"there is a bilaterian in that truck," Jun-Yuan Chen said as we watched the vehicle disappear around a bend in the road. Chen, a paleontologist at the Chinese Academy of Sciences in Nanjing, and I, along with Stephen Q. Dornbos, a colleague then at the University of Southern California, had just collected a truckload of black rocks from a 580-million- to 600-million-year-old deposit in Guizhou Province. Chen was sure they held something important.

We had come to Guizhou in 2002 to hunt for microscopic fossils of some of the earliest animals on earth. Specifically, we were hoping to find a bilaterian. The advent of bilateral symmetry--the mirror-image balance of limbs and organs--marks a critical step in the history of life. The first multicelled animals were not bilaterally symmetrical; they were asymmetrical aquatic blobs--sponges--that filtered food particles from currents they generated. Radially symmetrical aquatic creatures, the cnidarians, are slightly more complex; they have specialized stinging cells that can immobilize prey. Bilaterians constitute all the rest of us, from worms to human beings. During some stage in their life cycle, all display not only the crucial left-right balance but a multilayered body that typically has a mouth, gut and anus.

This article was originally published with the title The Early Evolution of Animals.

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