The evolution of birds is the result of a drastic change in how dinosaurs developed, Harvard scientists say.
They've based their conclusion on the similarities between bird skulls and those of juvenile dinosaurs.
Rather than take years to reach sexual maturity, as many dinosaurs did, some bird species take as little as 12 weeks to mature. This allowed 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," says Arkhat Abzhanov, associate professor of organismic and evolutionary biology.
"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."
The researchers carried out CT scans of dozens of skulls, including modern birds, theropods – the dinosaurs most closely related to birds – and early dinosaur species.
By marking various 'landmarks' – such as the orbits, cranial cavity and other bones in the skull – on each scan, they were able to track how the skull changed shape over millions of years.
And what they 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," says Abzhanov. "In the case of birds, we can see that the adults of a species look increasingly like the juveniles of their ancestors."
In the case of modern birds, he 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 as little as 12 weeks to reach maturity, allowing them to retain the characteristics of their juvenile ancestors into adulthood.
Ultimately, says Abzhanov, the way the bird skull evolved 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 says.
"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."