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Dinosaur and Paleontology Dictionary |
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The PrecambrianThe Precambrian is the geological time span that preceded the Cambrian period (before 540 million years ago). This time period comprises most of the Earth's history (roughly 7/8 of the time sime the Earth formed).
The Precambrian is defined as the time from when the Earth formed (about 4.5 billion years ago, the earliest-known rocks) until the beginning of the Cambrian period (540 million years ago). The Precambrian used to be defined as the time from when the Earth's rocks formed until the earliest life forms evolved, but as the date of the earliest fossils gets earlier and earlier, this definition has also changed.
The first life forms on Earth were simple. single-celled life forms (like blue-green (photosynthetic) algae, archaeans, and bacteria) that evolved in the seas. The earliest-known fossils are of cyanobacteria-like organisms that are roughly 3.5 billion years old. Microfossils from the Archean Eon of the Precambrian have been found in South Africa and Western Australia. The first billion years of life on Earth saw only these simple forms.
The Precambrian:
| EON | ERA | PERIOD | EPOCH | PIVOTAL EVENTS | |
| Proterozoic Eon 2.5 billion years ago to 540 mya |
- | Vendian Period 600 to 540 Million Years Ago |
Vendian biota (Ediacara fauna), multi-celled, animals appear, including sponges. A mass extinction occurred. The continents had merged into a single supercontinent called Rodinia. | ||
| - | First multicellular life: colonial algae and soft-bodied invertebrates appear. Oxygen build-up in the Mid-Proterozoic. | ||||
| Archeozoic Eon (Archean) 3.9 to 2.5 billion years ago |
- | - | "Ancient Life" - The first life forms evolve - one celled organisms. Blue-green algae, archaeans, and bacteria appear in the sea. This begins to free oxygen into the atmosphere. | ||
| Hadean Eon 4.6 to 3.9 billion years ago |
- | - | "Rockless Eon" - The solidifying of the Earth's continental and oceanic crusts. | ||
For a complete timeline of geologic time, click here.
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