Although a sparrow and a Tyrannosaurus Rex might appear to have nothing in common, evolutionary biologists see more that relates the two creatures than not. A new study, led by Harvard
scientists, has shown that modern birds are essentially living dinosaurs with skulls that are remarkably similar to those of their juvenile ancestors.
Arkhat Abzhanov, Associate Professor of Organismic and Evolutionary Biology and Bhart-Anjan Bhullar, a PhD student in Abzhanov laboratory and the first author of a paper published this week in Nature,
found evidence that the evolution of birds is the result of a drastic change in how dinosaurs developed.
Sexual maturity in many dinosaurs took many years to reach. Birds, however, sped up the clock, with some species taking as little as 12 weeks to mature, which allows them to retain the physical characteristics of baby dinosaurs.
"What is interesting about this research is the way it illustrates evolution as a developmental phenomenon," Abzhanov said. "By changing the developmental biology in early species, nature has produced the modern bird - an entirely new creature - and one that, with approximately 10,000 species, is today the most successful group of land vertebrates on the planet."
Mark Norell, chair of the Division of Paleontology at the American Museum of Natural History
and one of the paper's co-authors, said, "The evolution of the many characteristics of birds - things like feathers, flight, and wishbones - has traditionally been a difficult problem for biologists. By analyzing fossil evidence from skeletons, eggs, and soft tissue of bird-like dinosaurs and primitive birds, we've learned that birds are living theropod dinosaurs, a group of carnivorous animals that include Velociraptor."
"This new work advances our knowledge by providing a powerful example of how developmental changes played a major role in the origin and evolution of birds."
Comparing the skulls of dinosaurs and modern birds, the differences are readily apparent, with dinosaurs having distinctively long snouts and mouths bristling with teeth. Birds, on the other hand, have proportionally larger eyes and brains. The similarities however, show a surprising degree of similarity, which sparked the study.
"No one had told the big story of the evolution of the bird head before," Bhullar said. "There had been a number of smaller studies that focused on particular points of the anatomy, but no one had looked at the entire picture."
"What's interesting is that when you do that, you see the origins of the features that make the bird head special lie deep in the history of the evolution of Archosaurs, a group of animals that were the dominant, meat-eating animals for millions of years."
The scientists scanned dozens of skulls of both modern birds and dinosaur fossils using CT scanners. By marking various "landmarks" - such as the orbits, cranial cavity and other bones in the skull - on each scan, researchers were able to track how the skull changed shape over millions of years.
"We examined skulls from the entire lineage that gave rise to modern birds," Abzhanov said. "We looked back approximately 250 million years, to the Archosaurs, the group which gave rise to crocodiles and alligators as well as modern birds. Our goal was to look at these skulls to see how they changed, and try to understand what actually happened during the evolution of the bird skull."
What Abzhanov and colleagues found surprised them. While early dinosaurs, even those closely related to modern birds, undergo vast morphological changes as they mature, the skulls of juvenile and adult birds remain remarkably similar.
"This phenomenon, where a change in the developmental timing of a creature produces morphological changes is called heterochrony, and paedomorphosis is one example of it," Abzhanov explained. "In the case of birds, we can see that the adults of a species look increasingly like the juveniles of their ancestors."
Modern birds, Abzhanov said, change as the result of a process known as progenesis, which causes an animal to reach sexual maturity earlier. Unlike their dinosaurian ancestors, modern birds take dramatically less time to reach maturity, allowing birds to retain the characteristics of their juvenile ancestors into adulthood.
Ultimately, Abzhanov said, the way the bird skull changes in the developmental timeline, highlights the diversity of evolutionary strategies that have been used over millions of years.
"That you can have such dramatic success simply by changing the relative timing of events in a creature's development is remarkable," he said. "We now understand the relationship between birds and dinosaurs that much better, and we can say that, when we look at birds, we are actually looking at juvenile dinosaurs."
"It shows that there's so much for evolution to act upon," Bhullar agreed. "When we think of an organism, especially a complex organism, we often think of it as a static entity, but to really study something you have to look at its whole existence, and understand that one portion of its life can be parceled out and made into the entire lifespan of a new, and in this case, radically successful organism."