Final answer:
The Cryogenian period, occurring 850 to 630 million years ago, is known for the 'snowball Earth' phenomenon, where Earth was completely covered in ice and inhabited mostly by single-celled organisms.
Step-by-step explanation:
The period during which Earth experienced an ice age so severe that the planet was covered in snow and ice, known as snowball Earth, was called the Cryogenian period. This geologic period occurred about 850 to 630 million years ago and was characterized by an extremely cold global climate. During this time, Earth's oceans may have been completely frozen over at least once. The planet was primarily inhabited by single-celled organisms, such as bacteria and archaea, the most complex forms of life at that time.
These drastic climate events were primarily influenced by changes in the tilt of Earth's rotational axis due to the gravitational effects of other planets. According to geological evidence, these conditions arose well before the rise of more complex eukaryotic life and significantly predate periods like the Quaternary Period, which also featured glaciation events but had much more developed forms of life including large mammals.