New evidence is suggesting that the emergence of birds was from a drastic change in how dinosaurs developed. Rather than spending years to reach sexual maturity, they drastically shortened their development, essentially remaining juvenile dinosaurs their whole lifespan.
Some species of birds take as little as 12 weeks to reach maturity, as opposed to the very long development of some species of dinosaurs.
“What is interesting about this research is the way it illustrates evolution as a developmental phenomenon,” said Arkhat Abzhanov, associate professor at Harvard and study co-author. “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.”
There are some clear differences between birds and dinosaurs; birds have shorter snouts, less/no teeth, and they have proportionally larger eyes and brains. But those differences are all from the sexual characteristics of grown dinosaurs — the skulls of juvenile dinosaurs and birds are virtually identical.
“No one had told the big story of the evolution of the bird head before,” said Bhart-Anjan Bhullar, a Harvard PhD student and first author of the study. “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 perform the study, the researchers used CT scanners; scanning dozens of skulls from birds, theropods (the dinosaurs most closely related to birds), and from other early dinosaur species.
By keeping track of the different “landmarks” on the skull, such as the orbits, cranial cavity, and other bones; the researchers were able to track how the skull changed 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 is that while early dinosaurs undergo large morphological changes in their skull as they mature, the skulls of of adult and juvenile birds are nearly the same.
The process of a species changing to reach sexual maturity earlier is called progenesis. Different from their dinosaur ancestors, birds reach sexual maturity rapidly, as quickly as in 12 weeks.
The researchers of this study comment that this highlights the diversity of different evolutionary strategies that species use.
“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.”
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