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The evolution of birds began in the Jurassic Period, with the earliest derived from a clade of theropod named Paraves. Birds are categorized as a biological class, Aves. For more than a century, the small theropod dinosaur Archaeopteryx from the Late Jurassic period was considered to have been the earliest bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked reptile clade, the . Four distinct lineages of bird survived the Cretaceous–Paleogene extinction event 66 million years ago, giving rise to ostriches and relatives (Palaeognathae), waterfowl (Anseriformes), ground-living fowl (Galliformes), and "modern birds" (Neoaves).
Phylogenetics, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, Passer domesticus), and either Archaeopteryx, or some prehistoric species closer to Neornithes (to avoid the problems caused by the unclear relationships of Archaeopteryx to other theropods). If the latter classification is used then the larger group is termed Avialae. Currently, the relationship between non-avian dinosaurs, Archaeopteryx, and modern birds is still under debate.
We have had to stretch the definition of the class of birds so as to include birds with teeth and birds with paw-like fore limbs and long tails. There is no evidence that Compsognathus possessed feathers; but, if it did, it would be hard indeed to say whether it should be called a reptilian bird or an avian reptile.Huxley, T.H. (1876): Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46-138 original text w/ figures
Discoveries in northeast China (Liaoning Province) demonstrate that many small theropod dinosaurs did indeed have feathers, among them the compsognathid Sinosauropteryx and the Dromaeosauridae Sinornithosaurus. This has contributed to this ambiguity of where to draw the line between birds and reptiles.Norell, M & Ellison M (2005) Unearthing the Dragon, The Great Feathered Dinosaur Discovery Pi Press, New York, Cryptovolans, a dromaeosaurid found in 2002 (which may be a junior synonym of Microraptor) was capable of powered flight, possessing a sternal keel and ribs with uncinate processes. Cryptovolans seems to make a better "bird" than Archaeopteryx which lacks some of these modern bird features. Because some basal members of Dromaeosauridae, including Microraptor, were capable of powered flight, some paleontologists have suggested that Dromaeosauridae are actually derived from a flying ancestor, and that the larger members became secondarily flightless, mirroring the loss of flight in modern Palaeognathae like the ostrich.Paul, Gregory S. (2002). Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: Johns Hopkins University Press. 472 pp. The discoveries of further basal dromaeosaurids potentially capable of powered flight, such as Xiaotingia, has provided more evidence for the theory that flight was first developed in the bird line by early dromaeosaurids rather than later by Aves as was previously supposed.
Although (bird-hipped) dinosaurs share the same hip structure as birds, birds actually originated from the (lizard-hipped) dinosaurs if the dinosaurian origin theory is correct. They thus arrived at their hip structure condition independently. In fact, a bird-like hip structure also developed a third time among a peculiar group of theropods, the Therizinosauridae.
An alternate theory to the dinosaurian origin of birds, espoused by a few scientists, notably Larry Martin and Alan Feduccia, states that birds (including "dinosaurs") evolved from early archosaurs like Longisquama. This theory is contested by most other paleontologists and experts in feather development and evolution.
The Cretaceous saw the rise of more modern birds with a more rigid ribcage with a carina and shoulders able to allow for a powerful upstroke, essential to sustained powered flight. Another improvement was the appearance of an alula, used to achieve better control of landing or flight at low speeds. They also had a more derived pygostyle, with a ploughshare-shaped end. An early example is Yanornis. Many were coastal birds, strikingly resembling modern , like Ichthyornis, or ducks, like Gansus. Some evolved as swimming hunters, like the Hesperornithiformes – a group of flightless divers resembling and . While modern in most respects, most of these birds retained typical reptilian-like teeth and sharp claws on the manus.
The modern toothless birds evolved from the toothed ancestors in the Cretaceous. Meanwhile, the earlier primitive birds, particularly the Enantiornithes, continued to thrive and diversify alongside the through this geologic period until they became extinct due to the K–T extinction event. All but a few groups of the toothless Neornithes were also cut short. The surviving lineages of birds were the comparatively primitive Palaeognathae (ostrich and its allies), the aquatic Anseriformes, the terrestrial Galliformes, and the highly volant Neoaves.
The timing of divergence of these major groups are a matter of debate. It is agreed that modern birds originated in the Cretaceous and that the split between Galloanserae and Neoaves occurred before the Cretaceous–Paleogene extinction event, but there are different opinions about whether the radiation of the remaining neognaths occurred before or after the extinction event. This disagreement is in part caused by a divergence in the evidence, with molecular dating suggesting a Cretaceous radiation and the fossil record suggesting a Paleogene radiation. The latest attempts to reconcile the molecular and fossil evidence estimated the most recent common ancestor of modern birds at 95 million years ago and the split between Galloanseres and Neoaves at 85 million years ago. Notably, these studies show that the rapid proliferation of lineages in Neoaves seems to coincide with the Cretaceous–Paleogene extinction event, suggesting a role for ecological opportunity stimulating diversification in the aftermath of the mass extinction.
In contrast, another recent genomic study suggests that the Galloanserae and Neoaves diverged around the Early-Late Cretaceous boundary (100.5 million years ago), with the paleognaths and neognaths diverging even earlier (around 130 million years ago), and that most terrestrial neoavian orders gradually diverged from one another throughout the Late Cretaceous, roughly in sync with the concurrent radiation of Flowering plant. This would suggest that a majority of all terrestrial avian orders coexisted with the non-avian dinosaurs and are K-Pg extinction survivors. In contrast, most major radiations of seabirds and shorebirds (as well as in paleognaths, despite their ancient origins) were found to have only occurred after the K-Pg extinction event, and primarily after the Paleocene–Eocene Thermal Maximum. This clashes with previous studies that found a very rapid radiation of avian orders only after the K-Pg extinction. The results of this study have been disputed by other researchers, due to a lack of fossil evidence to support its conclusions.
The birds that survived the end-of-Cretaceous extinction were likely ground-dwelling (not arboreal) and thus persisted despite the worldwide destruction of forests.
An analysis of the variation of diversification rates through time further revealed a potential effect of climate on the evolution diversification rates in birds in which the generation of new lineages accelerates during periods of global cooling. This can be the result of climate cooling fragmenting tropical biomes and producing widespread allopatric speciation plus an effect of some lineages diversifying in the expanding arid and cool biomes.
Bird skull evolution decelerated compared with the evolution of their dinosaur predecessors after the Cretaceous–Paleogene extinction event, rather than accelerating as often believed to have caused the cranial shape diversity of modern birds. Text and images are available under a Creative Commons Attribution 4.0 International License.
Structural characteristics and fossil records have historically provided enough data for systematists to form hypotheses regarding the Phylogenetics relationships between birds. Imprecisions within these methods is the main factor for why a lack of exact knowledge with regards to the orders and families of birds exists. Expansions in the study of computer-generated DNA sequencing and computer generated phylogenetics has provided a more accurate method for classifying bird species - although DNA data studying can only go so far, and questions are still unanswered.
Another concern with evolutionary implications is a suspected increase in hybridization. This may arise from human alteration of habitats enabling related allopatric species to overlap. Forest fragmentation can create extensive open areas, connecting previously isolated patches of open habitat. Populations that were isolated for sufficient time to diverge significantly, but not sufficient to be incapable of producing fertile offspring may now be interbreeding so broadly that the integrity of the original species may be compromised. For example, the many hybrid found in northwest South America may represent a threat to the conservation of the distinct species involved.
Several species of birds have been bred in captivity to create variations on wild species. In some birds this is limited to color variations, while others are bred for larger egg or meat production, for flightlessness or other characteristics.
In December 2019 the results of a joint study by Chicago's Field Museum and the University of Michigan into changes in the morphology of birds were published in Ecology Letters. The study uses bodies of birds which died as a result of colliding with buildings in Chicago, Illinois, since 1978. The sample is made up of over 70,000 specimens from 52 species and spans the period from 1978 to 2016. The study shows that the length of birds' lower leg bones (an indicator of body sizes) shortened by an average of 2.4% and their wings lengthened by 1.3%. The findings of the study suggest the morphological changes are the result of climate change, demonstrating an example of evolutionary change following Bergmann's rule.
(2025). 9781119020677, Wiley. ISBN 9781119020677
Classification of modern species
The phylogenetic classification of birds is a contentious issue. Charles Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). A preponderance of evidence suggests that most modern bird orders constitute good clades. However, scientists are not in agreement as to the precise relationships between the main clades. Evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged.
Current evolutionary trends in birds
Evolution generally occurs at a scale far too slow to be witnessed by humans. However, bird species are currently going extinct at a far greater rate than any possible speciation or other generation of new species. The disappearance of a population, subspecies, or species represents the permanent loss of a range of genes.
See also
Further reading
(2025). 9780691264639, Princeton University Press. ISBN 9780691264639
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