The evolution of bilateral symmetry—the left-right balance of arms, legs, and organs that is a hallmark of all higher animals—was one of the greatest leaps in the history of life. In June a team of paleontologists identified the oldest example of such symmetry in a group of fossils excavated from a 600-million-year-old rock quarry in southern China.
Jun-Yuan Chen of the Nanjing Institute of Geology and Paleontology and his colleagues collected and studied samples of Vernanimalcula guizhouena, a microscopic animal that probably moved along the seafloor sucking in bacteria for food. Chen sees signs of a single anterior mouth as well as a set of paired digestive canals on either side of the gut. That would mean the first symmetrical animals appeared up to 30 million years earlier than previously known, well before the Cambrian Explosion around 540 million years ago, when a wide array of hard-bodied animals are first seen in the fossil record.
Some paleontologists suggest the perceived symmetry in V. guizhouena might simply be due to the petrification process. David Bottjer of the University of Southern California, who worked with Chen, responds that the V. guizhouena fossils were found in an unusual mineral setting that preserved them in exceptional detail. And a very ancient origin of symmetry makes sense: Because all but the most primitive animals are bilateral at some stage in their life, Bottjer says, “this basic feature must have been an early evolutionary innovation.”