Currently, fossil studies from around the world suggest that scientists generally believe dinosaurs are the ancestors of birds, often referring to birds as "living dinosaurs" or "flying dinosaurs." However, how did dinosaurs evolve from ground-dwelling creatures into the sky-dwelling beings we know as birds? What are the specific stages of this evolution? These questions remain a mystery.
At present, there is limited fossil evidence regarding the origins of birds. Due to their fragile bones and aerial lifestyle, the chances of fossilization are quite rare. Only five examples of primitive bird fossils have been discovered worldwide, all dating back about 150 million years and found in limestone layers in Bavaria, Germany. These fossils have been identified as Archaeopteryx, showcasing features similar to modern birds, such as clear feather imprints, differentiated primary and secondary flight feathers, and tail feathers. Its forelimbs evolved into wings, while its hind feet had four toes, with three pointing forward and one backward; the collarbone fused into a wishbone, and the pubis elongated backward, among other characteristics. Interestingly, these fossils also exhibit traits strikingly similar to reptiles, including teeth in its beak and three claws at the tips of its wings; its carpals and phalanges are separate, and it had a long tail made up of many vertebrae. Research indicates that it represents an intermediate stage in the transition from reptiles to birds, hence the name "Archaeopteryx." It is estimated that Archaeopteryx could achieve a minimum flight speed of 7.6 meters per second, allowing it to flap its wings but not sustain flight. How did Archaeopteryx transition from a terrestrial lifestyle to an aerial one?
For over a century, two main hypotheses have dominated the academic discussion regarding this mystery: the arboreal hypothesis and the terrestrial hypothesis. The arboreal hypothesis suggests that flight evolved from tree-dwelling creatures gliding down with the aid of gravity, while the terrestrial hypothesis posits that ground-dwelling creatures learned to fly through vigorous running.
Historically, the terrestrial hypothesis has held a dominant position in academia, gaining broader acceptance compared to the arboreal hypothesis. Professor Ken Dial from the University of Montana's Biological Flight Laboratory discovered that some young birds flap their wings while climbing slopes, aiding their ascent. Based on this observation, he speculated that the ancestors of birds learned to fly by flapping their wings while running.
Dr. Xu Xing from the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology believes that, logically, Professor Dial's terrestrial hypothesis is feasible. He stated, "Research on dinosaur behavior indicates that dinosaurs were typical ground-dwelling running animals. Fossil studies suggest that dinosaurs evolved all the necessary structures for flight while running and could reach the speeds required for takeoff. There are solid models and data to describe this process."
**Confuciusornis**
Confuciusornis was a bird the size of a magpie that lived in early Cretaceous China. They inhabited trees, fed on plants, and lived and bred in large groups. Some of these birds were likely males, adorned with long, extravagant tail feathers, possibly just for display, while others had short, stout tails. Confuciusornis exhibited stronger flying abilities than Archaeopteryx, showcasing features characteristic of transitional development. Its claws, flat breastbone, wrists, hips, and legs reminded scientists of Archaeopteryx, while its evolutionary traits included a deeper chest cavity, a toothless beak, and fused bones forming its tail. Some scientists classify Confuciusornis within the order Pseudornithes.
However, he also remarked, "Professor Dial's speculation is quite risky. We are interpreting ancient behaviors through a modern lens. The reasons behind ancient behaviors are numerous, and we do not know them all. (The terrestrial hypothesis) is biomechanically feasible."
While there is consensus that birds evolved from dinosaurs, some predict that certain dinosaurs had feathers, yet no fossil evidence had been found until now. Conversely, many fossils prove that dinosaurs had scales, resembling reptiles. Scientists hope to discover how the scales on dinosaurs transformed into feathers and whether dinosaurs had feathers at all. Over 1,200 genera of dinosaurs have been named worldwide, but many are invalid; currently, about 300 to 400 genera are recognized. In China, dinosaur fossils have been found in all regions except Hainan, Fujian, and Hong Kong and Taiwan. In terms of fossil quantity and variety, Yunnan, Sichuan, Xinjiang, Inner Mongolia, and Liaoning have the richest dinosaur fossil resources. Especially in recent years, discoveries in Liaoning are making China a center for global dinosaur research. Since 1996, fossils such as "Zhonghua Longbird," "Primitive Archaeopteryx," "Tail Feather Dragon," "Beipiao Dragon," "Chinese Bird Dragon," and "Small Dromaeosaur" have been discovered, all indicating that dinosaurs had feathers, some being primitive and others modern.
In contrast to the terrestrial hypothesis, the arboreal hypothesis has its advantages. Almost all animals related to gliding or flying live in trees, such as bats. Generally, the body structure of flying animal ancestors would not be fully adapted to flight, making it easier to initially glide with the help of gravity. Dr. Xu and his colleagues' paper provides new evidence for this viewpoint, highlighting the discovery of four-winged dinosaur fossils in western Liaoning, China. They believe that the ancestors of birds first learned to glide using gravity before evolving into flapping flight. The feather morphology and arrangement on the fore and hind limbs of these dinosaurs are identical to those of bird wings.
Professor Padian from the University of California, Berkeley, commented, "The potential significance of this discovery is akin to that of Archaeopteryx." Dr. Rine from the University of Leeds in the UK stated that the four-winged dinosaur is the most important discovery in the study of bird evolution since Archaeopteryx. However, currently, only the small dromaeosaur represents a necessary phase of gliding with four wings in the evolution towards birds, and further fossil evidence is needed to support this.
**Archaeopteryx Fossils**
The first Archaeopteryx fossil was discovered in 1860 in a quarry in Solnhofen, Bavaria, Germany. The original owner sold this fossil to the British Museum for 700 pounds. The second skeleton was found in 1877, initially purchased by a savvy collector for 140 marks, who immediately resold it. Due to its rarity, it was sold to the Humboldt Museum at Berlin University for 20,000 marks.
Some scientists have also suggested that the four-winged dinosaur fossils could be interpreted in other ways, meaning that the four wings may not necessarily represent a necessary phase in the evolution from dinosaurs to birds, but rather a side branch in the evolutionary process.
However, not all scientists agree with the inferences made by Xu Xing and others. Professor Paul Sereno from the University of Chicago believes that only by finding fossils of other dinosaurs with feathers on their legs can we confirm that the small dromaeosaur (Chinese four-winged dinosaur) represents a necessary phase in the evolution of birds.
When it comes to speculation versus facts, we lean more towards science. From the above content, we can see that although scientists currently believe that dinosaurs are the ancestors of birds, there is still insufficient evidence to prove this. Whether the ancestors of birds were indeed dinosaurs remains to be further examined.
