Human beings - along with all jawed vertebrates on Earth - are descended from a fossil fish that looks a lot like today's sharks.
A new analysis of the braincase of a 290-million-year-old fossil fish, Acanthodes bronni, has shed light on the point at which the sharks split off from the first bony fishes, from which we are descended.
"Unexpectedly, Acanthodes turns out to be the best view we have of conditions in the last common ancestor of bony fishes and sharks," says Dr Michael Coates of the University of Chicago.
"Our work is telling us that the earliest bony fishes looked pretty much like sharks, and not vice versa. What we might think of as shark space is, in fact, general modern jawed vertebrate space."
The group gnathostomes - meaning 'jaw-mouths' - includes tens of thousands of living vertebrate species, ranging from fish and sharks to birds, reptiles and mammals.
But while cartilaginous fish, which today include sharks, rays, and ratfish, are known to have diverged from the bony fishes more than 420 million years, little is known about what the last common ancestor of humans, manta rays and great white sharks looked like.
Coates and his team re-examined fossils of Acanthodes bronni, the best-preserved acanthodian species, creating highly-detailed latex molds of specimens revealing the inside and outside of the skull.
And their analysis, combined with recent CT scans of skulls from early sharks and bony fishes, showed a surprising similarity to sharks, despite the fact that evolutionalry analysis of Acanthodes bronni connected it to early bony fishes.
Some acanthodian species turned out to be primitive sharks, while others were relatives of the common ancestor of sharks and bony fishes. However, acanthodians as a whole, including our own earliest ancestors, appear to cluster with ancient sharks.
"It helps to answer the basic question of what's primitive about a shark." says Coates. "And, at last, we're getting a better handle on primitive conditions for jawed vertebrates as a whole."