Friday, June 15, 2012
There’s a good reason birds are so much cuter and less threatening than their scary ancestors—the dinosaurs—if new research is correct. It’s because birds are, in a sense, dinosaurs stuck in baby mode. “When we look at birds, we are actually looking at juvenile dinosaurs” to a great degree, said Arkhat Abzhanov of Harvard University, co-author of a report on the findings. Abzhanov and colleagues analyzed dozens of bird and dinosaur skulls. They found that rather than take years to reach sexual maturity, as many dinosaurs did, birds sped up the clock—some species take as little as 12 weeks to mature—allowing them to retain the physical characteristics of baby dinosaurs. The report appeared May 27 in an online edition of the journal Nature. In evolution, species change because some characteristics are more useful than others in a given environment. Thus individuals with more of those traits thrive, and through their offspring, spread those features through a population. Individuals lacking those traits gradually drop out. As this goes on, species can eventually become nearly unrecognizable compared to their old selves. Most evolutionary research has focused on the physical structure of organisms, but “what is interesting about this research,” Abzhanov said, is that it illustrates how great changes can occur “simply by changing the relative timing of events in a creature’s development.” Thus, he added, “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.” Dinosaurs have long snouts and mouths bristling with teeth, while birds have proportionally larger eyes and brains. But what inspired the study was the realization that skulls of modern birds and juvenile dinosaurs show surprising similarity, researchers said. “No one had told the big story of the evolution of the bird head before,” said Bhart-Anjan Bhullar, a Harvard doctoral 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. ... 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.” With colleagues at The University of Texas at Austin, the researchers conducted CT scans on dozens of skulls, ranging from modern birds to theropods—the dinosaurs most closely related to birds—to early dinosaur species. By marking various “landmarks” in the skull the scientists tracked how the overall shape 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.” It turned out, he said, that while early dinosaurs, even those closely related to modern birds, undergo vast structural changes as they mature, the skulls of juvenile and adult birds remain remarkably similar. In the case of modern birds, Abzhanov said, the change is the result of a process known as progenesis, which causes an animal to reach sexual maturity earlier. “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,” Bhullar said.