Plenty of people have a hard enough time accepting that humans and apes evolved from a shared ancestor, but what if it goes deeper than that? What if one of our earliest ancestors was a tiny, nondescript fish living in Siberian waters over 400 million years ago? That’s the case presented by researchers from Oxford University, who claim to have found a fossil that may be a shared ancestor of all jawed vertebrates.

“There are over 60,000 species of living jawed vertebrates, and they encompass pretty much everything you can think of [with a backbone] that lives on land or in the sea,” lead researcher Sam Giles told LiveScience. “But we don’t really know what they looked like when they split.”

What makes the specimen so fascinating is that it has characteristics of both cartilaginous (like sharks) and bony fishes. It’s known that both types of fish shared a common ancestor about 420 million years ago. This specimen, dubbed Janusiscus schultzei, appears to be one of them.

Using CT scans, Giles and her team took hundreds of images of the fossil (which was embedded in rock) in order to create a 3D model. The model revealed features that pointed towards the bony/cartilaginous amalgamation: Sensory canals along the skull like those used by bony fish to avoid predation, along with blood vessels located within the skull similar to those found in species like sharks and rays.

It’s long been assumed that more cartilage was the early default in the evolution process, making later cartilaginous species (like sharks) necessarily less-evolved. Not the case, says Giles.

“But what this animal tells us [is] that actually the last common ancestor of the two groups had lots of bone,” Giles said. “So rather than sharks being primitive, sharks are actually very highly evolved in their own way, and just as highly evolved as we are.”

With all this talk about the fossil’s significance for basically all living jawed vertebrates, it’s odd that there’s little examination of the jaw itself. That’s because, in an ironic twist, the jaw itself is missing from the specimen. Giles expressed frustration, noting that the jaw is probably unfindable, settled at the bottom of a river somewhere.