When most people think about human evolution, they look to our closest cousins the apes. But dial the evolutionary clock back just a bit more (by perhaps a few hundreds of thousands of years) and you’ll find the point where humans diverged from a more wormy cousin: acorn worms.
Acorn worms are humans’ closest invertebrate relatives. The last common ancestor between our two lineages lived about 570 million years ago. These worms are a group of invertebrates that burrow in the ocean floor. They get their name from their front end, which is an acorn-shaped, muscular proboscis they use to burrow. Many species of the worm filter seawater through gill slits to catch floating bits of food.
Their appearance may seem far removed from people, but the same genes that give rise to those gill slits are involved in shaping the human pharynx, the passageway leading to the esophagus and larynx that gives us the ability to chew, swallow and speak, writes Robert Sanders in a press release.
Now, new research from a team of international researchers shows that humans share about 70 percent of their genome with acorn worms, according to a new study published in the journal Nature. "It’s an ugly beast," says an author of the paper John Gerhart, of the graduate school at the University of California, Berkeley. But that doesn’t make the creatures any less important.
The researchers decoded the DNA of two species of acorn worms to investigate the roots of the vertebrate family tree. They teased out the shared genes comparing the DNA of the Atlantic acorn worm Saccoglossus kowaleskii and a tropical Pacific acorn worm Ptychodera flava with other animals.
Similarities actually extend beyond the critter's genes to an anatomical quirk seen early in development: Humans and other terrestrial creatures briefly sport vestigial pharyngeal gill slits as embryos. The gene cluster involved in this act is shared by all "deuterostomes" (a large group of animals that includes vertebrates as well as some invertebrates such as sea stars and yes, acorn worms).
If having such a homely cousin is unappealing, take comfort in the fact that 30 percent of the human genome isn’t shared with acorn worms. We likely have more genetic similarities with those whose evolutionary lineage diverged from ours more recently: attractive creatures like the blobfish, the naked mole rat and the probosics monkey.