Their populations crashed during the Cryogenian period 860 million years ago ( the Snowball Earth episodes ) . They proliferated in the Cambrian explosion and reached their highest diversity in the Paleozoic . Their populations crashed during the Snowball Earth episodes , when all or very nearly all of the Earth 's surface was covered by ice or snow , but they proliferated in the Cambrian explosion and reached their highest diversity in the Paleozoic .