How Did Birds Evolve From Dinosaurs?

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.

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, 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.

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

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.

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.

Feathers appeared long before flight. In 1996, fossil beds in Liaoning, China, yielded Sinosauropteryx, a small ground dwelling predator covered in simple filaments, the first non-avian dinosaur found with a feather-like covering. Many others have since turned up, some with simple fuzz, others with branched feathers too short or too symmetrical for flying. This suggests feathers first evolved for other purposes, such as insulation, display or brooding eggs, and were later taken over for flight. Filament-like coverings have even been reported in some plant-eating dinosaurs far from the bird line, though how they relate is still debated.

Fossils can preserve more than shape. Under powerful microscopes, some fossil feathers contain tiny structures called melanosomes, which hold pigment in living birds. Because their shape relates to colour, researchers have proposed colour patterns for several feathered dinosaurs, including a reddish brown tail with pale bands for Sinosauropteryx. Bone tissue offers another link. Female birds build a special calcium store called medullary bone inside their limb bones when laying eggs, and similar tissue has been reported in some non-avian dinosaurs. Each line of evidence is independent, which is why the combined case is so strong.

A common mistake is to picture pterosaurs, the winged reptiles of the dinosaur age, as early birds. They were not. Pterosaurs evolved powered flight separately, and earlier than birds, using a wing of skin stretched mainly along a hugely lengthened fourth finger, rather than feathers on a whole arm. They were close relatives of dinosaurs but not dinosaurs themselves, and they left no living descendants. Flight has evolved independently at least three times among vertebrates, in pterosaurs, birds and bats, each building a wing from a modified forelimb in its own way.

Among living animals, the closest relatives of birds are crocodiles and alligators. Both belong to the archosaurs, the group that also included dinosaurs and pterosaurs, and comparing them helps scientists infer what extinct dinosaurs were like. Crocodilians and birds both have a four-chambered heart, build nests and guard their eggs or young. Studies of alligators have even found that air flows through parts of their lungs in one direction, a feature once thought special to birds. Traits shared by both branches were probably present in their common ancestor, and so likely in dinosaurs too.

You can still see traces of the dinosaur past in living birds. The hoatzin, a leaf-eating bird of South American swamps and rivers, hatches with two claws on each wing. Its chicks use them to clamber through branches above the water, and if threatened they can drop into the water, swim, and climb back up again. The claws are lost as the bird matures. Some other birds, including ostriches, keep small claws on their wings into adulthood, a reminder of the clawed fingers seen in Archaeopteryx and its relatives.

Embryos tell a similar story. A bird's tail begins as a longer row of vertebrae, and the last several fuse during development into a short stub called the pygostyle, which anchors the tail feathers. Birds also still carry broken versions of genes once used to make tooth enamel, disabled many millions of years ago. In 2015 researchers who altered certain chemical signals in chicken embryos produced faces with rounder, snout-like structures in place of the usual beak, closer in shape to the faces of small feathered dinosaurs and early birds.

Behaviour can fossilise too, and one case corrected a famous mistake. In 1924 a toothless, beaked dinosaur found in Mongolia lying on a nest of eggs was named Oviraptor, meaning egg thief, because researchers assumed the eggs belonged to the horned dinosaur Protoceratops and that it had died raiding them. In the 1990s, expeditions to the Gobi Desert found an embryo of an oviraptor-like dinosaur inside one of these eggs, showing they were its own kind. They also found an adult of a related species, Citipati, preserved sitting on a nest with its arms spread around the eggs, just as a brooding bird does.

Arms held in that posture would have shielded the eggs with feathers, which suggests that brooding, and feathered arms used much like wings, existed before flight. Other fossils show similar continuity. A small Chinese dinosaur named Mei long, described in 2004, was preserved curled up with its head tucked beside its body and its legs folded beneath it, the roosting posture many birds use to conserve heat while they sleep. Studies of nests and bones also suggest that in some of these dinosaurs the fathers did much of the incubating, as male emus and several other ground nesting birds do today.

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.