Worms Help Explain Why Our Spines Are on Our Backs

What distinguishes humans and flies? There seem to be so many qualities that differentiate us from our annoying winged companions as to make the question seem trivial, yet to many biologists it is everything but. The question becomes particularly important when discussing the placement of the nerve cord, a rope ladder of nerve cell clusters known as ganglia that is part of an animal's' central nervous system.

Flies and many other insects have a ventral nerve cord (meaning it runs along their front side), whereas humans have a dorsal nerve cord (meaning it's on our backside). While this trivia may seem unremarkable, for biologists it poses a major evolutionary conundrum because ventral and dorsal nerve cords share a common origin. This suggests that the nerve cord has always been in the same place, but some animals' bodies flipped relative to the position of the nerve cord—flies to the front, humans to the back. The question is what happened genetically to cause this flip, and to search for answers, scientists are studying the genome of a bizarre creature known as the acorn worm.

In a paper published last week in Nature, a number of researchers led by the Okinawa Institute of Science and Technology's Oleg Simakov turned to two species from a group of exclusively marine invertebrates known as hemichordates to see what they might tell us about why our spinal cord is on our back rather than our stomach.

Turning to an ocean dwelling worm to figure out humanity's spinal mystery might seem like a strange choice, but as it turns out, we share quite a bit in common, genetically speaking. Acorn worms comprise one of two groups of hemichordates and are formally known as enteroneusts, solitary marine animals which burrow in soft sentiments and can grow up to 2 meters in length.

By studying the genomes of two species of acorn worms, Simakov and his colleagues were the first to publish a hemichordate genome, the significance of which has less to do with the hemichordates themselves than how they relate to other animals, particularly humans. What the team found is that the acorn worm genomes share many features with humans—they have about the same number of genes (around 19,000) and these genes are organized in a remarkably similar manner.

What this means is that many features of the chordate genome (a phylum which includes humans) are not chordate specific—rather these genetic traits were present much earlier in evolutionary history, as evidenced by the fact that they are shared with hemichordates such as the acorn worm.

While Simakov and his colleagues did not solve the mystery of why some deuterostomes (a superphylum which includes both chordates and hemichordates) developed a ventral nerve cord and some developed a dorsal nerve cord, their unprecedented hemichordate genome maps are an important step in that direction.

There are several reasons why it is so difficult to determine which genomic aspect gives rise to certain developmental, anatomical or functional traits. In the first place, many genome changes are neutral, evolutionary speaking: they neither increase fitness nor influence traits. Further, the function of the genome itself evolves, meaning that similar genomic features do not necessarily yield the same traits across species.

All of this goes to say that for as much as we know about genome evolution, we still have a long way to go before we really understand how certain genomic traits impact anatomical traits, such as nerve cord placement. What Simakov and his colleagues showed us is that striking anatomical traits do not necessarily arise from radically different genomes. To find out just what causes these developmental disjunctions we're going to need a lot more genomes, and in all probability, a lot more worms.