that animal has a better chance of surviving, reproducing and passing its genes along to its offspring. That’s why Darwin’s theory is often called “survival of the fittest,” with the “fittest” organisms having the traits best suited to their environments. And if an environment changes, the traits that give an organism an advantage will gradually evolve, too.

Over time, a series of small changes adds up, often resulting in an organism that looks nothing like its distant ancestors — like modern birds and their reptilian ancestors, the dinosaurs.

In the past three decades, scientists — especially paleontologists (PALE-ee-on-TOL-uh-jists), who study life in the past — have found additional fossils and learned more about the dinosaur-bird transition than they had in the previous two centuries. They discovered that all the important features of modern birds evolved one by one, often for different purposes, and that the transition from dinosaur to bird was so smooth that it is hard to identify the actual “first bird.”

For many years, people thought that only birds had feathers. But in the 1990s, paleontologists discovered that feathers emerged long before the evolution of birds — and early feathers didn’t even help the animals fly. Like all evolution, it took a series of gradual changes to create flight feathers.

At first, some small theropods 125 million years ago grew short, hair-like feathers on their heads, necks, and bodies — mainly to keep them warm. These feathers were different colors and arranged in various patterns, so they also may have helped the dinosaurs hide, recognize their own kind, or find mates. The small, non-avian theropod Sinosauropteryx, for example, rocked reddish-orange and white stripes!

Over time, some theropods grew fluffy or downy feathers, while others evolved a central quill — the hollow shaft or stalk of a feather — with fuzzy branches shooting off the sides. The hollow shafts may have dispersed heat, keeping dinosaurs from getting too hot. Later, the feathers filled in, becoming larger, lighter and more flexible. Even then, feathers were not just for flying. Fossils showed that 75 million years ago, Oviraptors spread their feathered arms around their egg-filled nests, much like chickens today keep their eggs warm.

Eventually, feathers helped dinosaurs get airborne. The early feathers probably just helped little dinosaurs leap into the air to catch flying insects. But larger feathers made flight possible — first helping the dinosaurs scramble up into trees, then glide from branch to branch. By the time Archaeopteryx came around, it sported large flight feathers on its wings, similar to those on modern birds. Though Archaeopteryx could not soar like an eagle, it was capable of short bursts of powered flight, like today’s pheasants. These abilities gave avian dinosaurs a huge advantage, because they could reach new habitats and food sources and escape predators more easily.

Feathers alone did not help the avian dinosaurs take to the skies. Their bodies also had to change. Here again, the changes probably served different purposes at first. The avian dinosaurs’ wrist joints changed so they could flex sideways — great for whipping their arms around to snatch prey and essential, later, for tucking their wings at rest and flapping more powerfully during flight. Meanwhile, their bodies grew smaller and their bones lighter, their arms became longer than their legs, and they evolved feathers on a shortened tail to help in flight.

Despite the wonder of wings, it may have been the development of beaks that helped some birds to survive the mass extinction 65 million years ago. Non-avian dinosaurs, toothed birds, and pterosaurs — the huge, flying reptiles that dominated the skies for millions of years — all went extinct then. Why did the beaked birds survive? It was not flight that gave them the crucial advantage then, but food. With their beaks, they could pluck up nuts and seeds from the ground — probably the only foods that survived the destruction caused by a massive asteroid impact, the cause of the mass extinction. Animals that needed to eat leafy vegetation or meat were out of luck.

Beaked birds were just one tiny twig on the dinosaurs’ branch of the tree of life, but they were the only dinosaurs to survive. Over time, they have evolved and diversified into around 10,000 species of modern birds today.