Flatworms Are Oldest Living Ancestors To Those Of Us With Right And Left Sides, Researchers Report In Science

Researchers agree that the first multicellular animals had circular shapes that were "radially symmetrical," just like the jellyfish or sea anemones of today. A crucial step in the evolution of more complex organisms was the transition to body shapes with bilateral symmetry. (For example, most of the features that allow an animal to move itself from one place to another--legs, fins, wings--can only develop on bilateral organisms.) In spite of the major evolutionary importance of this transition, until now scientists have known little about the earliest bilateral animals.

It has generally been thought that most bilaterians arose during a dramatic diversification of animal life (dubbed the Cambrian explosion) 540-500 million years ago. That's because the ancestors of nearly all major modern animal groups, or phyla, made an appearance in the fossil record during this period. However, evidence has been growing that there was an extended and fruitful period before the Cambrian when bilateral organisms may have arisen and diversified. The new study by Jaume Baguñà, of the University of Barcelona, and his colleagues supports this theory by identifying a group of contemporary flatworms called the Acoela as the living descendants of an early lineage from this pre-Cambrian time. (Flatworms, several of which are parasites, are only distantly related to, and much simpler than, the familiar earthworms.)

"It may well be that the origin of bilateral organisms occurred a bit earlier on, [before the Cambrian explosion,] from simple animals whose fossils have not been recovered," said Baguñà. "This may mean that we need to look more carefully at pre-Cambrian rocks or sediments to search for primitive bilateral animals."

Baguñà and his colleagues, Iñaki Ruiz-Trillo and Marta Riutort of the University of Barcelona and D. Timothy J. Littlewood and Elisabeth A. Herniou of the Natural History Museum in London, began their study with the intention of investigating a troublesome subgroup of flatworms that didn't fit neatly into its prescribed classification. The Acoela, as this subgroup is known, are unlike other flatworms for a number of reasons, particularly because they are unusually primitive. For example, other flatworms have digestive tracts of some sort, but the Acoela lack them altogether.

To determine whether the Acoela might not be better classified separately from the other flatworms, Baguñà and his colleagues took a molecular approach. Molecular studies such as this investigate the evolutionary relationships among taxonomic groups, based on the assumption that mutations in a gene occur at a constant rate. Once researchers have determined the sequence of a particular gene, they can compare the gene in a variety of organisms. If the sequences are significantly different, that implies that the organisms are more distantly related. In other words, more time has passed since they diverged from their common ancestor.

The research team sequenced the Acoela's "18S rDNA" gene, which had already been sequenced in many other animals, including other types of flatworms. The scientists then compared this sequence in a variety of animals, using a software program to create an evolutionary road map that showed the most likely relationships among the organisms. According to the results, the Acoela were the first group of organisms to split off from the radial organisms, well before the other flatworms arose in the midst of the Cambrian explosion. The scientists suggest that the Acoela should be classified in their own new phylum.

At some point during the period in which bilateral animals appeared, another important transition took place, this one involving a fundamental change in the way animals developed as embryos. Radial animals are also "diploblastic," meaning their tissues develop from two primary layers of embryonic cells. In contrast, tissues of the more complex "triploblasts" develop from three primary layers of embryonic cells. Because information from the pre-Cambrian is so sparse, researchers don't know yet the details of when or how this other change occurred-they just know that bilaterians were also triploblastic. "The finding of the Acoela well in the middle of the long branch separating diploblasts from triploblasts may be the first item to bridge the gap and to give us a better understanding of how the major body plans emerged," said Baguñà.