Sharks are often seen as "living fossils," examples of evolutionary excellence that have not altered their design significantly since they came into existence. Evolutionary biologists have theorized specifically that the creatures' respiratory systems, fed by efficient gills, were present in the species since they first diverged on Earth more than 400 million years ago. But researchers have recently discovered a fossil record that appears to refute that theory.

A study of the 325-million-year-old "shark-like" creature, published in scientific journal Nature, suggests that ancient sharks might have developed their gills after bony fish did. The authors of the study say the fossil, which represents the earliest identified cartilaginous fish with a preserved respiratory system, has a gill structure more like a modern bony fish than a shark.

The fossil has gills that make it look more like a bony fish than a shark

The scientists say the findings "invert the classic hypothesis, in which modern sharks retain the ancestral condition," suggesting that sharks evolved their gills after bony fish, honing them over millennia. The structure supporting these early gills is believed to have been essential in the evolution of jaws, a mutation that paved the way for the evolution of many land-based vertebrates, including humans. The scientists say the findings "profoundly affect our understanding of evolutionary history."