How Did Birds Evolve From Dinosaurs?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Birds did not evolve by dinosaurs turning into something else, but by one branch of small feathered theropods gradually acquiring the traits we now think of as bird traits. The evidence is now strong enough that most palaeontologists simply say birds are dinosaurs.
The outline: from theropods to birds
Birds descend from theropods, the group of mostly two-legged meat eaters that includes Velociraptor and Tyrannosaurus. Within that group, a smaller lineage became progressively lighter, more feathered and better at getting off the ground.
The change was gradual and no single fossil marks the moment a dinosaur became a bird. What the fossil record shows instead is a long series of animals with different combinations of the relevant traits, which is exactly what evolution predicts.
The anatomy they share
Many features once thought uniquely avian turn up in non flying dinosaurs. These include hollow bones with air spaces, a wishbone formed from fused collarbones, three main forward facing toes, and a backward folding wrist joint.
That wrist is particularly important. The motion that lets a bird fold its wing against its body is the same motion a small predatory dinosaur used to snap its arms inward when grabbing prey. The flight stroke was built from a hunting movement.
Archaeopteryx and its neighbours
Archaeopteryx, found in Germany and about 150 million years old, is the classic transitional fossil. It has fully formed flight feathers and a wishbone, alongside teeth, a long bony tail and clawed fingers.
It is no longer alone. Chinese fossil beds have produced many small feathered dinosaurs from either side of that transition, including four winged forms with long feathers on both arms and legs. The picture is now a bushy family tree rather than a single line.
How flight got started
There are two long standing ideas about the origin of flight, and the current view borrows from both:
- •From the ground up, with running animals using feathered arms for extra lift and control
- •From the trees down, with climbing animals gliding between branches
- •Wing assisted incline running, seen in living birds, where flapping helps grip steep surfaces
- •Flapping used first for balance and manoeuvring rather than sustained flight
Why birds survived the extinction
Not all birds survived the asteroid impact. Many early bird groups died out with the rest of the dinosaurs, and the survivors were a specific subset: small, beaked, toothless ground feeders that could live on seeds.
Seeds keep. When the sunlight failed and plants stopped growing, a seed eating bird could last where a leaf eater or a hunter could not. From that narrow survival came the eleven thousand or so bird species alive today.
When the idea was first proposed
The connection between birds and dinosaurs was suggested by Thomas Henry Huxley in the 1860s, shortly after Archaeopteryx was found, on the basis of skeletal similarities he noticed in a comparison of bones.
The idea then fell out of favour for most of a century, largely due to an influential argument that dinosaurs lacked collarbones and so could not be bird ancestors. That claim turned out to be simply wrong, and fossil discoveries from the 1970s onwards revived Huxley's position, which is now the mainstream view. It is a useful case study in how a correct hypothesis can be sidelined for decades by a single flawed objection.
The takeaway
Birds evolved from small feathered theropod dinosaurs through a long series of gradual changes, and the survival of a few seed eating species through the mass extinction is why dinosaurs are still with us.