What is the Evolutionary History of Birds?

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Evolutionary history of Birds.

The evolution of birds is thought to have begun in the Jurassic (a geologic period and system that extends from about 155 million years ago, with the earliest birds derived from theropod (a clade consisting of that suborder and its descendants (including modern birds)) dinosaurs. Clade is a group consisting of an organism and all its descendants. Birds are categorized as a biological class, Aves. The earliest known species of class Aves is ‘Archaeopteryx lithographica’, from the Late Jurassic period, though Archaeopteryx (earliest and most primitive bird) is not commonly considered to have been a true 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 Archosauria. The notion that the evolutionary history of birds began with dinosaurs was proposed by a devotee of Darwin, Thomas Henry Huxley, shortly after the 1859 publication of ‘The Origin of Species’, but it wasn't confirmed until the 1960s, when evolutionary biology and comparative anatomy advanced to the point of providing abundant evidence in favor of the relationship.


What is the Origin of Birds?

There is significant evidence that birds evolved from theropod dinosaurs, specifically, that birds are members of Maniraptora (a clade of dinosaurs which includes the birds and the dinosaurs), a group of theropods which includes dromaeosaurs (a family of bird-like theropod dinosaurs) and oviraptorids (a group of bird-like, herbivorous and omnivorous maniraptoran dinosaurs), 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. Recent discoveries in northeast China, demonstrating that many small theropod dinosaurs had feathers, contribute to this uncertainty. The basal bird Archaeopteryx, from the Jurassic, is well-known as one of the first fossiled remains to be found in support of evolution in the late 19th century, though it is not considered a direct ancestor of modern birds. Confuciusornis is another early bird; it lived in the Early Cretaceous (100 mya).


Both may be predated by Protoavis texensis (early bird), though the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor. Other Mesozoic birds include the Confuciusornis, the Enantiornithes (extinct group of primitive birds), Yanornis (an extinct genus of Early Cretaceous bird), Ichthyornis (toothed seabirds), Gansus (a genus of aquatic birds), and the Hesperornithiformes - a group of flightless divers. The recently (2002) discovered dromaeosaur Cryptovolans (a genus of feathered, dromaeosaurid, dinosaur from the Early Cretaceous of China) was capable of powered flight, possessed a sternal keel (an extension of the breastbone) and had ribs with uncinate processes (extensions of bone that in birds project caudally from the vertical segment of each rib). In fact, Cryptovolans makes a better "bird" than Archaeopteryx which lacks some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are actually basal birds whose larger members are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around.


What is the between Dinosaurs, Archaeopteryx, and modern birds?

Aves are 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). Modern birds (subclass Neornithes) are the most recent common ancestor of all living birds (class Aves) and all its descendants. 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. For many decades, the early evolutionary history of birds was unknown and a highly contentious topic. The evolutionary history of birds is long and complex, as they are the latest major branch of the sauropsid group. Previously, it was supposed that birds may have evolved from crocodilians or stem-group archosaurs in the early Mesozoic.


What is the classification of modern Bird species?

The phylogenetic (study of evolutionary relatedness among groups of organisms) classification of birds is a controversial issue. Sibley, an American ornithologist and molecular biologist’s classification of birds and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). Predominant evidences suggest that most modern bird orders constitute good clades. However, scientists are not in agreement as to the precise relationships between the orders; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged. As of the mid-2000s, new fossil and molecular data provide an increasingly clear picture of the evolution of modern bird orders, and their relationships. For example, the Charadriiformes (a diverse order of small to medium-large birds) seem to constitute an ancient and distinct lineage, while the Mirandornithes (a clade that consists of flamingos and grebes) and Cypselomorphae (a clade of birds) are supported by a wealth of anatomical and molecular evidence. On June 27, 2008, the largest study of bird genetics was published. It overturns several hypothesized relationships, and will likely necessitate a wholesale restructuring of the avian phylogenetic tree.


Where do the Flightless birds fit in the Evolutionary history?

Modern birds are classified as ‘Neornithes’, which are now known to have evolved into some basic lineages by the end of the Cretaceous. The Neornithes are split into the paleognaths and neognaths.

  • Paleognathae: 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, rheas, cassowaries, 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 lineage; in any case, the available data regarding their evolution is still very confusing.
  •  Neognathae: The basal divergence from the remaining Neognathes was that of the Galloanserae (fowl, refers to birds belonging to one of two biological orders, namely the gamefowl or landfowl (Galliformes) and the waterfowl (Anseriformes)), the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (chickens, turkeys,pheasants, and their allies).
  • 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 occurred before the K-T extinction event (the Cretaceous–Tertiary extinction event, which occurred approximately 65.5 million years ago was a large-scale mass extinction of animal and plant species in a geologically short period of time), 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 (a geologic period and system). Attempts made to reconcile the molecular and fossil evidence have proved controversial.


How did bird Feathers evolve?

Feathers may have evolved for thermal insulation, for flight, or for display. Early fossil birds had layers of bones that suggest annual growth rings. These suggest that birds were not constant high-temperature animals, not endothermic. The earliest birds look like they could fly (Archaeopteryx). This line of reasoning suggests that feathers evolved for flight rather than for thermoregulation. A third possibility is that they evolved for displays (sex and aggression). Though, there is only less evidence for this, it cannot be ruled out, for there is plenty of comparative evidence in some living reptiles with large flashy scales. Depending on the fossil record, early birds had feathers, but some fossil birds left no impressions of feathers. Also, some dinosaurs that were not birds had feathers. They had vane feathers, feathers with a shaft, barbs and vane. These led to the larger flight feathers of modern birds, which are asymmetric around shaft with the barbs and vane of leading edge shorter. These were used for balance, or warmth, or courtship display. Other pre-bird dinosaurs had insulating feathers (down, with barbs separate not linked together to form an aerodynamic surface). Some dinosaurs had both types of feathers, vane and insulating.


What is the Evolutionary trend in Birds currently?

  • Evolution generally occurs at a scale far too slow to be witnessed by humans. Yet, 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 allegations is a suspected increase in hybridization (sexual reproduction).
  • Forest fragmentation (occurring when forests are cut down in a manner that leaves relatively small) 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.
  • Some species, like the rock pigeon or several species of crows have been successful living in man made environments. 
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