Evolution of birds

Birds evolved from small carnivorous dinosaurs by the development of feathers into flight structures. The earliest known bird is the Late Jurassic Archaeopteryx, but almost immediately, birds became extremely diverse and numerous, with many of the modern bird groups being present by the Late Cretaceous period.

Theories of Bird Origins

The major groups of living birds have fossil records that can be traced back to the Early Tertiary period (later than 65 million years ago) and, in some cases, to the Late Cretaceous period (up to 75 million years ago). However, the first known fossil bird, Archaeopteryx, is from the Late Jurassic period (about 150 million years ago), evolving from theropod dinosaurs called Paraves, showing that almost half of the history of bird evolution had taken place before the modern groups appeared.

Interest in bird origins was sparked by the discovery in 1860 of a solitary bird feather in the Solnhofen Limestone of Bavaria, which is about 150 million years old (Late Jurassic). This was one year after the publication of Charles Darwin's On the Origin of Species by Means of Natural Selection (1859) and thus came at an opportune time in the study of evolution. Shortly after this, several skeletons were discovered in the same deposits, and the presence of feather impressions made it clear that these were the remains of true birds. The animal was named Archaeopteryx lithographica and is still the most primitive known member of the birds. Although it had feathers and wings and could fly, it also had several unbirdlike features, such as a toothed jaw and a long, bony tail.

Thomas Henry Huxley was the first person to make a connection between birds and dinosaurs. In 1870, he noted similarities between the hind legs of the theropod (carnivorous dinosaur) Megalosaurus and those of the ostrich and concluded that they were closely related. This link was contested on the grounds that both Megalosaurus and the ostrich were large and bipedal and that the similarities in leg structure might be caused by a similar mode of life. It was also pointed out that dinosaurs were even larger than ostriches and that none of them had been able to fly, so they seemed rather unlikely bird ancestors.

In 1926, Gerhard Heilmann's book The Origin of Birds renewed interest in the idea. He showed that birds were anatomically closer to theropods than any other group, except for the lack of clavicles in theropods—the structures that fuse in birds to form the wishbone. Since other reptiles had clavicles and theropods did not, he concluded that they had become lost secondarily in theropods. Therefore, they could not be the ancestors of birds, and the similarities must be because they were bipedal and shared a similar lifestyle. Surprisingly, information was available at that time to show that dinosaurs did have clavicles, as this structure had been illustrated in a description of the theropod Oviraptor, although it had been misidentified. Other views on the origin of birds were put forward during the next forty years, including the ideas that they had been derived from crocodiles, that they had been derived from an early archosaur (the group from which both dinosaurs and crocodiles are derived), and even that they were closely related to mammals. However, the idea of a bird-dinosaur relationship was revived by John H. Ostrom of Yale University in the 1960s. He had described a sickle-clawed theropod, Deinonychus, a predator from Montana's Early Cretaceous period (115 million years ago). Deinonychus's skeletal anatomy showed several features shared with birds—including Archaeopteryx—and other theropods but not with other reptiles. Based on these findings, Ostrom concluded that birds are descended from small theropod dinosaurs.

When Ostrom put forward his evidence for the theropod origin of birds, a new method of analyzing relationships between organisms was being developed. Called “cladistics,” this method groups organisms entirely on the presence of shared characters that are particularly informative. Thus, as evolution proceeds, new heritable traits emerge and are passed on; hence, two groups of animals sharing such new traits will be more closely related to each other than groups that share only the original traits. Analyses of these characters are presented as treelike diagrams called “cladograms” showing the order in which the new characters, and thus the new creatures, evolved. Each branching point in the diagram reflects the emergence of an ancestor that founded a group having advanced characters not present in groups that developed earlier.

Although Ostrom did not use cladistics during his analysis of bird-dinosaur relationships, a study carried out ten years later by Jacques A. Gauthier of the University of California at Berkeley showed that birds were most closely related to small carnivorous dinosaurs and particularly to sickle-clawed theropods such as Deinonychus.

Development of Birdlike Characteristics

Analysis of the features considered “birdlike” has shown that many of them appeared early in the history of bird ancestors and then were adapted later to support flight and an arboreal way of life. Some of these were already present in the earliest dinosaurs; for example, bipedality (the ability to walk on two limbs) and an upright stance were already present in the immediate ancestors of dinosaurs. In addition, dinosaur ancestors had a hingelike ankle joint and foot bones that were elongated so that they walked on their toes and placed their feet in line when walking. The earliest theropods had hollow bones and a lightened skull, features that provided a light skeleton as in birds. They also had long necks and held their backs horizontally as birds do. Many birdlike features of the limbs developed during the evolution of the theropods, including the reduction of the number of digits from five to three. Archaeopteryx also showed this feature, although in the later history of birds, the fingers fused together. The legs of early theropods were long, with a short thigh and long shin as is seen in birds, and the foot was birdlike, with only the three inner toes used for walking.

During theropod evolution, the arm became progressively longer, and the wrist became more flexible—features that were useful for prey capture but were also an advantage later in the development of flight. The shoulder girdle was also strengthened by elongation of the scapula (shoulder blade) and the coracoid (which forms part of the shoulder joint), as well as by the fusion of the clavicles (the collarbones) in the midline to form the wishbone. The breastbone, which was originally made of cartilage, formed a bony plate in advanced theropods. These features provided a stronger skeleton for the dinosaurs but would later form a solid support for the flight muscles in birds. It is also clear that the tail became shorter and stiffer in the theropods and that muscles that once attached between the leg and the tail later connected the pelvis to the tail, thus becoming ideally positioned for tail control during flight in birds.

Behavioral traits also seem to have developed first in the ancestors of birds. One of the most vivid examples is seen in a small theropod from the Gobi Desert called the Oviraptor (“egg stealer”), which was so named because it likely preyed on other dinosaurs' eggs. However, one specimen was found sitting on a nest full of eggs, apparently brooding them when it was overwhelmed by sand and died.

Origins of Feathers and Flight

A major diagnostic feature of birds is the presence of feathers. It is thought that these developed originally from long scales in the reptilian ancestors of birds. They may originally have served as heat shields to reduce heat flow into the body and then, after fraying along the edges, as an insulating covering to retain heat in these small, probably endothermic animals (animals whose internal temperature is maintained by food consumption). Evidence for the presence of feathers in dinosaurs was provided in 1996 and 1997 by Ji Qiang and Ji Shuan of the National Geological Museum of China. They published reports on fossils from Late Jurassic or Early Cretaceous rocks in Liaoning Province, one of which, named Sinosauropteryx, bears filamentous fringes along its back and on its body that could have consisted of precursors of feathers. However, Sinosauropteryx has short arms and other characteristics indicating that it is not a close relative of birds. A second animal, Protoarchaeopteryx, has short true feathers on its body and longer ones on its tail. Skeletal features indicate that it is probably a theropod closely related to birds.

There are two main views on how flight developed in the ancestors of birds. The arboreal hypothesis suggests that ancestral birds started by gliding from branches with the help of incipient flight feathers and then graduated to flapping flight as the feathers enlarged. Although this is an intuitively attractive idea, it is not supported by any arboreal adaptations in theropods or in Archaeopteryx. The cursorial hypothesis suggests that, as small dinosaurs chased prey along the ground, they developed lift from feathers along their arms as they stretched or jumped. As the feathers enlarged, lift would have increased incrementally until sustained flight was achieved. This hypothesis is supported by the terrestrial mode of life of theropods and by the fact that the structure of the arm in these dinosaurs, developed to grab prey, is preadapted to create the flight stroke in birds.

Later Evolution of Birds

Little was known of ancestral birds other than Archaeopteryx until the late 1970s, when members of a new group of birds, the Enantiornithes, were found in northwestern Argentina. The name means “opposite birds” because their ankles are fused at the opposite end from those of modern birds. They are now known from Spain, China, Mongolia, Madagascar, Argentina, North America, and Australia. They varied in size from vultures to sparrows and had teeth and claws on their wings in some cases, but they were all extremely capable fliers. A specimen from Spain with feather impressions shows the presence of an alula, a tuft of feathers on the first digit that is a key to maneuverability at slow speeds and is found in all modern birds. This 115-million-year-old bird, named Eoaluluavis, or “dawn alula bird,” represents the oldest record of modern flight.

Although Enantiornithes became extinct at the end of the Cretaceous (65 million years ago), at least four major lineages of living birds, including ancient relatives of shorebirds, seabirds, loons, ducks, and geese, were already present before the end of the Cretaceous. Evidence from calculated divergence times using deoxyribonucleic acid (DNA) studies suggests that others were also present. Thus, a picture is developing of a major radiation of birds through the Cretaceous while dinosaurs were already thriving in terrestrial environments. Although some bird groups did become extinct at the end of the Cretaceous (most notably the Enantiornithes), the overall picture of bird extinctions is spotty because of the low potential for preservation of small, fragile bird skeletons. Birds that frequent aquatic environments, where suitable environments for preservation exist, are better represented in the fossil record than other groups, and it will take many more discoveries to complete the picture of bird evolution.

One such discovery was published in a longitudinal study in 2019, which revealed that climate change is causing North American migratory birds to shrink. From 1978 to 2016, scientists collected over 70,000 birds of fifty-three species in Chicago, Illinois, that died after flying into buildings. Comparing these specimens revealed that, over forty years, their lower leg bones shrank by about 2.4 percent while their wings grew longer by 1.3 percent. This finding has important implications for understanding the evolution of birds and for understanding the implications of climate change on all living things.

Significance

Birds—an important and diverse group of organisms—are currently the most completely known of modern vertebrates. They pose several intriguing questions concerning the origins of flight and their relationship with other major groups. The earliest known bird, Archaeopteryx, from the Late Jurassic of Germany, could already fly, as demonstrated by the presence of flight feathers on the wings. However, it also retained a set of non-birdlike characters that link it closely to theropod dinosaurs. Discoveries from China have shown that small feather-covered dinosaurs were present at about the same time, and further discoveries from South America and Spain have shown that birds were diverse and numerous shortly afterward. The method by which flight developed in birds is still debated, as is the exact group from which they developed and the time of divergence.

Principal Terms

Archaeopteryx: the oldest known bird, fossils of which were discovered in the Upper Jurassic Solnhofen Limestone of Bavaria in 1860

cladistics: a method of analyzing biological relationships in which advanced characters of organisms are used to indicate closeness of origin

Cretaceous period: a period that lasted from about 146 to 65 million years ago, the end of which was marked by the extinction of the dinosaurs

Enantiornithes: a diverse group of Cretaceous birds that filled many of the avian niches now inhabited by modern bird groups

Jurassic period: a period of geological time that lasted from about 208 to 146 million years ago, during which birds originated

theropods: the group of carnivorous dinosaurs from which birds developed

Bibliography

Alexander, David E. On the Wing: Insects, Pterosaurs, Birds, Bats and the Evolution of Animal Flight. Oxford UP, 2015.

Benton, Michael J. Vertebrate Palaeontology. 4th ed., Blackwell Science, 2015.

Chiappe, Luis M. Glorified Dinosaurs: The Origin and Evolution of Birds. John Wiley and Sons, 2007.

Dyke, Gareth, and Gary Kaiser. Living Dinosaurs: The Evolutionary History of Modern Birds. John Wiley & Sons, 2011.

Foth, Christian, and Oliver W. M. Rauhut. The Evolution of Feathers from Their Origin to the Present. Springer, 2020.

Hand, Carol. The Evolution of Birds. Essential Library, an imprint of Abdo Publishing, 2019.

Kroodsma, Donald E., and Edward H. Miller. Ecology and Evolution of Acoustic Communication in Birds. Cornell UP, 2020.

"The Origin of Birds." Berkeley, evolution.berkeley.edu/what-are-evograms/the-origin-of-birds. Accessed 20 July 2024.

Padian, Kevin, and Luis M. Chiappe. “The Origin of Birds and Their Flight.” Scientific American, Feb. 1998, pp. 28-37.

Sayol, Ferran, et al. "Anthropogenic Extinctions Conceal Widespread Evolution of Flightlessness in Birds." Science Advances, vol. 6, no. 49, 2020. doi:10.1126/sciadv.abb6095.

Wicander, Reed, and James S. Monroe. Historical Geology. 8th ed., Brooks/Cole, Cengage Learning, 2016.