During PM’s recent reader trip to Gansbaai, where our guests got up close and personal with at least 12 individual great white sharks in a cage diving experience, I asked a passing researcher if she could tell us when the last common ancestor of sharks and humans was alive, guessing it was about 360 million years ago. (She wasn’t sure, but neither did she cackle or roll her eyes.)
Diving into the Net on my return home, I discovered that I was out by about 70 million years – give or take a few days. Not only that, but the common ancestor of all jawed vertebrates on Earth – including humans – resembled a shark (no real surprise there). This comes from an analysis of the braincase of a 290-million-year-old fossil fish that has long puzzled palaeontologists.
Recent research on Acanthodes bronni, a fish from the Paleozoic era, sheds light on the evolution of the earliest jawed vertebrates and offers a new glimpse of the last common ancestor before the split between the earliest sharks and the first bony fishes – the lineage that would eventually include human beings.
Says Michael Coates, professor of organismal biology and anatomy at the University of Chicago and senior author of the study published in Nature: “Unexpectedly, Acanthodes turns out to be the best view we have of conditions in the last common ancestor of bony fishes and sharks. 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, mammals and humans. Cartilaginous fish, which today include sharks, rays and ratfish, diverged from the bony fishes more than 420 million years ago. But little is known about what the last common ancestor of humans, manta rays and great white sharks looked like.
Coates and colleagues Samuel Davis and John Finarelli found answers to this mystery in an unexpected place: the acanthodians, extinct fishes that generally left behind only tiny scales and elaborate suites of fin spines. But armed with new data on what the earliest sharks and bony fishes looked like, Coates and colleagues re-examined fossils of Acanthodes bronni, the best-preserved acanthodian species.
Their revision of the lineage of early jawed vertebrates will allow palaeontologists to dig into deeper mysteries, including how the body plan of these ancient species transformed over the transition from jawless to jawed fishes. “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.”
(Source: University of Chicago Medicine.)