Precambrian / Cambrian.
Precambrian: a supereon.
Cambrian: the first period of the Paleozoic Era.
The last period of the Precambrian supereon: the Ediacaran period (one of three periods of teh Neoproterozoic Era.
The Neoproterozoi Era was in turn one of three eras of the Proterozoi Econ. The Proterozoic Eon was one of three eons of the Precambrian supereon. Since there has been only one eon since, the Phanerzoic Eon (the eon of visible life), three of four eons occurred before the current (and last) eon of visible life, th Phanerzoic Eon.
For the last 150 years, it was felt that life began in the Ediacaran period and then was wiped out at the end of the Precambrian, before life came roaring back in the "Cambrian explosion."
Now, there is science suggesting life began before the Ediacaran.
A digression: if the Ediacaran was the last period before most of life was wiped out, it can be considered "similar" to the Cretaceous period when dinosaurs were wiped out.
So, now the period before the Ediacaran, the Cryogenian (from Greek cryo, meaning "cold"; and Greek genesis, meaning "birth"); a geologic period that lasted from 720 to 635 million years ago. It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran.
Prior to the Ediacara one can't find fossils, so one looks for biomarkers.
Sterane: a biomarker; found in the Huqf Supergroup in Oman, at least 650 million years old; some argue that these steranes are unique to a particular group of sponges; not all agree; a recent study suggests steranes may suggest single-celled forms of amoebas.
Oldest candidate for animal fossils: China; Lantian Formation; possibly 635 million years old; related to corals or jellyfish; appear to be tentacles but may not be animals at all.
Oldest animal remains that almost everyone agrees on: Newfoundland; 571 million years ago; shortly after the last regional "Snowball Earth" glaciation that encased much of the planet in thick ice.
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