Birds and dinosaurs have many similarities that point to a close evolutionary relationship. Both groups are archosaurs, meaning they descend from a common ancestor that lived during the Triassic period over 200 million years ago. Since the 19th century, scientists have proposed that birds evolved from theropod dinosaurs, a group that includes giants like Tyrannosaurus rex as well as smaller predatory dinosaurs. But even with so many anatomical and behavioral similarities between birds and theropod dinosaurs, determining their exact evolutionary relationship has been challenging for scientists.
Evidence birds evolved from dinosaurs
There is now overwhelming evidence that birds evolved from small feathered theropod dinosaurs sometime during the Jurassic period. Some key pieces of evidence are:
Feathers
Feathers are a unique evolutionary innovation found only in birds. However, feathers have also been discovered in many dinosaur fossils, suggesting they evolved in the common ancestor of birds and dinosaurs. Intriguingly, some non-avian dinosaur fossils preserve feathers almost identical in structure to modern bird feathers. This suggests the early theropod dinosaurs had plumage very similar to the feathers of modern birds.
Skeletons
Birds and theropod dinosaurs share dozens of skeletal similarities, including hollow bones, three-toed feet, wishbones, and many unique wrist bones. In fact, a velociraptor skeleton is practically indistinguishable from an archaeopteryx (early bird) skeleton when feathers are removed. This provides compelling evidence that they share a relatively recent common ancestor.
Lungs
Birds have a very efficient lung design that allows for extended flight at high altitudes. Remarkably, scientists have found that many theropod dinosaurs like tyrannosaurs and velociraptors also possessed a similar flow-through lung design with air sacs, suggesting this key adaptation for flight originally evolved in dinosaurs.
Feature | Birds | Theropod Dinosaurs |
---|---|---|
Feathers | Yes | Some species (e.g. velociraptors) |
Three-toed feet | Yes | Yes |
Hollow bones | Yes | Yes |
Wishbone | Yes | Yes |
Flow-through lungs | Yes | Some species (e.g. tyrannosaurs) |
Behavior
Birds and their dinosaur ancestors share some behaviors, including nesting and brooding over eggs. Fossils reveal that many dinosaur species built nests, incubated eggs, and cared for hatchlings much like modern birds. The occurrence of such avian-like behaviors in dinosaurs provides further evidence of their close evolutionary relationship to birds.
Are birds just modern dinosaurs?
Based on the many unique features shared by birds and theropod dinosaurs, most paleontologists consider birds to be the only surviving dinosaur lineage. Essentially, birds are just a subgroup of modern dinosaurs that evolved flight capabilities.
Under this view, non-avian dinosaurs like T. rex are thought to have gone completely extinct 66 million years ago, while the avian dinosaurs (the birds) survived and continued to diversify. So rather than thinking of birds as descendants of dinosaurs, it’s more accurate to think of them as specialized flying dinosaurs.
How birds evolved from dinosaurs
The earliest birds like Archaeopteryx evolved from small feathered theropods called paravians in the Late Jurassic period around 150 million years ago. They retained many anatomical features of their dinosaur ancestors, including teeth, claws, and a long bony tail.
Over millions of years, birds gradually evolved distinct adaptations for flight like broad wings, a keeled breastbone, and fused hand bones. By the time of Ichthyornis in the Late Cretaceous period around 80 million years ago, early birds had evolved into virtually modern-looking seabirds complete with beaks and only stubby tail bones.
So in essence, the first birds were small feathered dinosaurs with wings. Over time, anatomical features from their dinosaurian heritage disappeared as adaptations for flight became more prominent.
Time period | Species | Dinosaur or bird? |
---|---|---|
Late Jurassic (150 mya) | Archaeopteryx | Transitional species with both dinosaur and bird features |
Early Cretaceous (120 mya) | Microraptor | Feathered dinosaur closely related to birds |
Late Cretaceous (80 mya) | Ichthyornis | Extinct seabird with modern anatomy |
Today | Crows | Modern bird |
How dinosaurs gave rise to modern bird diversity
The asteroid impact that triggered a mass extinction 66 million years ago wiped out all non-avian dinosaurs. But some lineages of birds managed to survive the extinction event that ended the Cretaceous period.
These bird survivors experienced an explosive diversification in the following Cenozoic era, giving rise to the huge variety of bird species we see today. Having already evolved adaptations for flight earlier in the dinosaur lineage, birds possessed a huge advantage over grounded competitors following the mass extinction. They quickly adapted to fill newly vacant ecological niches, spurring rapid diversification into the thousands of bird species present today.
So in summary, birds owe their remarkable diversity mainly to two key factors:
Feathers and flight
The early evolution of feathers and flight capabilities gave birds a distinctive advantage in dispersing to new locations, avoiding predators, and exploiting food resources. Flight allowed birds to survive the post-dinosaur extinction and spread rapidly to recolonize the world.
Mass extinction of competitors
When the dinosaurs went extinct 66 million years ago, birds faced less competition and could adaptively radiate into vacant niches around the world. This ecological opportunity prompted the explosive diversification seen during the Cenozoic into modern bird lineages.
Conclusion
Birds and dinosaurs share a common ancestor and are part of the same evolutionary lineage. Birds evolved from small feathered theropod dinosaurs during the Jurassic period. Over time, they adapted for flight by developing wings, fused bones, beaks, and other avian features. After non-avian dinosaurs went extinct at the end of the Cretaceous, birds underwent adaptive radiation in the Cenozoic era to yield the huge diversity of bird species we see today. So while dinosaurs may be extinct (except for birds!), we can still marvel at the dinosaurs flying around us today. From hummingbirds to ostriches, penguins to eagles, birds provide a living glimpse of the dinosaurs that vanished long ago.