Scientists Trace Back Earliest Evolution of Human Teeth to Ancient Species of Reptile

The discovery of a 300-million-year-old extinct reptile has provided evolutionary insight into the predatory nature of mammals. A study conducted by University of Bristol researchers and published in Royal Society Open Science found that the teeth of the now-known extinct species, known as Shashajaia, laid the foundation for incisor, canine and molar teeth that all mammals, including humans, now possess. Dr. Suresh A. Singh, postdoctoral research associate in the Department of Earth Sciences at the University of Bristol and study co-author, told Newsweek that Shashajaia sits at or close to the base of the therapsid evolutionary tree "and is therefore on our own evolutionary lineage as we descend from therapsids."

"Shashajaia appears to show that its branch of synapsids had evolved tooth differentiation [heterodonty]—this was passed on to their descendants and subsequent species have built on those basic adaptations, eventually resulting in the incisors, canines, and molars that we have in our mouths," Singh said. According to Singh, the study began due to his colleague, Dr. Adam Huttenlocker, discovering the fossil during fieldwork in summer 2019. He is described as an "expert" on synapsids, or mammals and their extinct relatives. "Adam came to me in the summer of 2020 as I was finishing up my Ph.D. because my specialty is studying broad patterns of evolution and ecology across multiple different animal groups through time," Singh said. "I've been studying synapsid jaw biomechanics and Adam knew I had the wider data and analytical knowledge he needed to better understand and highlight the importance of new species within the wider context of synapsid evolution." Huttenlocker, an assistant professor of clinical integrative anatomical sciences at the University of Southern California and the study's lead author, could see from the anatomy that Shashajaia had a clearly defined canine-type tooth, set apart from the other front teeth. This is typical of more advanced synapsids known as therapsids, or "mammal-like reptiles," Singh said, though it has never been seen before in a non-therapsid synapsid. When the fossil was fully excavated, it was revealed that the teeth toward the back of the jaw were also stubbier. "Canine-like teeth in small sphenacodontids like Shashajaia might have facilitated a fast, raptorial bite in riparian habitats where a mix of terrestrial and semi-aquatic prey could be found in abundance," Huttenlocker said.

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Singh said paleontologists have long thought that therapsids evolved and split off the evolutionary tree from other synapsids around 300 million years ago, based on when their nearest relatives, the sphenacodontids, evolved—animals like the famous sail-backed Dimetrodon. However, he said, the oldest therapsid fossils come from around 270 million years ago "so there's a big gap in the fossil record." Shashajaia was discovered in summer 2019 at a fossil site in the Valley of
the Gods, Utah—a site discovered in 1989 by Dr. David Berman, known for his work with The Carnegie Museum of Natural History. The full name of the new species is Shashajaia bermani, meaning "Berman's bear heart" in honor of Berman and the local Navajo people of the discovery site. "Dr. Berman led many of the digs in the Bears Ears area of southern Utah, which is why we decided to name the new species after him and his extraordinary career spanning 51 years studying these early land animals," Singh said. "This area has seen strong interest over the years from paleontologists, especially those working on early tetrapods as the rocks here archive the final stages of the Late Paleozoic Ice Ages, so understanding changes in its fossil assemblages through time will shed light on how climate change can drastically alter ecosystems in deep time, as well as in the present." It isn't that unusual for humans to share common ancestry with reptiles due to millions of years of evolutionary traits that become advantageous or disadvantageous," Singh said. However, he said the trait of heterodont teeth and it appearing this early in history, "was remarkable." "For me, the most important finding is that we are now filling in the gap in our evolutionary tree," he said. "We now know that we evolved from something that probably looked very similar to Shashajaia. The comparative analytical part of the study tells us more about our origins; our earliest ancestors were eking [out] a living as small predators and one of the hallmarks of mammals, heterodont teeth, likely evolved as a way of improving prey capture for these little carnivores."