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Scientists: birds are just baby dinosaurs, in a way

May 31, 2012
Courtesy of The University of Texas at Austin
and World Science staff

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.

Skulls of three types of ar­chosaur—al­li­ga­tor, prim­i­tive di­no­saur, and ear­ly bird. The left col­umn rep­re­sents ju­ve­niles and the right col­umn rep­re­sents adults. (Im­age cour­te­sy U. of Tex­as at Aus­tin)
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.

* * *

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There’s a good reason birds are so much cuter and less threatening than their scary ancestors, the dinosaurs, new research suggests. It’s because birds are in, 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 that appeared May 27 in an online edition of the journal Nature. Abzhanov and colleagues analyzed dozens of bird and dinosaur skulls to reach their conclusions. 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. In evolution, species change because some characteristics are more useful than others. Thus individuals with more of those traits thrive and spread those features through a population. Individuals lacking those features 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 is the way it illustrates evolution as a developmental phenomenon,” Abzhanov said. That is, you can get vast changes “simply by changing the relative timing of events in a creature’s development.” In this way, 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. 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.” 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 researchers 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.