In most narratives, the story of evolution is the story of organisms emerging from the ocean and eventually populating the land.

But for some species that evolution also involved a return trip. Dozens of major mammal and reptile groups ultimately made their way back to the beach and into the water. A new Yale study has undertaken the task of explaining when and how this happened — and which species fully re-committed to the life aquatic.

The study appears in the journal Current Biology.

“These secondarily aquatic groups adapted in strikingly similar ways to their new aquatic home — evolving flippers and a suite of other features that made them better swimmers,” said lead author Caleb Gordon, who earned his doctorate as a student in Yale’s Graduate School of Arts and Sciences (GSAS) earlier this year and is now a postdoctoral researcher at the Florida Museum of Natural History. “As a result, they’ve become textbook examples of convergent evolution, which can tell us a lot about the processes driving and constraining adaptive change in response to similar environmental cues.”

For the study, the researchers analyzed hundreds of specimens in the collection of the Yale Peabody Museum and dozens of other institutions around the world, taking new measurements (more than 11,000), photographs, and CT scans. The team also made use of classic paleontology methods, phylogenetic machine-learning algorithms, and even World War II-era naval statistics — all to reconstruct one of the most significant evolutionary transitions in natural history.

“Reconstructing the lives of extinct life forms in a scientifically rigorous way, as opposed to just telling stories, is a precise and delicate undertaking,” said Bhart-Anjan Bhullar, associate professor of Earth and planetary sciences in Yale’s Faculty of Arts and Sciences (FAS), associate curator of vertebrate paleontology and vertebrate zoology at the Peabody Museum, YIBS faculty affiliate, and senior author of the new study.

“It requires a careful interweaving of data from modern organisms, of which our understanding is necessarily far deeper, and knowledge of these living organisms’ genealogy relative to fossil forms,” he said. “Caleb’s work accomplishes all of this and more, and on a massive evolutionary scale.”

The lack of a robust, comprehensive explanation for when various animal groups fully returned to the water has divided the scientific community for years. Researchers had to rely upon their own interpretations of fragmentary fossil samples that had uncertain relationships to one another — with some features suggesting the animals lived on land and other features suggesting they lived in the water.

“In these cases, paleontologists are often stuck, as different lines of evidence disagree about what the ancient animal was like,” Gordon said. 

To help solve this challenge, he developed a method that used simple but powerful machine-learning models, trained on modern species, to predict the aquatic habits and associated soft-tissue adaptations of ancient extinct species with historically contentious ecologies.

For more information, click here for an article published by Yale News or here for the full study in Current Biology.