Research led by a Yale Ph.D. student has demonstrated a key link between the evolution of the brain and the skull, dating back from ancient dinosaurs all the way to today’s reptiles.

The study centered around the concept of paedomorphism, the idea that certain organisms retain juvenile traits from their evolutionary ancestors. Looking at how the brain and skull have evolved over millions of years has shed light on how different regions of the skull have changed, answering long-standing questions in the field of paleontology about the evolution of various bones. This research falls under the relatively new field of evolutionary developmental biology, or “evo-devo”, in which scientists consider how the evolutionary lineage of an organism relates to its development.

“What we saw was that different regions of the brain and different bones were actually linked together,” said first author Matteo Fabbri GRD ’21, a member of the Bhullar Lab at Yale.

By performing CT scans of the skulls of numerous dinosaurs and other animals, the researchers were able to look at how exactly different bone regions evolved over time. Assembling all of the samples needed for this work required a large-scale collaboration between many institutions with differing resources, according to Arkhat Abzhanov, a researcher at Imperial College London who worked with the Yale team.

By looking at the bones of extinct species and the embryos of modern reptiles and birds, the researchers concluded that the evolution of the brain was responsible for the skull changes that occurred over many millennia, as opposed to the reverse process taking place.

“Being able to use two different yet complementary approaches to understand how species evolved across deep time scales is to me the highlight of this project,” said Nicolás Mongiardino Koch GRD ’21, another author on the paper.

To accomplish their goal, researchers had to assemble rare fossils from around the world, and then process and analyze the data, Abzhanov said.

As a result of this large-scale survey of various samples, they were able to solve other questions that have plagued the field of paleontology, such as the disappearance over millions of years of a separate postparietal — a bone found in the skull of some early reptiles, which was removed through evolution.

According to Fabbri, studying this relationship between evolution and development could hold the key to understanding why modern animals have evolved to be what they are today — and the skull is the most important step of this process.

“From an evolutionary perspective, the head can tell you a lot about an animal,” Abzhanov said. “If you compare skull shapes, they are quite similar to a very juvenile dinosaur ancestor.”

He added that evolution will sometimes select for this more juvenile structure because it is smaller, lighter and faster, which is especially advantageous for modern birds that need to remain small and agile.

Now that a link has been established between brain evolution and the skull, an important next step would be to figure out the actual genetic changes that took place to cause this evolution, according to Fabbri. He explained that while the scientific community does not have access to the DNA of ancient reptiles, evo-devo techniques allow researchers to study the development of modern embryos to investigate the effects of evolution.