The recent sequencing of the genomes of two marine worms helped tell the story of the 570-million-year evolution of gills into the human pharynx.
The two species of acorn worm retain similarities to the first animals to develop pharyngeal or "gill" slits, the University of California at Berkeley reported. These ancient ancestors eventually developed into the first chordates, which are animals with backbones and hollow vertebrates like what is seen in the human anatomy. Acorn worms and the human lineage are believed to have diverged about 570 million years ago, and since then pharyngeal slits for filtering food evolved into gills, and then evolved again into the human upper and lower jaw and pharynx. Vestigial gills can be seen in early embryos, but usually disappear as the pregnancy progresses.
"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."
Surprisingly, about 70 percent of human genes have counterparts in acorn worms. The researchers compared clusters of the two worms' genomes with the sequences of many other animals, and found clusters of genes on the same chromosome in humans can also be found clustered on the acorn worm genome. A cluster of six ordered genes that are involved in the development of the pharynx and gill slits were found in both acorn worms and vertebrates. This could mean pharyngeal gill slits are an "evolutionary innovation" that defines the entire group.
The team also found deuterostomes (comprised of chordates, acorn worms, and echinoderms) have more than 30 genes that have no counterpart in non-deuterostomes but are resembling those found in algae and bacteria. The genes play a role in modifying the sugars that coat cells. The researchers believe these genes either are very ancient and were lost in other animals, or were acquired by "horizontal transfer" from algae and bacteria early in deuterostome evolution. It is extremely rare to find bacterial-to-animal gene transfers.
The findings were published in a recent edition of the journal Nature.