Thoughts of the family tree may not be uppermost in the mind of a person suffering from a slipped disc, but those spinal discs provide a window into our evolutionary past. Humans are part of a group of animals called chordates, whose defining feature is a rod of cartilage that runs lengthwise along the middle of their body, under their spinal chord. This structure, called the notochord, was the first vertebrate skeleton. It is present in human embryos, and is replaced with the backbone as we develop, with the cartilage reduced to those tell-tale discs. Since starfish, sea urchins and related animals have no such structure, scientists assumed the notochord had emerged in a relatively recent ancestor, after our branch of the evolutionary tree split away from the \u2018starfish branch\u2019.<\/p>\n\n\n\n
People simply haven\u2019t been looking beyond our direct relatives, but that means you could be fooled<\/p><\/blockquote>\n\n\n\n
\u201cPeople simply haven\u2019t been looking beyond our direct relatives, but that means you could be fooled, if the structure appeared earlier and that single group lost it,\u201d says Detlev Arendt<\/a> from EMBL Heidelberg, who led the research. \u201cAnd in fact, when we looked at a broader range of animals, this is what we found.\u201d<\/p>\n\n\n\n
Antonella Lauri and Thibaut Brunet, both in Arendt\u2019s lab, identified the genetic signature of the notochord \u2013 the combination of genes that have to be turned on for a healthy notochord to form. When they found that the larva of the marine worm Platynereis dumerilii<\/em> has a group of cells with that same genetic signature, the scientists teamed up with alumnus Philipp Keller\u2019s group<\/a> at the Howard Hughes Medical Institute in Janelia Farm, to use state-of-the-art microscopy to follow those cells as the larva developed.<\/p>\n\n\n\n
They found that the cells form a muscle \u2013 which they named the axochord \u2013 that runs along the animal\u2019s midline, precisely where the notochord would be if the worm were a chordate.<\/p>\n\n\n