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Flatworms Are Oldest Living Ancestors To Those Of Us With Right And Left Sides, Reported In The 19 March Issue Of Science

This news release is also available in Spanish.

Washington DC - A team of scientists from Spain and the UK has determined that a certain curiously primitive group of flatworms are the oldest living ancestors to all "bilateral" animals-that is, those with a right and left side. These worms were previously thought to belong to a much younger group of organisms, and their newfound identity also implies that bilateral organisms began making their debut on Earth earlier than previously thought. The finding is reported in the 19 March issue of 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 Bagu, 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 lineag
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Contact: Gabriel Paal
gpaal@aaas.org
202-326-6421
American Association for the Advancement of Science
19-Mar-1999


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