Before a bird hatches, its hip resembles a dinosaur’s pelvis and turns into a bird just days before hatching, according to research from scientists at Yale University.
“It’s amazing that we have anatomy in living animals that we previously thought were locked up in the fossil record,” said Christopher Griffin, a postdoctoral researcher and lead author of a paper published in the journal Nature. “But it turns out that they are present — temporarily present, but present — in birds.”
That means the anatomy “can be studied directly,” Griffin said. “You can really look at the genetic changes and the genetic architecture that underlie this anatomy. You can’t do that with a dinosaur that is 250 million years old.” There is no DNA in a fossil, he said.
Griffin and Bhart-Anjan Bhullar, assistant professor of Earth and planetary sciences at Yale and senior author, along with others explored how a bird’s pelvis closely resembles that of a Tyrannosaurus rex or other dinosaur until late in its development, just before it changes from cartilage to bone, a phase known as “terminal addition.”
Although all vertebrates develop in the same way, the dramatic change in the egg is a new insight, made possible by technology that allows the scientists to look at the bones, muscles and nerves of the embryo even at the cellular level, Griffin said.
“That’s why this hasn’t been noticed before,” Griffin said. “Since Aristotle, people have been studying bird embryos.”
“It’s still the age of dinosaurs,” Bhullar said. “I think vertebrates are interesting by nature. … They are certainly the most complex life form that has ever appeared on Earth.”
And birds have unique features that make them even more fascinating, Bhullar said. “Among the vertebrates, birds are extremely successful, and there are countless superlatives that apply only to them.”
To be able to fly, “one of the most demanding forms of travel, but also the most powerful” [and] transcendent,” the bodies of birds are “the most exquisitely crafted bodies of all vertebrates,” Bhullar said.
But their abilities also “limit the body shape of birds in a way,” he said. “There are characteristic shapes of all bird body parts and they all have to work together. You can immediately recognize a bird’s hip or a bird’s arm. They inherited that from their dinosaur ancestors.”
As dinosaurs evolved into birds, so did the changes in their anatomy, Bhullar said. “The beginnings of becoming a bird involved a series of adaptations that allowed early dinosaurs to walk on two legs, and these adaptations were very substantially concentrated in the hip or pelvis,” he said.
The changes were “basically an extension of everything,” so that a bird’s pelvis extends over most of its rigid, compact body. Along with that came the expansion of the muscles needed to support the bird’s weight.
“In birds, it has expanded further than other dinosaurs,” Bhullar said.
That is in contrast to lizard legs, for example, which “are not muscular, but they are a bit cylindrical and lean”. Chicken thighs, on the other hand, have meat on the bone.
The change from dinosaur to bird hip so late in its development is also unusual. “In any case, my expectation was that most of these changes would be in place early on,” Bhullar said. “The sooner you screw up a process, the more it ends up messed up.”
As vertebrates develop from a single cell, the genes tell the cells how to change and what to become: skin, bone, or brain. “During the editing you have to change the instructions,” Bhullar said.
He expected changes going so far back in evolutionary history to occur early in the embryo’s development. “We had previously found that to be the case … for the platypus, but it turns out not to be the case for the pelvis,” he said. “During the life of every single bird in the little microcosm that is the egg, there is a passage through stages similar to those of their dinosaur ancestors.”
Since this is true, it means that any bird genome still has the instructions to make “dinosaur-like hips,” Bhullar said. Usually, genetic instructions that are no longer used are “distorted because nothing works to stabilize them,” he said. “They are thrown in an attic somewhere and left to break down.”
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Other structures in the bird embryo also resemble those of dinosaurs, but they “appear and then disappear, so they are transient,” Bhullar said. One is the pubis, which is located at the end of the pubic bone and which scientists believed to have disappeared 65 million years ago.
“The assumption was that there was nothing on Earth that had that particular structure at any point during its lifetime,” Bhullar said.
The technology the research team used is called CLARITY, Griffin said, and consists of three steps: making the embryo transparent, then staining it so that the nerves, muscles and bones glow under a laser, and finally viewing it through a confocal microscope.
Bhullar said he considers himself both a historian and a scientist.
“Basically what my lab does is we take an honest look at the modern world and we look at the existing biodiversity and we try to identify those vertebrates that have had a disproportionate impact on the modern world and the carpet of life on the surface. of the planet,” he said.
“Indisputably, the blooming of birds is one of the great events in the history of life on Earth.”
Ed Stannard can be reached at: estanard@courant.com203-993-8109.