The Great Transitions is a three part documentary covering evolutionary origins of land vertebrates tetrapods, birds and Humans. In this second installment of the series, paleontologist Julia Clarke tells how birds evolved from dinosaurs.
When the first Archaeopteryx fossil was discovered in a quarry in Germany in the early 1860s it created a lot of excitement. The discovery came only a few years after the publication of the Origin of Species by Charles Darwin. Since then dozens of Archeopteryx fossils have been unearthed. Approximately 150 million year old (late Jurassic period) Archeopteryx had many properties that indicated birds descended from a group of reptiles. But which group of reptiles? Thomas Henry Huxley published a report in 1867 and listed 14 anatomical features shared by reptiles and birds. These characters don’t exist in mammals. Huxley’s report was foundational in forming the hypothesis that birds may have descended from bird-hipped dinosaurs. The hypothesis is now well supported and is a pillar of evolutionary theory. The most naive reasoning would be to think that birds may have evolved from flying reptiles like the pterosours. After comparing the wing bone structures palentologists quickly realized that pterosours cannot be the ancestors of birds.
Scientists have identified many shared features between birds and two-legged carnivorous dinosaurs called theropods. Archeopteryx was undisputedly a bird with feathers. However, at the same time it had many theropod dinosaur characteristics. Unlike modern birds, Archaeopteryx had small teeth. It also had a long bony tail which modern birds don’t. Isn’t it ridiculous that the most famous therapod of all times the T-Rex has a wishbone (furcula) like the birds do? Additionally modern bird skulls are curiously resembling baby dinosaurs. In 2012, based on a detailed analysis of therapod dinosaur skulls a group of scientists concluded that birds living today are neotenized version of dinosaurs. The concept of neoteny (retention of juvenile characters) also extends into human evolution. Biologists are now accumulating more evidence that human evolution is shaped for quite a bit based on retention of juvenile characteristics such as ability to digest milk (lactose tolerance) in adulthood or having larger brains by prolonged childhood period (leaving more time for brain to develop). You can watch an articulated video on this subject by a Scientific American blogger Karen Straughan.
Among all, feathers are the defining characteristic of all birds. When, how and why did feathers evolve? Feathers evolved before flight and has many functions other than flight. Feather provide insulation, ornamentation for attracting mates and can also be used for camouflage especially by females who should get less attention from predators during nesting.