Modern birds are very different from their vertebrates. Until recently, their origin was one of the great secrets of biology. Feathers, toothless beaks, hollow and deep sternum bones are only part of the special features that other animals do not have. Birdwatching doesn’t explain how they got feathers and flying ability. Over the past decades, new discoveries and new research methods have appeared that have enabled paleontologists to make several discoveries. Fossils found in the region of China and South America, as well as samples stored in museums, which can now be studied using new technologies, have caused yet another wave of interest in studying the history of bird evolution. Thanks to this, a theory of their origin from small predatory dinosaurs that lived on Earth at the end of the Jurassic period has previously appeared.
Beginning of bird research
Scientists began to speculate on the evolution of birds, starting from the time that Charles Darwin presented his theory of evolution in the work “On the Origin of Species”. In 1861, a year after the publication of the Darwin tract, an ancient bird feather was found in Bavarian limestone sediments, which were about 150 million years old. The following year, the skeleton of an animal was found that had bird wings and feathers, a very long bony tail and a jaw with teeth. It was found in the same region. The petrified skeleton was called Archeopteryx. He became the first found oldest animal with plumage. The skeletal anatomy of archeopteryx gave clear evidence that dinosaurs were the ancestors of birds, but in 1861, scientists still could not establish this connection. Then began the study of the evolution of birds and the hunt for the ancestors of modern species of birds.
The anatomical structure of archeopteryx
The fossil was discovered in the early 1860s. For a long time, the famous find from the Late Jurassic was unique. She was the only one who could give some information about how the evolutionary transition from reptiles to birds could occur, since it combined the characteristics of both birds and reptiles. The structure of the archeopteryx had much in common with the structure of birds. For example, feathers along the front legs, which became wings. But, unlike modern species, teeth and a bony tail were also present.
The first theories about jurassic animals
It is worth noting that many of the bones of the animal, including the forepaws, shoulder girdle, pelvis and legs, were distinct, not fused, like its descendants. A few years later, Thomas Henry Huxley became the first scientist to find a connection between the structure of birds and dinosaurs. He compared the hind limbs of a giant dinosaur with an image of an ostrich and noted 35 signs proving that they are related to each other. Huxley presented his findings to the Geological Society. But in 1870 in London, paleontologist Harry Govier Seeley decided to challenge the hypothesis of the origin of birds and their relationship with dinosaurs. Seeley suggested that the hind limbs of an ostrich and a dinosaur might look similar only because these animals were large and bipedal, and their hind limbs were used under similar conditions. In addition, dinosaurs were even larger than ostriches, and none of them could fly. The scientist had a question, how then can birds fly if they evolved from a dinosaur.
Gerhard Heilman Theory
The mystery of the origin of birds again interested scientists after about half a century. In 1916, Gerhard Heilmann, a medical doctor with an interest in paleontology, published a brilliant book in Danish about the history of the origin of birds, which in 1926 was translated into English under the title “The Origin of Birds”. Heilman showed that the birds were anatomically more like theropod dinosaurs than any other fossil group. But there was one inevitable discrepancy: theropods clearly lacked clavicles, the same ones that merge into a little fork in birds.
Deficiencies of the Gerhard Heilman theory
Due to the fact that other reptiles had collarbones, Heilman suggested that theropods, in particular the Triassic pseudosuchia, lost them for some reason. For him, this loss meant that birds could not develop from this species of dinosaurs. Therefore, he was convinced, as it later turned out, erroneously that the function of the clavicles, lost during evolution, could not be restored. Birds, he argued, were to evolve from a more archaic reptile group that had collarbones.
As Seely had previously, Heilman concluded that the similarities between birds and dinosaurs should simply reflect the fact that both groups were bipedal. His findings influenced paleontological research and have been a priority for a long time, although the new information has refuted some facts. But until now, some scientists adhere to it. Two separate studies showed that theropods actually had clavicles. In 1924, an anatomical drawing of an unusual theropod with parrot plumage was published. He was called an oviraptor, and his existence disproved Heilman’s theory.
New evidence
In 1936, Charles Kamp from the University of California at Berkeley found the remains of a small theropod of the early Jurassic period and wanted to put an end to the story with collarbones. But the proof of the fallacy of Heilman's theory was few recognized. Recent studies have found clavicles in many theropods. Based on the structure of birds at present, as well as as a result of a number of studies, Archeopteryx was called an intermediate link in the history of the evolution of birds. But the question remained: between which particular reptiles? Further fossil birds are found in layers below the Early Cretaceous. Most of them have been discovered since the beginning of the 1990s, mainly in China. Meanwhile, more than thirty species of birds from the Cretaceous are known. In addition, a large number of small dinosaurs were found, moving on two legs with features characteristic of ancient birds. Thus, the gap between dinosaurs and birds due to the study of fossils has become much smaller.
Theory of Ostrom
Finally, centuries after Huxley’s controversial presentation to the Geological Society of London, John H. Ostrom of Yale revived the idea that birds were associated with theropods. He bluntly suggested that the birds are their direct descendants. In the late 1960s, Ostrom described the skeletal anatomy of the predatory theropod deinonych, which was the size of a human teenager and lived on Earth about 115 million years ago, in the Early Cretaceous era. In his further published works, Ostrom continued to define the anatomy of birds and a group of features that they, including the Archeopteryx, shared with Deinonychus and other theropods, but not with other reptiles.
Based on these data, he concluded that birds come directly from small theropod dinosaurs. After putting forward the theory, Ostrom continued to gather his evidence of the origin of birds from theropods. He used a new method to decipher relations between organisms, using it in natural history museums in New York, Paris and other places. This method is called cladistics or phylogenetic systematics. Since it became the standard for comparative biology, its use strictly substantiated the conclusions of Ostrom.
1970s research
In the 1970s, scientists continued to be interested in some common unique features of the archeopteryx, which make it related to some animals of the Jurassic period. Among them were those very small carnivorous dinosaurs that move on two legs, which were called theropods. Having carefully studied these traits, scientists again put forward the theory that, perhaps, they were the ancestors of birds. Leading paleontologists built evolutionary trees, and after that they turned out to be even more convinced that they were right.
What is a cladogram?
These trees, otherwise known as the cladogram, are the modern gold standard in the analysis of evolutionary relationships between animals. The method itself was called cladistic. Practitioners of cladistics determine the evolutionary history of a group of animals by studying various types of characters. In the process of evolution, a new, genetically determined trait may appear in some animal, which will be transmitted to its descendants. Consequently, paleontologists may conclude that two groups that uniquely share a set of such new or acquired traits are more closely related to each other than to animals that lack these traits. Nodes or branch points on the cladogram indicate the appearance of a line with a new set of derived features. As a result, the birds seemed to scientists just a branch on the tree of dinosaur life. In the works of J. Ostrom in the mid-1970s, theropods received the status of the most likely group for the ancestors of birds. Therefore, today the idea that birds are feathered dinosaurs is practically not disputed among evolutionary theorists.
Features of the Cladistic Method
Traditional methods of grouping organisms according to the principle of similarity and difference can exclude a species from a group only because it has features that were not found in other members. In contrast, cladistic groups were based solely on certain groups of similarities that are particularly informative. This method begins with the Darwinian commandment that evolution continues when a new hereditary trait appears in some organisms, which is transmitted genetically to its descendants. The precept indicates that the two groups of animals sharing such new features are more closely related to each other than those that share only the original, but not derivatives, inherited. By defining common derivative traits, cladistic practices can determine relationships among the organisms under study. The results of such analyzes, which are usually investigated, can be presented in the form of a cladogram. A tree diagram depicting the order in which new characteristics and new creatures appeared reflect the order in which evolution took place.
The reason for the appearance of feathers in birds
As the birds evolved and moved away from dinosaurs called theropods, many of their features changed and improved. If you take a closer look, it becomes quite obvious that the set of these characteristic features has long developed and served to perform a certain function. For example, the appearance of plumage in a small theropod was associated with the need to isolate certain areas of the skin and at first looked more like hair. The first bird feathers may have had various color patterns. There is a version that they were intended for camouflage, recognition of their appearance and other functions.
Comparison of the anatomical structures of birds and dinosaurs
The comparative anatomy of birds and theropods helped to connect them with each other. She also revealed some ways to change these characteristics, as dinosaurs became closer to birds, and birds turned out to be more modern. For example, in the pelvic area, the pubic bone, originally directed forward, later moves vertically or backward. In the front legs, the relative bone proportions remained fairly constant in the early birds, but the wrist changed. In some species, the bone in the wrist took the shape of a crescent. As a result, its shape contributed to the possibility of take-off. The wide boomerang-shaped fork of the first feathered dinosaurs became thinner and formed a deeper arc when this feature became necessary for the flight.
Common signs of dinosaurs and birds.
Recent Gauthier studies demonstrate that many functions that are traditionally considered to belong only to birds actually appeared before them, theropods of their ancestors. Many of these properties have helped their original owners to survive. These same features and some others were eventually used or transformed for flight and life on trees. The bird characteristics of theropods that evolved before birds did not appear immediately, and some were present before the theropods themselves appeared. They were still in the dinosaurs that existed before. For example, the immediate ancestor of theropods was bipedal and moved like a bird. He was small and carnivorous. He had forepaws, like early birds. In addition, the second, and not the third finger, as in other reptiles, was longer. In the ancestors of dinosaurs, the ankle joint became articulated, and the bones of the foot became elongated. Many of the changes in leg structure are believed to have affected the increase in stride length and running speed. This property will one day help bird theropods fly.
The world's largest flying bird and smallest
Giant birds lived on Earth over 6 million years ago. The largest flying bird in the world was argentavis. He lived in Argentina and belonged to the falcon family. The adult bird reached 2 meters in height, and its skull was about half a meter in length. The huge wings in the wings were like a three-story house. The smallest bird exists now. This is a well-known species - a hummingbird-bee. The length of his body usually does not exceed 6 cm. Hummingbird habitat is the steppe regions of the Brazilian province of Minas Gerais. The smallest bird eats nectar of flowers and very quickly flaps its wings, like a bee, and therefore received this name.