How fish grow a spine

The first vertebrates arose some 525 million years ago, during huge outburst of evolutionary creativity known as the Cambrian explosion. They would have been something like a rudimentary fish, filter-feeders of the seabed with skeletons made of cartilage.

About half of modern vertebrates are fish, while the other half are tetrapods, or four-limbed creatures: amphibians, reptiles, birds and mammals.

While all have backbones by definition, not all grow their backbones in the same way.

All land-dwelling vertebrates (including humans) grow their spines from a group of cells called somites. Ray-finned fish (which have fins of skin supported by bony spines), on the other hand, grow their spines using two different types of cell: somites and another kind called chordoblasts.

Scientists have wondered what this meant for the original vertebrates: did they have use both types of cells, or somites alone?

New research on the spinal development of the little skate, a cartilaginous fish (which are more distant relatives of the tetrapods than are the ray-finned fish), has shed some light on the question.

Using fluorescent cell tracking, Katharine Criswell and Andrew Gillis of Cambridge University determined that the skate’s spine grows from somites alone. This led them to conclude that the common ancestor of all jawed vertebrates used somites, and ray-finned fish later evolved a separate process of spinal development.

“Backbone development in sharks and skates is more similar to humans than it is to bony fishes like zebrafish and salmon,” says Criswell. “It refutes the old notion of sharks and skates as ‘primitive’ fishes.”“Backbone development in sharks and skates is more similar to humans than it is to bony fishes like zebrafish and salmon,” says Criswell. “It refutes the old notion of sharks and skates as ‘primitive’ fishes.”

The research is published in Proceedings of the Royal Society B.