Based on the known fossil record, the first true primitive forms of multicellular life apparently developed around 650 million years ago, although some scientists classify a certain 1.2 billion-year-old red algae as a “taxonomically resolved” multicellular organism. (Humans are considered multicellular organisms, complete with 100 trillion cells that make up our bodies.)

One of the first such organisms is thought to have been a primitive form of sponge. The first fossil records of burrows are also found around the same time. These multicellular organisms are called Ediacaran fauna or assemblages (they are named after the Ediacaran hills in Southern Australia). Most have large surface areas, perhaps in response to their need to absorb oxygen, as there were very small concentrations of this gas present in the atmosphere at that time. They appear to have lived in shallow marine environments.