Birds evolved from small dinosaurs about 155 million years ago, with feathers and the ability to glide and eventually fly. The most famous fossil indicating an evolutionary link between dinosaurs and birds is Archaeopteryx. The extinction of dinosaurs and pterosaurs left the skies open, allowing birds to become a major source of food for carnivorous or omnivorous mammals.
Birds are the third major evolutionary branch of sauropsids (reptiles), the first being non-archosaur reptiles and the second being archosaurs (which includes dinosaurs and crocodilians). The birds are highly evolved archosaurs and therefore look nothing like their closest living relatives, the crocodilians. The evolutionary history of birds is long and complex, as they are the last major branch of the sauropsid group. Previously, it was assumed that birds may have evolved from stem-group crocodilians or archosaurs in the early Mesozoic.
Birds evolved from small dinosaurs about 155 million years ago, right in the middle of the Mesozoic era. This is the scientific consensus, formed relatively recently: for many decades, the early evolutionary history of birds was unknown and a highly controversial topic. The idea 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 On the Origin of Species, but was not confirmed until the 1960s, when evolutionary biology and comparative anatomy advanced to the point of providing abundant evidence for the relationship.
The evolutionary history of birds is rooted in feathered dinosaurs, animals we didn’t even know existed until a series of pivotal discoveries in China in the 1990s. Apparently, some dinosaurs’ feathers grew so long that they could use them to glide and eventually fly. This led to a number of evolutionary changes in the direction of modern birds: hollow bones, higher metabolic rate, beaks, etc. The evolution of birds from the reptilian lineage is an example of a warm-blooded creature evolving from cold-blooded creatures, something that also happened when mammals evolved from cold-blooded synapsids, our distant ancestors.
The most famous fossil indicating an evolutionary link between dinosaurs and birds is Archaeopteryx, a bird-dinosaur that lived in the Middle Mesozoic. It includes numerous intermediate characteristics between dinosaurs and birds. While it had feathers, broad wings, and the ability to fly, much of its anatomy had more in common with theropod dinosaurs, such as small teeth, a long bony tail, and clawed forelimbs.
The evolutionary history of birds got a new start when the main predators and competitors of birds, dinosaurs and pterosaurs (flying reptiles), became extinct, leaving the skies open. Today, birds are a major source of food for carnivorous or omnivorous mammals such as felines and humans.
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