The newly sequenced genomes of two marine worms are shedding light on the 570-million-year evolution of gills into the pharynx that today gives humans the ability to bite, chew, swallow and speak.
An adult female acorn worm, Saccoglossus kowalevskii, with eggs, collected near Woods Hole, Massachusetts. The series of parallel lines on the body below the orange collarare the pharyngeal slits used for filtering food from seawater. The gill slits were the origin of fish gills and the human pharynx. Chris Lowe photo, Stanford University.
The draft genome sequences of two species of acorn worm, which live in U-shaped burrows in shallow, brackish water, are the first genomes of hemichordates, which retain similarities to the first animals to evolve pharyngeal or gill slits. Those ancestors eventually gave rise to chordates: animals with backbones and hollow nerve cords, like humans and other vertebrates.
Since acorn worms and the human lineage diverged 570 million years ago, pharyngeal slits for filtering food evolved into gills for extracting oxygen, and later into todays human upper and lower jaw and pharynx, which encompasses the thyroid gland, tongue, larynx (voice box) and various glands and muscles between the mouth and the throat. Humans and other terrestrial vertebrates actually initiate vestigial gills while embryos, though they disappear quickly and rarely persist in infants.
The presence of these slits in acorn worms and vertebrates tells us that our last common ancestor also had them, and was likely a filter feeder like acorn worms today, said Daniel Rokhsar, one of the leaders of the sequencing effort and a UC Berkeley professor of molecular and cell biology and of physics. Acorn worms are marine invertebrates that, despite their decidedly non-vertebrate form, are nevertheless among our closest invertebrate relatives.
Its an ugly beast, acknowledged John Gerhart, senior author of the report and a professor of the graduate school at UC Berkeley. Acorn worms look very different from chordates, which makes it especially surprising that they and chordates, like humans, are so similar on the genomic, developmental and cell biological levels.