Origin of birds

The scientific question of which larger group of animals birds evolved within has long interested scientists. Today, most scientists agree that birds are a group of maniraptoran theropod dinosaurs that originated during the Mesozoic era.

A close connection between birds and dinosaurs was first suggested in the nineteenth century after the discovery of the primitive bird Archaeopteryx in Germany. Birds and extinct non-avian dinosaurs share many special features in their skeletons. Scientists have also found fossils of more than thirty species of non-avian dinosaurs that had feathers. Some very small dinosaurs, like Microraptor and Anchiornis, had long feathers on their arms and legs that formed wings. Evidence also shows that birds and dinosaurs shared traits such as hollow, pneumatized bones, stones in their digestive system to help with eating, building nests, and caring for their eggs.

Although there has been debate about the origin of birds, only a few scientists today disagree that birds came from dinosaurs. They suggest birds may have come from other types of archosaurian reptiles. Even among those who agree birds came from dinosaurs, there is still discussion about the exact steps that led to the first birds. How birds first learned to fly is another interesting question with several different ideas about how it happened.

Research history

Scientific study of birds began after Charles Darwin published his famous book in 1859. In 1860, scientists in Germany found a fossilized feather, naming it Archaeopteryx. Soon after, they found a nearly complete skeleton. This animal looked like a bird but had features of reptiles, such as clawed wings and a long tail.

Thomas Huxley, a strong supporter of Darwin’s ideas, noticed that Archaeopteryx looked very much like certain dinosaurs. He suggested birds might have evolved from dinosaurs. Later, in the early 1900s, a scientist named Gerhard Heilmann thought birds came from different ancient reptiles, not dinosaurs, because he couldn’t find certain bones in dinosaur fossils.

In the 1960s, a new dinosaur called Deinonychus was discovered. This helped scientists see many similarities between birds and theropod dinosaurs. In the 1990s, amazing fossils from China showed dinosaurs with feathers, making the link between birds and dinosaurs even clearer.

Phylogeny

Further information: Coelurosauria, Maniraptora, and Avemetatarsalia

Scientists have long wondered where birds came from. Many now believe birds are a special kind of dinosaur that lived long ago. One important fossil, called Archaeopteryx, was found in Germany and looks like a mix between a bird and a dinosaur. This fossil helps scientists understand how birds evolved from dinosaurs. Today, most experts agree that birds are part of a group called coelurosaurian theropod dinosaurs.

Features linking birds and dinosaurs

Many body parts are shared by birds and other meat-eating dinosaurs.

Feathers

Main article: Feathered dinosaur

Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The first fossil was found in the Solnhofen limestone in southern Germany. Archaeopteryx is a special fossil, with body parts clearly in between those of non-bird meat-eating dinosaurs and birds. Since the 1990s, more fossils of feathered dinosaurs have been found, giving even stronger proof of the close link between dinosaurs and modern birds.

Skeleton

Because feathers are often linked with birds, feathered dinosaurs are sometimes called the "missing link" between birds and other dinosaurs. But the many bone parts shared by the two groups give the strongest proof for paleontologists.

Lungs

Heart

Sleeping posture

Reproductive biology

Brooding and care of young

Gizzard stones

Molecular evidence

Origin of bird flight

Main article: Origin of avian flight

Debates about how birds learned to fly have been around almost as long as we’ve known birds came from dinosaurs, starting with the discovery of Archaeopteryx in 1862. Since then, two main ideas have led the discussion. The "from the ground up" theory suggests birds evolved from small, fast land predators that ran. The "from the trees down" theory suggests flight began with gliding from trees. A newer idea, "wing-assisted incline running" (WAIR), combines these, suggesting wings helped dinosaurs run up steep slopes.

In March 2018, scientists shared that Archaeopteryx could likely fly, though not quite like modern birds.

Cursorial ("from the ground up") theory

This theory was first suggested by Samuel Wendell Williston and later expanded by Baron Nopcsa. It suggests that fast-running animals with long tails used their arms to balance. Over time, their arm scales grew into feathers, possibly to help catch insects. As these animals began leaping farther, their wings helped them glide. Nopcsa thought flight developed in three steps: first passive gliding, then flapping flight, and finally soaring.

Recent studies show feathers existed long before flight. Feathers are common in many small dinosaurs, and early birds likely used them for warmth or display. The modern version of this theory says birds came from small, feathered dinosaurs that ran on the ground, using their wings for balance before evolving flight.

Wing-assisted incline running

The wing-assisted incline running (WAIR) idea came from watching young chukar chicks. It suggests wings helped dinosaurs run up steep slopes, like tree trunks, to escape danger. This theory is a type of the "from the ground up" idea. However, early birds like Archaeopteryx lacked the special shoulder setup modern birds use for powerful wing movements, so they probably couldn’t run up slopes this way.

Arboreal ("from the trees down") theory

This theory says bird ancestors were small dinosaurs that lived in trees. They had feathers that later changed to help them glide from branch to branch. Over time, these wings became better for controlling their moves, leading to flight. Several small, feathered dinosaurs found from the Jurassic and Cretaceous periods show signs they might have lived in trees and could glide.

Diminished significance of Archaeopteryx

Archaeopteryx was the first known feathered animal from the Mesozoic era and was central to early debates about bird flight. It had brain structures and feathers like modern birds, but lacked the shoulder setup for strong flapping flight, meaning it might have only glided.

Since the 1990s, many other feathered dinosaurs have been found. These show possible early steps toward flight, making Archaeopteryx less unique in the story of how birds learned to fly.

Secondary flightlessness in dinosaurs

Some scientists suggest certain non-flying meat-eating dinosaurs might have evolved from flying ancestors. However, most recent studies say Archaeopteryx is more closely related to modern birds than to these other dinosaurs. Evidence suggests early dinosaurs might have glided with four wings, and some heavy dinosaurs later lost their ability to fly.

In Euornithes, the earliest clear example of a dinosaur that lost flight is Patagopteryx.