Evolution of birds
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The evolution of birds began in the
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
Period, with the earliest
bird Birds are a group of s constituting the Aves , characterised by s, toothless beaked jaws, the of eggs, a high rate, a four-chambered , and a strong yet lightweight . Birds live worldwide and range in size from the to the . There ar ...

bird
s derived from a
clade A clade (), also known as a monophyletic group or natural group, is a group of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell ...

clade
of
theropod Theropoda ( from Ancient Greek, Greek 'wild beast' and 'foot'), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three-toed limbs. Theropods are generally classed as a group of saurischian d ...
dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) an ...

dinosaur
s named
Paraves Paraves are a widespread group of theropod dinosaurs that originated in the Late Jurassic period. In addition to the extinct dromaeosaurids, troodontids, anchiornithids, and scansoriopterygidae, scansoriopterygids, the group also contains th ...
. Birds are categorized as a biological class, Aves. For more than a century, the small theropod dinosaur ''
Archaeopteryx lithographica
Archaeopteryx lithographica
'' from the
Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period (geology), Period, and it spans the geologic time scale, geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratu ...
period was considered to have been the earliest bird. Modern phylogenies place birds in the dinosaur clade
Theropoda Theropoda ( from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mil ...

Theropoda
. According to the current consensus, Aves and a sister group, the
order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, and the habit of achieving a ...
Crocodilia Crocodilia (or Crocodylia, both ) is an Order (biology), order of mostly large, predatory, List of semiaquatic tetrapods, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous Period (g ...

Crocodilia
, together are the sole living members of an unranked "
reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtl ...

reptile
" clade, the
Archosaur Archosauria ("ruling reptiles") is a clade A clade (), also known as a monophyletic group or natural group, is a group of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. ...
ia. Four distinct lineages of bird survived the
Cretaceous–Paleogene extinction event near Drumheller Drumheller is a town within the Red Deer River The Red Deer River is a river in Alberta ("Strong and free") , image_map = Alberta in Canada 2.svg , Label_map = yes , coordinates ...
66 million years ago, giving rise to ostriches and relatives (
Paleognathae Palaeognathae , or paleognaths (from Ancient Greek ''palaió-'' "old" + ''gnáthos'' "jaw") is one of 2 living clades of birds – the other being Neognathae. Together, these two clades form the clade Neornithes. Palaeognathae contains five Extan ...
), ducks and relatives (
Anseriformes Anseriformes is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, ...

Anseriformes
), ground-living fowl (
Galliformes Galliformes is an order (biology), order of heavy-bodied ground-feeding birds that includes turkey (bird), turkeys, chickens, Old World quail, quails, and other landfowl. Gallinaceous birds, as they are called, are important as seed dispersers a ...

Galliformes
), and "modern birds" (
Neoaves Neoaves is a clade that consists of all modern Bird, birds (Neornithes or Aves) with the exception of Paleognathae (ratites and kin) and Galloanserae (ducks, chickens and kin). Almost 95% of the roughly 10,000 known species of modern birds belong ...
).
Phylogenetically In biology, phylogenetics (from Greek language, Greek wikt:φυλή, φυλή/wikt:φῦλον, φῦλον () "tribe, clan, race", and wikt:γενετικός, γενετικός () "origin, source, birth") is a part of systematics that addresse ...

Phylogenetically
, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the
house sparrow The house sparrow (''Passer domesticus'') is a bird Birds are a group of warm-blooded vertebrates constituting the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * ...

house sparrow
, ''Passer domesticus''), and either ''Archaeopteryx'', or some prehistoric species closer to
Neornithes Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

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 dinosaurs, ''Archaeopteryx'', and modern birds is still under debate.


Origins

There is significant evidence that birds emerged within
theropod Theropoda ( from Ancient Greek, Greek 'wild beast' and 'foot'), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three-toed limbs. Theropods are generally classed as a group of saurischian d ...

theropod
dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 annum, million years ago, although the exact origin and timing of the evolution o ...

dinosaurs
, specifically, that birds are members of
Maniraptora Maniraptora is a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all its lineage ( ...
, a group of theropods which includes
dromaeosaur Dromaeosauridae is a family (biology), family of feathered Theropoda, theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period (geology), Period. The name Dromaeosauridae means ...

dromaeosaur
s and oviraptorids, among others. As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes less so. This was noted in the 19th century, with
Thomas Huxley Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist specialising in comparative anatomy. He is known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution. The stories regard ...
writing:
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 46-13
original text w/ figures
/ref>
Discoveries in northeast
China China (), officially the People's Republic of China (PRC; ), is a country in . It is the world's , with a of more than 1.4 billion. China spans five geographical and 14 different countries, the in the world after . Covering an area of ap ...

China
(
Liaoning Liaoning (), is a coastal province in Northeast China Northeast China, is a geographical region of China. It usually corresponds specifically to the three province A province is almost always an administrative division within a country ...
Province) demonstrate that many small theropod dinosaurs did indeed have feathers, among them the
compsognathid Compsognathidae is a family of coelurosaurian theropod dinosaurs. Compsognathids were small carnivores, generally conservative in form, hailing from the Jurassic and Cretaceous Period (geology), Periods. The bird-like features of these species, al ...
''
Sinosauropteryx ''Sinosauropteryx'' (meaning "Chinese reptilian wing", ) is a compsognathid dinosaur Dinosaurs are a diverse group of reptiles of the clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural g ...

Sinosauropteryx
'' and the
microraptoria Microraptoria (Greek, μίκρος, mīkros: "small"; Latin, raptor: "one who seizes") is a clade of basal Dromaeosauridae, dromaeosaurid theropod dinosaurs. The first microraptorians appeared 125 million years ago in China. Many are known for long ...
n Dromaeosauridae, dromaeosaurid ''Sinornithosaurus''. This has contributed to this ambiguity of where to draw the line between birds and reptiles. ''Cryptovolans'', a dromaeosaurid found in 2002 (which may be a junior synonym of ''Microraptor'') was capable of powered flight, possessed a sternal keel (bird), keel and had ribs with uncinate processes of ribs, uncinate processes. ''Cryptovolans'' seems to make a better "bird" than ''Archaeopteryx'' which lacks some of these modern bird features. Because of this, some paleontologists have suggested that
dromaeosaur Dromaeosauridae is a family (biology), family of feathered Theropoda, theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period (geology), Period. The name Dromaeosauridae means ...

dromaeosaur
s are actually basal birds whose larger members are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is currently inconclusive, but digs continue to unearth fossils (especially in China) of feathered dromaeosaurs. At any rate, it is fairly certain that flight utilizing feathered wings existed in the mid-Jurassic theropods. The Cretaceous Unenlagiinae, unenlagiine ''Rahonavis'' also possesses features suggesting it was at least partially capable of powered flight. Although ornithischian (bird-hipped) dinosaurs share the same hip structure as birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs if the dinosaurian origin theory is correct. They thus arrived at their hip structure condition Analogy (biology), 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 maniraptoran "dinosaurs") evolved from early archosaurs like ''Longisquama''. This theory is contested by most other paleontologists and experts in feather development and evolution.


Mesozoic birds

The basal bird ''Archaeopteryx'', from the
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
, is well known as one of the first "Transitional fossil, missing links" to be found in support of evolution in the late 19th century. Though it is not considered a direct ancestor of modern birds, it gives a fair representation of how flight evolved and how the very first bird might have looked. It may be predated by ''Protoavis, Protoavis texensis'', though the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor. The skeleton of all early bird candidates is basically that of a small theropod dinosaur with long, clawed hands, though the exquisite preservation of the Solnhofen Plattenkalk shows ''Archaeopteryx'' was covered in feathers and had wings. While ''Archaeopteryx'' and its relatives may not have been very good fliers, they would at least have been competent gliders, setting the stage for the evolution of life on the wing. The evolutionary trend among birds has been the reduction of anatomical elements to save weight. The first element to disappear was the bony tail, being reduced to a pygostyle and the tail function taken over by feathers. ''Confuciusornis'' is an example of their trend. While keeping the clawed fingers, perhaps for climbing, it had a pygostyle tail, though longer than in modern birds. A large group of birds, the Enantiornithes, evolved into ecological niches similar to those of modern birds and flourished throughout the Mesozoic. Though their wings resembled those of many modern bird groups, they retained the clawed wings and a snout with teeth rather than a beak in most forms. The loss of a long tail was followed by a rapid evolution of their legs which evolved to become highly versatile and adaptable tools that opened up new ecological niches. The Cretaceous saw the rise of more modern birds with a more rigid ribcage with a keel (bird anatomy), 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 shorebirds, like ''Ichthyornis'', or ducks, like'' Gansus''. Some evolved as swimming hunters, like the Hesperornithiformes – a group of flightless divers resembling grebes and loons. 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 pterosaurs through this geologic period until they became extinct due to the Cretaceous–Paleogene extinction event, K–T extinction event. All but a few groups of the toothless
Neornithes Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

Neornithes
were also cut short. The surviving lineages of birds were the comparatively primitive
Paleognathae Palaeognathae , or paleognaths (from Ancient Greek ''palaió-'' "old" + ''gnáthos'' "jaw") is one of 2 living clades of birds – the other being Neognathae. Together, these two clades form the clade Neornithes. Palaeognathae contains five Extan ...
(ostrich and its allies), the aquatic Anseriformes, duck lineage, the terrestrial Galliformes, fowl, and the highly volant
Neoaves Neoaves is a clade that consists of all modern Bird, birds (Neornithes or Aves) with the exception of Paleognathae (ratites and kin) and Galloanserae (ducks, chickens and kin). Almost 95% of the roughly 10,000 known species of modern birds belong ...
.


Adaptive radiation of modern birds

Modern birds are classified in
Neornithes Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

Neornithes
, which are now known to have evolved into some basic lineages by the end of the Cretaceous (see ''Vegavis''). The Neornithes are split into the paleognaths and neognaths. The paleognaths include the tinamous (found only in Central and South America) and the ratites, which nowadays are found almost exclusively on the Southern Hemisphere. The ratites are large flightless birds, and include ostriches, Rhea (bird), rheas, Cassowary, cassowaries, Kiwi (bird), kiwis and emus. A few scientists propose that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages. In any case, the available data regarding their evolution is still very confusing, partly because there are no uncontroversial fossils from the Mesozoic. Phylogenetics, Phylogenetic analysis supports the assertion that the ratites are polyphyletic and do not represent a valid grouping of birds. The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the
Anseriformes Anseriformes is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or waste, ...

Anseriformes
(ducks, goose, geese and swans), and the
Galliformes Galliformes is an order (biology), order of heavy-bodied ground-feeding birds that includes turkey (bird), turkeys, chickens, Old World quail, quails, and other landfowl. Gallinaceous birds, as they are called, are important as seed dispersers a ...

Galliformes
(chickens, Turkey (bird), turkeys, pheasants, and their allies). The presence of basal anseriform fossils in the Mesozoic and likely some galliform fossils implies the presence of paleognaths at the same time, in spite of the absence of fossil evidence. The dates for the splits are a matter of considerable debate amongst scientists. It is agreed that the Neornithes evolved in the Cretaceous and that the split between the Galloanserae and the other neognaths – the
Neoaves Neoaves is a clade that consists of all modern Bird, birds (Neornithes or Aves) with the exception of Paleognathae (ratites and kin) and Galloanserae (ducks, chickens and kin). Almost 95% of the roughly 10,000 known species of modern birds belong ...
– occurred before the
Cretaceous–Paleogene extinction event near Drumheller Drumheller is a town within the Red Deer River The Red Deer River is a river in Alberta ("Strong and free") , image_map = Alberta in Canada 2.svg , Label_map = yes , coordinates ...
, but there are different opinions about whether the radiation of the remaining neognaths occurred before or after the extinction of the other dinosaurs. This disagreement is in part caused by a divergence in the evidence, with molecular dating suggesting a Cretaceous radiation, a small and equivocal neoavian fossil record from Cretaceous, and most living families turning up during the Paleogene. Attempts made to reconcile the molecular and fossil evidence have proved controversial. One hypothesis as to how modern birds survived the Cretaceous–Paleogene extinction event, Cretaceous–Paleogene mass extinction when other dinosaur species did not could be related to their ability to adaptively radiate. Due to the fact that the avian ancestors of modern birds did not take up all of the niche space where other species did fill up their niche space, birds could have been able to produce a higher level of ecological diversity and innovation that helped them to faster adapt to different environments. These rates of evolution could in part be due to their small body sizes. The authors of a May 2018 report in ''Current Biology'' think that the birds that survived the end-of-Cretaceous disaster were Neornithes, Neognathae (Galloanserae + Neoaves), not tree-living, and could not fly far, because of the worldwide destruction of forests and that it took a long time for the world's forests to return properly. Virtually the same conclusions were already reached before, in a 2016 book on avian evolution. In August 2020 scientists reported that bird skull evolution decelerated compared with the evolution of their
dinosaur Dinosaurs are a diverse group of reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) an ...

dinosaur
predecessors after the Cretaceous–Paleogene extinction event, rather than Evolutionary radiation, accelerating as often believed to have caused the cranial shape diversity of modern birds.


Classification of modern species

The phylogenetic scientific classification, classification of birds is a contentious issue. Charles Sibley, 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. Structural characteristics and fossil records have historically provided enough data for systematists to form hypotheses regarding the Phylogenetics, phylogenetic 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.


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. Another concern with evolutionary implications is a suspected increase in Hybrid (biology), 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 hummingbirds 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 of Natural History, 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 span 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.


See also

* Aurornis * Lack's principle * Origin of avian flight * Origin of birds * Survival of the fittest *Genomic evolution of birds


References


Further reading

* Jarvis, Eric D., et al. "Whole-genome analyses resolve early branches in the tree of life of modern birds." ''Science'' 346. December 12, 2014, pp. 1320-1331. * * {{Authority control Evolution of birds, Prehistoric birds, . Jurassic events, Birds