Precambrian. About 700 million years ago, one of the most significant - and most mysterious - events in the history of life on Earth occurred. Suddenly, there was more to life than just single-celled microbes. Within a few tens of millions of years, an extraordinary array of large animals appeared, armed with jaws and claws and eyes and brains.
Yet we still know surprisingly little about the origin of multicellular animals. "The different branches of the animal tree evolved very rapidly in a short period, a long time ago," says Nicole King, an evolutionary biologist at the University of California, Berkeley.