Birds evolved from dinosaurs, based on the information from both fossils and DNA which has helped largely to clarify the relationship between birds and dinosaurs and help define the timeline over which birds most likely originated and diversified from dinosaurs. Here we discuss some some history of evolution
Birds are theropod dinosaurs
When dinosaur originated?
Nearly all experts now are confident that birds are the living descendants of dinosaurs, a diverse group of reptiles that originated about 230 million years ago and subsequently became the dominant group of land animals until their mass extinction about 66 million years ago that ended the Cretaceous Period (The Cretaceous is a geological period that lasted for about 110 million years from about 145 to 66 million years ago. It is the third and final period of the Mesozoic Era, also the longest).
Birds and dinosaur earlier link
Earlier researchers often suggested that birds might be more closely related to various non‐dinosaur groups of reptiles or that birds represent a very early offshoot of the dinosaurs. Many lines of evidence including some traits seen in fossils and shared aspects of their inferred physiology and behavior now firmly support the idea that birds form a subgroup of the dinosaurs, that arose fairly late from within the broader dinosaur radiation.
Even though birds are traditionally classified into a Class Aves (Warm-blooded vertebrates, characterised by feathers, toothless beaked jaws, the laying hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong but lightweight skeleton) that is separate from the Class Reptilia (Reptiles, are a group of tetrapods with an ectothermic metabolism and amniotic development. Living reptiles comprise four orders: Crocodilia, Testudines, Squamata, and Rhynchocephalia), in evolutionary terms dinosaurs and birds are both more accurately considered a subset of the reptiles, a group that also includes living crocodiles, turtles, snakes, and lizards. Reptile diversity was even higher in the past because the group includes not only all dinosaurs but also several other long-extinct lineages such as ichthyosaurs, plesiosaurs, and flying pterosaurs.
Therefore, it is technically correct to say that “birds are living dinosaurs” or that “birds are reptiles,” but it is incorrect to claim as it still happens in the popular media that these opinions are scientifically controversial.
Does some researchers think birds are not dinosaurs?
Their cornerstone argument was based on the opinion that dinosaurs had lost the fragile bones called clavicles, which are present in more ancient groups of birds and also in reptiles, which have fused clavicles that form the furcula, or wishbone. Under the assumption that birds would not have re‐evolved a complex structure that was lost in their immediate ancestors, birds were thought to be descended from one of the older, non‐dinosaur reptile groups with clavicles. For 50 years, this targeted focus on clavicles and a few other similar traits have distracted researchers from the many other features shared by birds and dinosaurs.
Clavicles of birds in dinosaurs
At the same time, new fossil discoveries expanded our knowledge diversely in the area of both dinosaurs and early birds.
A critical later finding was that dinosaurs did have clavicles and that the fusion of clavicles into a furcula is a trait found not only in birds but also widely found in the theropod group of dinosaurs that includes the well‐known tyrannosaurs, among others. The presence of clavicles was overlooked earlier because these fragile bones do not fossilize well, and because some dinosaur clavicles were misidentified.
Modern birds and theropod dinosaur
A debate has persisted longer over which subgroup within the dinosaurs gave rise to birds. Earlier it was thought that modern birds were more closely related to crocodiles than to dinosaurs, but this possibility has been refuted by new fossil evidence. Alternatively, birds might be embedded deeply within the theropod dinosaurs, within the dinosaurs, or within or near one of the other dinosaur groups.
Variants of this latter possibility have long been championed by the paleontologist, among most other avian paleontologists there now is nearly universal agreement that birds originated from within the theropods.
History of theropod dinosaurs
The earliest theropod dinosaur fossils date from about 225 million years ago the Late Triassic Period, where several major groups of dinosaurs first arose. Throughout the subsequent 160 million years of the Cretaceous and Jurassic Periods, theropods were the dominant terrestrial carnivores. The most well-known theropod species was the fearsome Tyrannosaurus rex.
However, although many theropods were huge meat eaters, the theropods also included a diverse range of small-bodied dinosaurs, some of which were likely insectivores or herbivores. Within the bipedal theropods, birds probably arose within a group termed the Maniraptora, which was distinguished by bird‐like features such as long arms. Fossil evidence suggests that some non‐avian maniraptorans could also fly or glide; indeed, because of their similarities to birds, there is still debate about whether some maniraptoran fossils should perhaps be reclassified as early birds. More generally, however, the lineage that ultimately gave rise to modern birds likely was just one of many similar maniraptoran lineages that had features once thought to be found just in birds and their direct ancestors.
From the broader archosaur group which once included the non-avian dinosaurs and other extinct groups, only birds and crocodiles have survived to the current day. Therefore, among living organisms, crocodiles and birds are each other’s closest present‐day relatives.
Feathers and other dinosaur traits in birds
Birds and theropod dinosaurs share many traits, some of which are also found in other dinosaur groups. From a historical perspective, feathers are one of the most surprising features that birds and other dinosaurs share in common because the presence of feathers was long thought to be a trait that unambiguously differentiated birds from all other animals.
Starting in the 1990s, an important series of fossil discoveries produced increasingly clear evidence that most lineages of theropod dinosaurs sported feather‐like quills and plumes. Evidence of feathers now has been found in dozens of fossil theropods from China as well as a few from North America and Europe; feather‐like structures also have been identified in some non‐theropod dinosaur fossils. Although today feathers are a feature restricted to living birds, in the past feathers were widespread across the dinosaur ancestors and relatives of birds.
In addition to feathers, birds and theropod dinosaurs share many different morphological traits, including the furcula formed by fused clavicles. Like birds, some theropods had bones that were tube‐shaped and hollow, or pneumatized. Feathers and pneumatized bones were long considered avian adaptations arising from selection for flight, but we now know that these features evolved in non‐flying dinosaurs, the ancestors of birds. Instead, the hollow bones of theropods may be evidence of a respiratory system in which the spaces within the bones serve as part of the air-sac network, similar to that of modern birds. Comparative studies suggest that pneumatized bones evolved uniquely in different groups of theropods; in the largest such dinosaurs, these bones may help to reduce the weight of the huge skeletal structures needed to support their massive bodies. In the non-avian theropod lineages related to birds, these lighter bones and the air spaces were more likely related to an active, fast-moving lifestyle.
Although no ancient dinosaur DNA survives to be analyzed, fossil evidence suggests that birds and theropods share the trait of having a small genome relative to most other vertebrates. In an elegant paleontological detective story, researchers realized that in living vertebrates, the size of certain bone cells correlates strongly with the size of an animal’s genome: larger genomes result in larger bone cells. The diameter of these bone cells can be measured in some particularly well‐preserved dinosaur fossils, which shows that genome size became reduced during the early evolution of theropods. This reduction is however another trait that birds probably inherited from their theropod ancestors.
Birds and the other theropod dinosaurs further share many physiological and behavioral traits:
An upright, bipedal posture; the rapid growth rate of young individuals- als; and details of the structures of their calcified eggs. Fossil trackways of dinosaur footprints and fossilized theropod nests show that at least some theropods were social and they likely guarded or incubated their eggs, similar to most modern birds. Debate persists about whether these other theropod dinosaurs, like modern birds, were endothermic metabolically able to maintain their core body temperature at a certain level higher than their surroundings but it is also possible that they share this trait.