Feathered dinosaurs and their evolution into birds may seem like a relatively new idea, but it is about as old as Charles Darwin’s theory of evolution itself. In 1861, two years after On the Origin of Species was published, paleontologist Christain Erich Hermann von Mayer unearthed what would be called Archaeopteryx, a dinosaur-like bird dated to around 150 million years ago. This skeleton, the first avian fossil ever discovered, ignited the paleontological community with the idea that birds and dinosaurs were related. Evolution itself was still a new idea, and the bird-dinosaur debate became a battleground for Darwin’s theory.

Early proponents of the bird-dinosaur link included anatomist Thomas Huxley, a friend and strong proponent of Darwin's work, who squared off against the anti-Darwin Richard Owen. Owen and other skeptics believed the similarities between dinosaurs and birds were matters of convergent evolution, without any relation. Those that did believe in the link were still riddled with their own questions and doubts, namely how did flight evolve? On this question, two camps emerged, the ground-up, which believed feathers evolved to aid in running and jumping while hunting prey, or the trees-down group, which thought feathers aided in gliding down from trees.

The debate around dinosaurs and birds continued for decades, culminating in Gerhard Heilmann’s The Origin of Birds, published in English in 1926. The book made such contributions as dismissing the link between birds and pterosaurs (arguing that their similarities is a matter of convergent evolution). However, Heilmann ultimately argued against a direct dinosaur-bird link, as he mistakenly believed that dinosaurs lacked clavicles, which would have been a distinct anatomical difference between them and birds. The Origin of Birds seemed to have finally settled the question of dinosaurs and birds, but unfortunately its conclusion was incorrect.

Further studies into dinosaurs and birds slowed in the following decades, along with much of dinosaur paleontology, until its resurgence in the 1960s with the Dinosaur Renaissance. The advent of DNA sequencing in the intervening years firmly established the connection between dinosaurs and living birds. Interestingly, many of the more recent finds are from the Cretaceous, unlike Archaeopteryx, which is dated to the earlier Jurassic. These finds include the Dromaeosaurus, Troodonts, and Oviraptorosaurs. The most closely related dinosaur group to birds is the mighty theropods, with the T-Rex’s closest living relative being the chicken.

The question of dinosaurs and birds being related is a fairly settled debate, but questions still abound on the specifics. The origin of flight question remains split between the ground-up and trees-down groups, but it is now known that feathers evolved independent of flight. Quill knobs, small depressions in bone where ligaments connect the skeletal system to feathers, have been found on many theropods. These feathers had a variety of uses, including thermoregulation to survive at lower temperatures. It is possible that feathers’ original use was not for flying at all, and that flight was an accident of evolution.

Another thorny question in the connection between dinosaurs and birds is the evolution of fingers. Dinosaur’s reptilian ancestors had five fingers, which scaled down to three in theropods (losing the ring and pinky). Birds also have three similar digits in their wings, but their loss is equivalent to the thumb and pinky, a distinct difference in anatomy. To account for this discrepancy, the frameshift hypothesis argues that the same fingers are being expressed in different positions and that the genetic mechanism that spawns off the fingers do not tether them to a fixed position.