Nearly 600 million years ago, early life was on the move. In the sedimentary layers of Uruguay’s Tacuarí Formation, geologist Ernesto Pecoits discovered trace fossils of an animal whose left side mirrored its right – this is the oldest known instance of bilateral symmetry. Many of the early forms of multicellular life were radially symmetrical, like a pie cut into four, five, six, or eight equal slices. Today, however, most animals exhibit bilateral symmetry – our right sides reflect our left sides.

If you’re the sort of animal that moves slowly, floats with the flow, or just stays put (think starfish, jellyfish, or sea anemone), there’s a good chance that you are radially symmetrical. If you’re a trail blazer, however, you’re probably bilateral. With a streamlined bilateral body, you’re better equipped for forward motion. This increased mobility can help you catch prey, and it can help you avoid becoming prey.

Unlike body fossils, trace fossils, like the ones Pecoits found in Uruguay, are not the mineralized remains of an organism – they’re the preserved imprints of an organism’s activities. These traces can be trails, footprints, bite marks, and even coprolites (fossilized poop!). The fossils that Pecoits found are burrows, bored into the sediment floor of Tacuarí’s shallow sea 585-600 million years ago. By analyzing the characteristics of these burrows, Pecoits deduced that their source was an early bilaterian eumetazoan. The subkingdom Eumetazoa includes nearly all animal life, with the exception of sponges and a few other groups of organisms.

According to Pecoits, the eumetazoans that left these traces behind were oxygen-breathing grazers, no bigger than a centimeter, which propelled themselves through the sediment by pushing off with their feet-like appendages. Prior to Pecoits’ finding, the oldest known instance of bilateral symmetry was believed to have occurred 30 million years later. This discovery pushes back the date for the evolution of bilateral symmetry, granting us new knowledge about a crucial adaptation in the early development of life on Earth.

Read more at Live Science.

Read the original article in the journal Science.

Published On: November 27, 2012

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