In the nineteenth century, when geologists were developing the geological timescale, the newly defined Cambrian Period marked a profound change in rocks. Cambrian and younger formations were rich in fossils, whereas older Precambrian rocks appeared to be free of fossils and had historically been assigned to the Azoic Eon (literally “time without life”). As discussed in Chapter 7, we now know that the Precambrian has a rich history of single-celled colonial life. It is also clear that metazoans, or multi-celled animals, appeared in the Neoproterozoic Era as the Earth was coming out of the Cryogenian snowball Earth period, which was the focus of Chapter 8.