'Two-Faced' Fossil Shows Humans Evolved from Cartilaginous Fishes

A "two-faced" fossil found about four decades ago in Siberia and dating back 415 million years is just revealing its secrets, showing that humans and other jawed vertebrates actually evolved from cartilaginous fishes, according to new research.

The fish fossil (Janusiscus schultzei) was originally believed to belong to the bony fishes known as osteichthyans, a group that includes cod, tuna and all land-dwelling creatures with backbones. But now a new, high-tech look inside its skull shows structures also resembling fish made of cartilage (chondrichthyans) such as sharks and rays. The findings indicate that this "two-faced" fish is a common ancestor of both bony and cartilaginous fishes, and provides clues as to how early jawed vertebrates looked.

"This 415 million year-old fossil gives us an intriguing glimpse of the 'Age of Fishes', when modern groups of vertebrates were really beginning to take off in an evolutionary sense," Dr. Matt Friedman of Oxford University, who was involved in the study, said in a statement. "It tells us that the ancestral jawed vertebrate probably doesn't fit into our existing categories."

Chondrichthyans have long been considered the common ancestor of jawed vertebrates, causing many to see sharks and manta rays as more primitive creatures than bony animals. However, after conducting X-ray computed tomography (CT) to create a virtual, 3D image of J. schultzei, researchers are turning this prior notion on its head.

Now, the discovery of certain cartilage features, in addition to an external skeleton and large bony plates, which characterize bony fish, possibly make this 415-million-year-old fossil one of the earliest common ancestors of jawed vertebrates like humans (despite the fact that it itself is missing a jaw).

"This mix of features, some reminiscent of bony fishes and others cartilaginous fishes, suggests that humans may have just as many features that you might call 'primitive' as sharks," Friedman added.

The research was published in the journal Nature.

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