Ancient near-mammalian fossil gives insight to brain evolution

Sarah Bloodworth

Imagine if the average human gave birth to over 30 babies at a time. Ouch, right?

According to a hypothesis in a study from the journal Evolution, mammals bear smaller litters because somewhere down the evolutionary line, mammals developed bigger brains in exchange. UT researchers found extraordinary evidence of this, according to a paper published in Nature in late August.

Jackson School researchers discovered a rare fossil of an extinct mammal precursor and her 38 babies, according to the paper. The researchers said it serves as one of the best developments in our knowledge about brain evolution — and it was almost left behind.

The fossil didn’t look promising at first, said Timothy Rowe, geology professor and director of the Vertebrate Paleontology Laboratory. He found the fossil about 18 years ago in Arizona. Yet the specimen, Kayentatherium wellesi of the tritylodontid family, a mammal-like species that lived about 185 million years ago during the Jurassic Period, was on his shopping list.

“The head was busted apart,” Rowe said. “We almost left it behind. But it was well-preserved enough to tell what it was, so we made the decision to collect it anyway.”

Rowe said it was thanks to the careful eyes of UT fossil preparator Sebastian Egberts and the scanning skills of recent UT graduate Eva Hoffman they found the tiny babies that belonged to the tritylodontid.

“(Egberts) came running into my office with a little piece of shale with tiny teeth,” Rowe said. “The whole thing was the size of a pen,” Rowe said. “It seemed clear right away we had a litter. We stopped everything and starting scanning for more bone.”

The researchers used an updated CT scanner to reveal more and more tritylodontid babies. While doing so, they uncovered more about the evolution of brains, according to Hoffman.

“Having a big brain is advantageous probably because the ancestors of mammals were able to process more information,” Hoffman said. “We think it’s likely that there’s a transition to having a smaller number of babies that will survive in exchange for a bigger brain.”

Hoffman added that the fossil represents a transitional moment in mammalian evolution since the specimen had both primitive features, such as a small brain and large litter size, along with modern mammalian features, such as teeth that fit precisely together.

“It would be impossible for us to evolve our giant brains if, in the ancestry of mammals, there wasn’t a transition to a bigger-brained animal,” Hoffman said. “(The fossil) laid the groundwork for huge brains to evolve in humans later.”

Rowe has been tracking brain evolution for years and, according to him, it is the advancements in technology such as CT scans that will allow them to continue to put the pieces of the puzzle of evolution together.

“I’d never dream that I would find something like this, but there were times that I thought this would kill me, with all these moving parts,” Rowe said. “I often wonder how many of these kinds of finds we’ve missed without the technology.”

He added that there is more stories to come out of this fossil.

“We will keep scanning these fossil for years and years,” Rowe said. “No more needles and grinding wheels … the Nature paper is really just the start.”