Friday, June 15, 2012

Scientists: birds are just baby dinosaurs, in a way

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.

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