Precambrian is an era which includes the Proterozoic eon and the Archean eon spanning about 2000 years each. Single celled organisms like blue-green algae and bacteria first appeared during this period. Towards the end of the Archean eon the first multicellular, soft bodied animals began to emerge such as jellyfish, seapens, and worms.
Precambrian spans from the formation of the Earth around 4500 million years ago to the evolution of abundant macroscopic hard-shelled fossils, which marked the beginning of the Cambrian, the first period of the first era of the Phanerozoic eon, some 542 million years ago.
Remarkably, little is known about the Precambrian despite it making up roughly seven-eighths of the Earth's history, and what little is known has largely been discovered in the past four or five decades.
The Precambrian could be divided into five "natural" eons, characterized as follows.
1. a period of planetary formation until a giant Moon-forming impact event.
2. the Late Heavy Bombardment period.
4. a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formations due to increasing atmospheric oxygen content.
5. a period of continued iron banded formation until the first continental red beds.
6. a period of modern plate tectonics until the first animals.
The Hadean refers to the geologic eon before the Archean. It extends back to the Earth's formation, and ended roughly 3.8 billion years ago (3800 million years ago), though the date varies according to different sources. The name "Hadean" derives from Hades, Greek for "unseen" or "Hell" and suggesting the underworld or referring to the conditions on Earth at the time.
Archean refers to the time before the Proterozoic, 2500 million years ago. At the beginning of the Archean, the Earth's heat flow was nearly three times higher than it is today, and was still twice the current level by the beginning of the Proterozoic. The Earth's crust was not only thinner than it is today, but probably broken up into many more plates, with numerous hot spots, rift valleys, and transform faults. There were no large continents until late in the Archean; small protocontinents were the norm.
The Archean atmosphere apparently lacked free oxygen. Temperatures appear to have been near modern levels, although astronomers think that the sun was about one-third dimmer. This is thought to reflect larger amounts of greenhouse gases than later in the Earth's history.
The Late Heavy Bombardment (LHB) was a period approximately 3.8 to 4 billion years ago during which the Moon, Earth, Mercury, Venus, and Mars were subjected to many impacts from space. The evidence for this and the dating comes mainly from analysis of the craters of the Moon and Moon rocks. It formed some craters the size of continents.
This bombardment came after a relatively calm period of several hundred million years. It is not yet clear exactly what brought about the renewed bombardment. One possibility is that Jupiter's orbit shrank, causing it to clean out the outer edges of the asteroid belt, a portion of which would have been sent careening into the inner solar system. Or somewhere in the Solar System a big collision created a lot of flying fragments which became new asteroids.
Recent computer models also suggest that resonances and perturbations caused by the four large outer planets settling into their current orbital configurations could have displaced large volumes of material into the inner solar system.
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The Proterozoic Eon extended from 2500 Ma to 542 million years ago. The Proterozoic is the most recent part of the old informal Precambrian time.
The Phanerozoic (occasionally Phanaerozoic) Eon is the period of geologic time during which abundant animal life has existed. It covers roughly 545 million years and goes back to the time when diverse hard-shelled animals first appeared. The Phanerozoic eon is still ongoing. Its name derives from the Greek meaning visible life, referring to the large size of organisms since the Cambrian explosion.
