Sea squirts are oddballs. They spend their first days of life in the ocean as swimming tadpoles. Then something weird happens: the tadpoles attach to rocks and undergo a drastic metamorphosis, transforming themselves into tubular creatures fixed to the ocean floor.

An evolutionary comparison of the human and sea squirt genomes may reveal when specific genes appeared in the human genome, says Daniel Rokhsar of JGI, noting that the goal of the project is to better understand human biology.

The sea squirt genome contains roughly 16,000 genes—half the number found in mice and humans, but in line with other invertebrates like the fruit fly. It was sequenced using the whole-shotgun sequencing method. The genome is extremely compact, nearly 20 times smaller than the human genome. Ciona intestinalis has many of the same genes as humans, but has single gene copies instead of the multiple copies found in higher organisms. Gene duplication is thought to lead to an increased number of genes and the evolution of more specialized and complex traits.

"Despite how simple and humble it looks, the sea squirt has rudiments of vertebrate organs," says Levine.

Researchers found sea squirt genes that match human thyroid hormone genes, yet much of the genome seems tailored to the life of a sea squirt. Indeed, it appears that sea squirts and vertebrates have used the same "raw materials" of an ancestral genome to "their unique purposes and needs," the scientists write Science The team found genes involved in producing the tough sheath—made of cellulose—that encompasses the sea squirt. This was unexpected because only plant and bacteria are known to produce cellulose.

      A multitaxon molecular study in 2010 examined the hypothesis that sea squirts are descended from a hybrid between a chordate and a likely extinct protostome ancestor, a hybridization event which supposedly took place at a time before the diversification of nematodes and arthropods.