30 May 2012
Birds are baby dinosaurs, say Harvard scientists
by Will Parker
Birds and dinosaurs are more closely related than previously thought, say scientists who claim that modern birds are, essentially, living juvenile dinosaurs. According to Harvard's Arkhat Abzhanov, Associate Professor of Organismic and Evolutionary Biology, the evolution of birds is the result of a drastic change in how dinosaurs developed. Rather than taking years to reach sexual maturity, as many dinosaurs did, birds sped up the developmental clock, allowing them to retain the physical characteristics of baby dinosaurs.
Abzhanov says the research, appearing in Nature, illustrates evolution as a developmental phenomenon. "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."
Harvard co-researcher Bhart-Anjan Bhullar explained how it was the realization that the skulls of modern birds and juvenile dinosaurs are surprisingly similar that sparked the study. "No one had told the big story of the evolution of the bird head before," said Bhullar. "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."
To investigate, the researchers turned to an unusual methodology. Using CT scanners, they scanned dozens of skulls, ranging from modern birds to theropods (the dinosaurs most closely related to birds) to early dinosaur species. Tracking the changes in salient features - such as the orbits, cranial cavity and other bones in the skull - on each scan, the researchers were able to map 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 the researchers found was surprising. 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."
For modern birds, Abzhanov says, the change is 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 - just 12 weeks in some species - to reach maturity, allowing birds to retain the characteristics of their juvenile ancestors into adulthood.
How the bird skull evolved through changes in the developmental timeline highlights the diversity of evolutionary strategies, Abzhanov claims. "That you can have such dramatic success simply by changing the relative timing of events in a creature's development is remarkable. 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."
"There's so much for evolution to act upon," Bhullar added. "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."
Source: Harvard University