February 9, 2025, 4:00 pm | Read time: 6 minutes
We learned at school that there are five different classes of animals. These are mammals, birds, fish, reptiles and amphibians. However, this knowledge, long enshrined in textbooks, does not seem to be true, and birds and reptiles are so similar that it is almost impossible to distinguish between them.
Please don’t laugh, but I took three advanced courses in my high school graduation exams. Simply because I couldn’t decide between English, German, and Biology, I still have my old biology textbooks on my shelf today and sometimes consult them. If you open one of them, you will inevitably find the chapter on evolution: the family tree of species by Carl Linné. The Swedish naturalist categorized animals and plants into different systems, including kingdom, class, order, genus, family, and species. Does that ring a bell for you, too? Then please forget it right now. Because the classification of species that is still taught in schools has no longer been a common opinion among researchers since the 1940s. Thus, the realization that birds are actually reptiles isn’t even a new concept. Nevertheless, it has fundamentally changed my understanding of nature.
The Surprising Connection Between Birds and Dinosaurs
Birds belong to the class Aves, while reptiles are traditionally considered a separate group of vertebrates. But from an evolutionary perspective, these boundaries are not as clear as textbooks have long made them out to be. “I would say that any modern biologist would say, or should say, that birds are reptiles,” said Martin Stervander, evolutionary biologist and senior curator of birds at National Museums Scotland, in the science magazine LiveScience.
The key lies in their evolutionary history: scientists have unequivocally proven that birds are direct descendants of dinosaurs. More precisely, they descend from the theropod dinosaurs, which include the famous Tyrannosaurus rex, but above all, various raptors. As early as the 19th century, scientists noticed that there were striking similarities between birds and small carnivorous dinosaurs, the velociraptors.
The prehistoric bird Archaeopteryx, which lived about 150 million years ago, exhibited both reptilian characteristics, like teeth and a long tail, and avian features, including feathers and wings. The idea that birds could be related to dinosaurs already emerged at this time. DNA analyses as we know them today began around the 1940s and replaced the Linnaean system. The phylogenetic tree is used instead. This tends to see genetic nodes in the ancestors of today’s animals but no longer works with rigid classifications. In biology, the term ‘taxa’ is now used rather than ‘species’.
Why Does That Make Birds Reptiles?
From a strictly biological standpoint, the term ‘reptiles’ is somewhat outdated, yet it remains in common usage. Classically, it covers animals such as snakes, lizards, and turtles. These animals have dry skin with horny scales and regulate their body temperature via the environment, i.e., they are cold-blooded.
“All these animals – snakes, turtles, crocodiles, and birds, as well as dinosaurs when they were alive – have a common ancestor,” says Stervander. “This simply means that birds are reptiles by definition.” According to modern systematics, reptiles also include a group that was overlooked – even by Linné: the dinosaurs. Since birds are their direct descendants, they are nothing more than a highly specialized group of reptiles in purely evolutionary terms.
However, the relationship was not always so easy to recognize. “The reason birds appear so different from the rest of the reptiles living today is that all the evolutionary intermediates are extinct, so we don’t have a direct comparison,” explains Klara Widrig, a postdoctoral researcher in the Department of Vertebrate Zoology at the Smithsonian’s National Museum of Natural History in Washington, D.C., in the magazine LiveScience.
Common Features of Birds and Reptiles
So, let’s take a closer look at the anatomy and physiology of birds and reptiles today. The similarities speak for themselves:
- Scaly feet: the scales on the legs of birds resemble those of reptiles, as they have the same origin.
- Eggs with a hard shell: Both birds and reptiles lay eggs with a calcareous shell – an adaptation to life on land.
- Skeletal structure: Many bone features, including the three-toed foot structure and the special arrangement of the hip bones, show a close relationship to dinosaurs and, thus, to reptiles.
- Air sacs: The highly developed respiratory system of birds with air sacs originated in theropod dinosaurs.
The Evolution of Birds – a Slow Process
Unraveling the genetics of birds and dinosaurs, and above all, verifying whether they had feathers, was somewhat more difficult. Fossil finds from China, in particular, have made a decisive contribution to deciphering the evolution of birds. Particularly noteworthy is the discovery of Zhenyuanlong suni, a feathered theropod closely related to Velociraptor. Although it had short arms and could not fly, it had a plumage that resembled modern birds. How do we know this? The animal was preserved by a volcanic eruption in such a way that feathers could be seen in dinosaurs for the first time.
“The spectrum includes nine-metre-long, down-covered relatives of the ancestors of Tyrannosaurus rex, dog-sized herbivores with simple, long, thick spiky hairs and crow-sized gliders with real wings,” wrote paleontologist Stephen Brusatte in his report on the impressive finds in the science magazine Spektrum.
The fossils unearthed in China’s Liaoning province demonstrate that feathers didn’t emerge abruptly with the advent of the first birds; rather, they existed in their dinosaur ancestors. However, it is difficult to say what purpose the feathers served. Today, they fulfill several purposes. They keep birds and their eggs warm and are presented for reproductive success, but of course, they serve one purpose above all: flight.
According to Brusatte, it is difficult to say which of these functions evolved first. “These animals could not fly,” the paleontologist continues. “They had no wings; moreover, such creatures would not have been able to withstand a current of air. Their purpose must have been different. They probably kept those little dinosaurs warm.”
Study Shows Decades of Assumption Disproved! Large Animals Do Get Cancer Faster
Study shows Dogs can remember the names of toys for up to two years
Tradition Why Icelanders are throwing puffins off cliffs
What Does This Mean for Our Understanding of Nature?
The fact that birds are reptiles from a scientific point of view shows how much our understanding of the animal world has changed. The division into classic groups – mammals, birds, reptiles, amphibians – is based on external characteristics, while modern science is increasingly looking at evolutionary relationships. “Until the 1940s, biologists thought that birds were a completely separate group because they are warm-blooded and feathered,” says Widrig on LiveScience. “But through genetic studies, it became clear that they fit seamlessly into the reptilian family tree.”
For the perception of birds, this also means that when we see a pigeon on the street today or observe a titmouse in the garden, we are actually looking at the last living dinosaurs. The next encounter with a bird could, therefore, be seen with completely new eyes – as a small piece of prehistoric times that still populates the earth.
The evolution of birds is a fascinating example of the gradual change of living beings over millions of years. It shows that major changes do not occur abruptly but in small steps. For instance, birds didn’t emerge suddenly from nothing; they are the result of a lengthy series of adaptations, making them the ultimate surviving dinosaurs of our era.