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Evo­lu­tion of Birds caused by changes in Dinosaurs development

pub­lished 31 May 2012 | mod­i­fied 05 Decem­ber 2012

Researchers from Har­vard Uni­ver­sity, The Uni­ver­sity of Texas at Austin and sev­eral other insti­tutes have found evi­dence that the evo­lu­tion of birds is the result of a dras­tic change in how dinosaurs developed.

Sci­en­tists have long under­stood that mod­ern birds descended from dinosaurs. 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 results of the study appeared May 27 in an online edi­tion of the jour­nal Nature.

What is inter­est­ing about this research is the way it illus­trates evo­lu­tion as a devel­op­men­tal phenomenon
Arkhat Abzhanov, asso­ciate pro­fes­sor at Har­vard, 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.”

While it’s clear sim­ply from look­ing at the skulls of dinosaurs and mod­ern birds that the two crea­tures are vastly dif­fer­ent – dinosaurs have dis­tinc­tively long snouts and mouths bristling with teeth, while birds have pro­por­tion­ally larger eyes and brains – it was the real­iza­tion that skulls of mod­ern birds and juve­nile dinosaurs show a sur­pris­ing degree of sim­i­lar­ity that sparked the study.

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 tackle the prob­lem, the researchers turned to an unusual method­ol­ogy. Using CT scan­ners at Har­vard and The Uni­ver­sity of Texas at Austin, they scanned dozens of skulls, rang­ing from mod­ern birds to theropods – the dinosaurs most closely related to birds – to early dinosaur species. By mark­ing var­i­ous “land­marks” – such as the orbits, cra­nial cav­ity and other bones in the skull – on each scan, researchers were able to track how the skull changed shape 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.”

dinosaurs vs birdsWhat the researchers 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. 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. Unlike their dinosaurian ances­tors, mod­ern birds take dra­mat­i­cally less time – just 12 weeks in some species – to reach matu­rity, allow­ing birds to retain the char­ac­ter­is­tics of their juve­nile ances­tors into adulthood.

Ulti­mately, Abzhanov said, the way the bird skull evolved – through changes in the devel­op­men­tal time­line – 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. 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.
(Arkhat Abzhanov)

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.”

The above news item is reprinted from mate­ri­als avail­able at Jack­son School of Geo­sciences via Sci­enceDaily. Orig­i­nal text may be edited for con­tent and length.

(Source: JSG News, 27.05.2012)

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