Because life on
Earth developed only once all organisms are interrelated, and birds are no exception. The closest living relatives of birds are crocodiles, alligators and caimans (Crocodilia). The group uniting
birds and crocodilians is referred to as Archosauria. Turtles and lizards are the next taxa most closely related to Archosauria, respectively.
Dating back to the year 1758 when the tenth edition of the
Systema Naturae of the famous Carl Linnaeus was published, birds are referred to by zoologists as Aves. There is, however, a dissent among
present-day scientists on the proper definition of the term. Personally, I prefer considering Aves as being represented by the last (i.e. the most recent) common ancestor of all extant birds and
all its descendants. According to this definition, the term Aves is synonymous to modern (or crown) birds. For historical reasons, however, many avian researchers still use the term Aves in a
broader sense to include the famous Archaeopteryx and other bird-like fossils; in this case modern birds are referred to as
Fossils not included in modern or crown birds but still closer to this group than to any other group of
living organisms are treated as members of the avian stem group. Stem-group fossils either pertain to the stem line itself, in which case they represent direct ancestors of the crown birds, or to
extinct side branches. Stem and crown birds together constitute the Pan-Aves, or total birds. Pan-Aves originated ~250 million years ago (mya), when the last common archosaurian ancestor split into two species,
one giving rise to the crocodilian and the other to the bird lineage.
Reconstructions of phylogenetic relationships among representatives of stem
groups are exclusively based on fossilized bones. Nuclear and mitochondrial DNA sequences, routinely used to reconstruct relationships among living groups, are usually not available. As a
consequence, inferring relationships among members of the avian stem-group is inherently problematic and the figure below depicts just one possible hypothesis. Despite this limitation, a
relatively coherent picture of stem avian evolution has developed over the last few decades:
Reconstruction of the evolutionary history of birds. Evolution from the earliest avian ancestors towards modern birds progresses along the avian stem-line (red) from Pan-Aves towards Aves. A number of stemline clades havn't been named yet (n.n.). For some clades the supposed age (in million years: Ma) is given in parentheses (green). Note that the popular Dinosauria clade comprises modern birds and almost all stem-birds (except Pterosauromorpha, Lagerpetonidae and Marasuchus). This means that dinosaurs didn't become extinct (a popular misconception), but are represented in the present-day fauna by modern birds. In fact, dinosaurs probably never were as diverse as they are today being reperesented by approximately 10.500 bird species.
The successive transformation into more and more bird-like
creatures started with the enlargement of the rear limbs. Initially, stem-birds possibly ran on their hind legs only for short periods, while moving on all four legs most of the time.
Bipedalism may have evolved in order to enhance the acceleratory phase when trying to escape from predators. In the course of evolution, stem-birds became lighter and
smaller, showing increased levels of activity and higher body temperatures. All living birds have body temperatures above 40° Celsius.
Along with increased body-temperature birds developed proto-feathers for better insulation and possibly display. Later, the forelimbs
became significantly longer than the hindlimbs, and the long bony tail disappeared.
Ultimately, complex contour feathers developed and birds learned to fly as an effective way of escaping from earth-bound predators. Considering the clumsy flight of extant tinamous and landfowl, avian flight almost
certainly evolved from the ground with a subsequent gliding phase.
Early dinosaurs had a global distribution. However,
since almost all non-avian fossils of the Eumaniraptora (with the remarkable exception of
Enantiornithes and Patagopteryx) were found in North America and Eurasia, it seems plausible to assume that eumaniraptorans evolved in the northern
hemisphere after the supercontinent Pangaea broke apart into a northern (Laurasia) and a southern (Gondwana) hemisphere.
Agnolin, F.L., Motta, M.J., Brissón Egli, F., Lo Coco, G. and Novas, F.E. (2019), Paravian phylogeny and the dinosaur-bird transition: an overview, Front.
Earth Sci. 6: 252. DOI: 10.3389/feart.2018.00252. (pdf)
Baron, M.G., Norman, D.B. and Barrett, P.M. (2017), A new hypothesis of dinosaur relationships and early dinosaur evolution, Nature 543, 501-506. DOI:
Barrett, P.M., Evans, D.E. and Campione, N.E. (2015), Evolution of dinosaur epidermal structures, Biol. Lett. 11, 20150229. DOI: 10.1098/rsbl.2015.0229.
Bell, A., and L.M. Chiappe (2015), A species-level phylogeny of the Cretaceous Hesperornithiformes (Aves: Ornithuromorpha): Implications for body size
evolution amongst the earliest diving birds. Journal of Systematic Palaeontology: 1. (abstract)
Benson, R.B.J. (2018),
Dinosaur macroevolution and macroecology. Annu. Rev. Ecol. Evol. Syst. 49, 379-408. DOI:
Brusatte, S.L. (2013), The phylogeny of basal coelurosaurian theropods (Archosauria: Dinosauria) and patterns of morphological evolution during the
dinosaur-bird transition. PhD thesis at the Columbia University. (pdf)
Brusatte, S.L., G. Lloyd, S. Wang, and M. Norell (2014), Gradual assembly of avian body plan culminated in rapid rates of evolution across dinosaur-bird
transition. Current Biology 24, 2386-2392. DOI 10.1016/j.cub.2014.08.034. (pdf)
Brusatte, S.L., J.K. O'Connor, and E.D. Jarvis (2015), The origin and diversifiaction of birds. Current Biology 25, R888–R898. (pdf)
Cau, A., T. Brougham, and D. Naish (2015), The phylogenetic affinities of the bizarre Late Cretaceous Romanian theropod Balaur bondoc (Dinosauria,
Maniraptora): dromaeosaurid or flightless bird? PeerJ 3:e1032. DOI
Chatterjee, S. (2015), The Rise of Birds: 225 Million Years of Evolution. John Hopkins University Press, Baltimore, 2nd edition. (link)
Clemente, C.J., P.C. Withers, G. Thompson, and D. Lloyd (2008), Why go bipedal? Locomotion and morphology in Australian agamid
lizards, J.Exper. Biol. 211, 2058-2065. DOI: 10.1242/jeb.018044. (pdf)
Crawford, N.G., B.C. Faircloth, J.E. McCormack, R.T. Brumfield, K. Winker, and T.C. Glenn (2012), More than 1000 ultraconserved elements provide
evidence that turtles are the sister group to archosaurs. Biol. Lett. DOI: 10.1098/rsbl.2012.0331 (pdf)
Gauthier, J., and K. de Queiroz (2001), Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name "Aves", In: New Perspectives on the Origin and
Early Evolution of Birds, Proc. Int. Symp. in Honour of John H, Ostrom, J. Gauthier, and L.F. Gall (eds), New Haven, Yale University Press, 7-41. (pdf)
Huang, J., X. Wang, Y. Hu, J. Liu, J.A. Peteya, and J.A. Clarke (2016), A new ornithurine from the Early Creataceous of China sheds light on the evolution of
early ecological and cranial diversity in birds, PeerJ 4:e1765. (pdf)
Ksepka, D.T. (2014), Evolution: a rapid flight towards birds. Current Biology 24, 1052-1055. http://dx.doi.org/10.1016/j.cub.2014.09.018. (pdf)
Langer, M.C., M.D. Ezcurra, J.S. Bittencourt, and F.E. Novas (2010), The origin and early evolution of dinosaurs. Biol. Rev. 85, 55-110. (abstract)
Langer, M.C., M.D. Ezcurra, J.S. Bittencourt, O.W.M Rauhut, M.J. Benton, F. Knoll, B.W. McPhee, F.E. Novas, D. Pol and S.L. Brusatte (2017), Untangling
the dinosaur family tree. Nature 551, E1-E3
page. DOI: 10.1038/nature24011. (abstract)
Lee, M.S.Y., C. Cau, D. Naisch, and G.J. Dyke (2014), Morphological clocks in paleontology, and a Mid-Cretaceous origin of crown-Aves, Syst. Biol. 63(3),
Makovisky, P.J., and L.E. Zanno (2011), Theropod diversity and the refinement of avian characteristics. In: Living Dinosaurs: The Evolutionary History of
Modern Birds, Chaper 1, Dyke, G. and Kaiser, G. (eds). John Wiley & Sons, Ltd. (link)
Marsola, J.C.A., and M.C. Langer (2018), Dinosaur origins. Model. Earth Sys. Environ., available online 29 May 2019. DOI: 10.1016/B978-0-12-409548-
Nesbitt, S.J. (2011), The early evolution of archosaurs: relationships and the origin of major clades. Bull. Am. Mus. Nat. Hist., 352, 292 pp. (abstract)
O´Connor, J.K., D.Q. Li, M.C. Lamanna, M. Wang, J.D. Harris, J. Atterholt, and H.-L. You (2015), A new Early Cretaceous enantiornithine (Aves,
Ornithothoraces) from Northwestern China with elaborate tail ornamentation, J. Vertebr. Paleontol. 36(1): e1054035. 2016. (abstract)
Parry, L.A., M.G. Baron, and J. Vinther (2017), Multiple optimality criteria support Ornithoscelida. R. Soc. open sci. 4, 170833. DOI:
Pol, D. and O.W.M. Rauhut (2012), A Middle Jurassic abelisaurid from Patagonia and the early diversification of theropod dinosaurs. Proc. R. Soc. (B) 279,
Rauhut, O.W.M., and C. Foth (2020), The origin of birds: current consensus, controversy, and the occurrence of feathers. In: The Evolution of Feathers, pages
27-45, Foth, C. and Rauhut, O. (eds), Springer, Cham. (link)
St.John, J.A. et al. (2012), Sequencing three crocodilian genomes to illuminate the evolution of archosaurs and amniotes. Genome Biology 13, 415. (pdf)
Wang, M., X. Zheng, J.K. O'Connor, G.T. Lloyd, X. Wang, Y. Wang, X. Zhang, and Z. Zhou (2015), The oldest record of Ornithuromorpha from the early
Cretaceous of China. Nature Communications, 6: 6987 | DOI: 10.1038/ncomms7987. (pdf)
Wang, Y.-M., J.K. O´Connor, D.-Q. Li, and H.-L. You (2015), New information on postcranial skeleton of the Early Cretaceous Gansus yumenensis (Aves:
Ornithuromorpha), Historical Biology 28(5), 1-14. (abstract)
Wang, M., Z. Zhou, and S. Zhou (2015), A new basal ornithuromorph bird (Aves: Ornithothoraces) from the Early Cretaceous of China with implication for
morphology of early Ornithuromorpha, Zool. J. Linn. Soc. 176, 207-223. (pdf)
Wang, M., and H. Hu (2017), A comparative morphological study of the jugal and quadratojugal in early birds and their dinosaurian relatives, Anatom. Rec
300, 62-75. (pdf)
Wang, M., T.A. Stidham, and Z. Zhou (2018), A new clade of basal Early Cretaceous pygostylian birds and developmental plasticity of the avian shoulder
girlde, PNAS, 115(42), 10708-107113. DOI: 10.1073/pnas.1812176115. (pdf)
Xu, X, Z. Zhou, R. Dudley, S. Mackem, C.-M. Chuong, G.M. Erickson, and D.J. Varricchio (2014), An integrative appproach to understanding bird origins,
Science 346, 1341-1352. (abstract)
Zhou, Z. (2014), Dinosaur evolution: feathers up for evolution. Current Biology 24, 751-753. DOI: 10.1016/j.cub.2014.07.017. (pdf)