Exploring What Drives Evolution

By Jack Lee

Dr. Watanabe recently presented a webinar entitled “How Head Structures Diversify: Investigating Skull and Brain Evolution Using High-Density Shape Analysis” as part of the “Inspiring Scientific Curiosity and Discovery” series hosted by the American Association for Anatomy, The Anatomical Record and Developmental Dynamics.

Watch the webinar here.

Ever since he was a child, Aki Watanabe wanted to be a scientist.

“I was really into dinosaurs and paleontology in general when I was a kid,” said Watanabe, an assistant professor of anatomy at New York Institute of Technology. These interests waned as he grew older. But in college, he began working in Paul Sereno’s Fossil Lab at the University of Chicago. The experience rekindled his love of prehistoric animals. It also sparked a passion for studying anatomy.

Aki Watanabe

Watanabe been tackling big evolutionary questions ever since. He has explored how the brains of birds evolved from their dinosaur ancestors. He has also investigated why snakes and lizards developed such vastly different skull shapes. Like Charles Darwin, Watanabe is fascinated by how “endless forms most beautiful” have come to be.

In recognition of his research contributions, the American Association for Anatomy awarded Watanabe the 2022 W.M. Cobb Award in Morphological Sciences.

Quantifying Shape

Watanabe uses quantitative approaches to understand anatomical variation. His research relies on morphological data captured through computed tomography (CT) and 3D surface scanners. The CT technology uses x-rays to generate detailed 3D images of internal structures of specimens.

Geometric morphometric methods, or statistical analyses of shape, allow Watanabe to quantify anatomical shapes. These methods typically involve placing coordinate points on features, such as the different bones that make up the skull. He and his collaborators have helped develop approaches for collecting high-density shape data, where hundreds or thousands of equally spaced points could be projected onto a specimen. These rich anatomical datasets enable researchers to investigate how biological shapes have changed through evolutionary or developmental time.

“While collecting new data is an important task of scientists, we also try to make sure that the data that we’re collecting are accurately capturing the information we want.”

Akinobu Watanabe

Watanabe uses these techniques to study the animals that have intrigued him since childhood. “I saw the power and the versatility of the method and applying it to the organisms I love,” he said.

He developed a computational tool to calculate how many of the anatomical landmarks are needed to accurately characterize the shape of structures under study. This tool can help reduce the time needed for data collection by showing how many landmarks could be removed without sacrificing important anatomical variation. It can also determine whether incomplete or poorly preserved specimens could be incorporated into the analyses.

“While collecting new data is an important task of scientists, we also try to make sure that the data that we’re collecting are accurately capturing the information we want,” Watanabe said.

Squamate Skulls

Watanabe and his colleagues used these quantitative approaches to investigate the evolution of squamate skulls. Squamates are a diverse group of reptiles that includes snakes and lizards. The researchers compared CT scans of skulls across this group. They found that snakes and lizards have fundamentally different cranial architectures. But, surprisingly, interactions between corresponding bones, such as those in the back of the skull, continue to be shared.

“If a bone that makes up that part of the skull … changes shape, the other bones in that area change shape in a very predictable way,” he said. Based on a regression analysis of ecological factors, the researchers propose that diet and habitat greatly influenced skull evolution. Snakes, for example, show a correlation between diet and the shape of bones important for gape widening.

“Selection acted on snakes’ and lizards’ skulls in a very different manner,” Watanabe said, and this is a big reason why their skulls look so different even though they share a common pattern of interactions between individual skull bones.

Bird Brains

Watanabe uses similar techniques to study birds, the modern-day descendants of the dinosaurs that captivated him as child. He and his colleagues analyzed endocasts (internal molds of the brain cavity) to investigate how the avian brain evolved over time. In particular, they investigated how distinct regions of the brain vary across the tree of life.

“Different parts of the brain definitely evolved at different times along the dinosaur-bird transition,” Watanabe said. Modern birds also have more integrated brains than non-avialan dinosaurs, which mean that the evolution of different brain regions are tightly linked to one another. This contrasts with what scientists have long thought about how humans attained large brains, through expansion of the neocortex, independent of other brain regions.

“This historical notion that you need to have a more modular brain structure to have bigger brains … that’s not the only way of getting one,” Watanabe said. Potentially, birds evolved their large brains through a completely different process.

“It makes you rethink what it means to be birdbrained, which is not a nice thing to say colloquially,” Watanabe said. “But scientifically, it’s a compliment, because we know birds are smart and equipped with very large brains for their body size”

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