DNA studies have caused a tiny marine flatworm to take a large leap down the tree of life. In today's Science , biologists report that the worms, called acoels, may be a missing link between the first simple organisms like jellyfish and the more advanced animals, like arthropods and vertebrates.
So far, acoels had been placed in the platyhelminths, a group comprising not just flatworms but also tapeworms and liver flukes. But anatomical features like simpler brains, kinked cilia, and a different pattern of development had suggested that acoels might not be related to other flatworms. So Jaume Baguñà, a geneticist at the University of Barcelona in Spain, and Timothy Littlewood, a molecular biologist at the Natural History Museum in London, decided to use molecular studies to evaluate the acoel's placement on the tree of life. The duo obtained DNA from 18 acoel species from around the world and sequenced the genes for the 18S ribosomal RNA subunit from each. They then compared these data to the same genes from other platyhelminths and from both simpler and more complex organisms.
They had to drop the 16 fast-evolving acoel species from the analysis, because their DNA sequences were so different from those of simpler organisms that the phylogenetic analyses would be suspect. But when they used only the two slow-evolving species, "the acoels dropped out completely from the rest of the platyhelminths," notes Mark Martindale, a developmental biologist from the University of Hawaii, Honolulu, and into a lower position on the animal tree of life.
They may be living relics of the transition between simple, radially symmetric organisms like the jellyfish and more complex bilaterally symmetric organisms. Acoels differ from simpler jellyfish in having three types of cells, instead of just two; but the new cell type, the mesoderm, is not organized into a distinct gut, the way it is in other bilateral animals. Acoels also differ from other bilateral animals in the way their cells divide during development. But because they represent an ancient group of organisms, the acoels likely have many of the same genes as the earliest bilateral animal. For those reasons, these new results are "quite exciting," says David Jablonski, a paleontologist at the University of Chicago. "We might have one bilateral survivor giving us a living window on early metazoan life."