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Birds are the only dinosaurs that have survived into modern times. Why is that? Of all the dinosaur species, how did they manage to make it through the catastrophic events of 65 million years ago, whereas all their fellow dinos perished? A new study, published May 27 in Nature, hints at an evolutionary phenomenon that may have played to birds' advantage: They are, it seems, baby dinosaurs whose biology prevents them from ever growing up. (Scientific American is part of Nature Publishing Group.)
Bhart-Anjan Bhullar and his doctoral advisor, Arkhat Abzhanov, posit that birds may have evolved from dinosaurs by a process known as paedomorphosis, whereby an organism retains juvenile traits even after it becomes sexually mature. "They certainly look to have some paedomorphic characteristics," says Jack Horner, a Montana State University paleontologist who studies dinosaur growth and development and was not involved in the study. Paedomorphosis "is not uncommon in evolution and speciation," he adds.
Bhullar and Abzhanov reached this conclusion by comparing the skulls of birds and dinosaurs across phylogenies, or related groups, and at different developmental stages. To quantitatively compare cranial geometries, they scanned the skulls of theropod dinosaurs (which are thought to be birds' ancestors), crocodiles and alligators (dinosaurs' cousins), early transitional birds such as Archaeopteryx, and modern birds. Then they created digitized versions of each skull and mapped out cranial landmarks, such as nostril tips, eye socket dimensions and places where bones meet.
Their measurements showed that whereas a typical non-avian dinosaur began life with a round head, large eyes and a big brain (relative to the rest of the body) then later developed an elongated snout and smaller relative brain size, birds kept their baby faces.
If birds did evolve by paedomorphosis, they join species such as axolotls. These salamanders evolved to retain tadpolelike gills and fins and, unlike most other amphibians, remain aquatic into adulthood. This feature appears to be due to a hormone disruption. By adding thyroid hormone into their water, researchers have caused axolotls to metamorphose into terrestrial salamanders.
But why would it be advantageous for adult animals to look like kids? Greg Erickson, who studies evolutionary morphology at The Florida State University and was not involved in the study, says that paedomorphosis can help a species to develop new adaptations and exploit new niches. In particular, he suggests that paedomorphosis may have enabled birds to develop larger eyes, which aid in spatial assessment during flight as well as a high brain-to-body-mass ratio, which may contribute to intelligence.
An even simpler explanation is that kids are small and, in times of environmental stress, small is good. Bhullar cites an example of Temnospondyli—large primitive amphibians that were common before 120 million years ago. Catastrophic events killed off most of the temnospondyli, except for a few paedomorphic species. "The interesting thing to me is that [after the catastrophe] these little paedomorphic animals were at the base of a giant radiation," Bhullar says. He suggests that a similar phenomenon may have occurred during the catastrophic events that killed the dinosaurs—being small may have been an advantage, because smaller animals require less food and can more easily hide. "Everything that lived on land and weighed more than one kilogram perished," Bhullar says. "The only dinosaurs that survived were the paedomorphic ones." And after many of those larger species went extinct, the little dino may have been better placed to exploit the new niches that opened up.
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13 Comments
Add CommentI thought that only the smallest dinosaurs needed good insulation with feathers, just as the smallest mammals have the thickest fur. So that, in a prolonged 'winter' the small would survive while the poorly insulated huge ones would not. Is this just a simpler version of your story?
Reply | Report Abuse | Link to thisthrow a little bird food now and then. help them continue to survive.
Reply | Report Abuse | Link to thisGood Lord! What ever happened to dinosaurs became birds? Don't tell me I know...finding them in the same place and from the same time put that one to bed.
Reply | Report Abuse | Link to thisSo now we're to believe they survived because they had the same characteristics as a late teen age adult, i.e., an organism (who) retains juvenile traits even after it becomes sexually mature. And some confirmation is gotten from the Montana State University paleontology department that 'discovered' that blood and related biologicals last millions of years when they cut into a fossil and find them.
I'm an old field engineer. If I had tried to solve problems and fix systems using inconsistent ideology as a basis, the U.S. would have lost the use of a number of very expensive war machines...if not worse.
It is a fictional story alright! The utter complexity and fine tuned synergism found among so many species pretty well precludes the notion of random chance mutations being responsible for all of biology.
Reply | Report Abuse | Link to thisEven though Dawkins insists that evolution is not random, there is no reasonable source for all the coherent code that directs the constitution and development of all living things. The only so called naturalistic source of new code is random mutations. The other option is an Intelligent Designer. Just like the Bible teaches.
Unless, of course, you believe in mother earth intelligently hatching out all the myriad species we see.
Does anyone realize that while there are fewer than 474 quadrillion seconds of time in 15 billion years, there are more than 1,152 quadrillion different ways to randomly arrange only 30 nucleotides in any DNA sequence?
Millions of years and all the dreaming in the world just doesn't cut it!
Please go back to Biology 101 before coming on here and spewing quack science again. If you refuse to learn about natural selection and survival of the fittest, and how they completely invalidate the "randomness" argument, then don't try to act like you know. If for some reason I'm off the mark, pick up a copy of the Selfish Gene to learn some real facts, and maybe a scientific theory or two.
Reply | Report Abuse | Link to thisIs paedomorphosis the same as neoteny?. Salut +
Reply | Report Abuse | Link to thisAs a Catholic physician, I know that anyone who thinks human design is intelligent must know very little about either design or anatomy.
Reply | Report Abuse | Link to thisWe are the only mammalthat habitually stands erect and that causes all sorts of problems. Anyone who has an inguinal hernia, uterine prolapse, hemorroids or varicose veins can testify.
There's so much more. Look an octopus in his intelligent eye, and know that eye is quite similar--but far from identical--to your own.
Well said.
Reply | Report Abuse | Link to thisI think that both paedomorphosis and neoteny refer to the retention of juvenile characters by adult individuals, but there may be subtle differences. Neoteny is the attainment of sexual maturity during the larval stage (as in the axolotl) while paedomorphosis suggests an increased capacity for further evolution.
Reply | Report Abuse | Link to thisActually the process is not random. Mutations may be, but the tendency for some mutations to be adaptive and most to simply kill their carrier, is not random at all. The "intelligence" behind nature is that all things living today have adapted to the rules of life in the universe and on this planet. Living things have molded themselves to "fit" nature, NOT the other way around.
Reply | Report Abuse | Link to thisThat is to say, WE molded ourselves to fit, not nature nor any 'intelligent' being.
Reply | Report Abuse | Link to thisI'm in complete agreement, but 'quizzical' probably isn't even listening. Willful ignorance of observed facts is a major problem for those who try to understand and explain the universe.
Reply | Report Abuse | Link to thisWhat is the supporting evidence tha paedomorphosis was the basis for the survival of an entore species over another?
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