Fossil evidence has allowed researchers to peer into the evolutionary past of the spinal cord.
New research suggests remnants of the first vertebrate skeleton are older than previously believed, the European Molecular Biology Laboratory (EMBL) reported.
Humans are part of an animal group called chordates which are defined by a rod of cartilage, called the notochord, that runs along the middle of the body below the spinal cord. The notochord was believed to be the first vertebrate skeleton and is present in human embryos before being replaced by the backbone and reduced to discs. Starfish, sea urchins and related animals do not possess this structure, so scientists assumed it had emerged in fairly recent ancestors after our evolutionary tree split from that of the starfish.
"People simply haven't been looking beyond our direct relatives, but that means you could be fooled, if the structure appeared earlier and that single group lost it," says Detlev Arendt from EMBL, who led the study. "And in fact, when we looked at a broader range of animals, this is what we found."
The researchers identified the genetic signature of the notochord and found the larva of the marine worm Platynereis has a group of cells with the same genetic signature. They followed these cells as the larva developed and found they formed muscles that ran along the creature's midline where a notochord would be if the worm were a chordate. The team dubbed the muscle the axochord.
Research revealed that most of the animal groups that sit between Platynereis and chordates on the evolutionary tree also possess this axocord. The structure most likely first emerged in an ancient ancestor before the different animal groups branched out on their separate evolutionary paths. This would explain why the lancelet amphioxus, a "primitive" chordate, possesses a notochord with both cartilage and muscle. The amphioxus could be a living example of the transition from muscle-based midline to cartilaginous notochord.
The findings were published Sept. 12 in the journal Science.
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