When geologists were first cataloging the world, the rocks with fossils in them were the easy ones. Because fossils change consistently over geologic time as species evolve and go extinct, fossils are good signs of a rock's age. In the 1800s geologists established a fossil-based framework of geologic age periods, marching back in time from the present Quaternary Period to the Cambrian Period.
Fossils seem to disappear in rocks older than the Cambrian, thus the periods from the Cambrian forward are all put in the Phanerozoic ("visible life") Eon. Older rocks, without these fossil signposts, for many years were lumped together as "pre-Cambrian."
In the 20th century, we found ways of dating rocks based on chemistry rather than fossils. From these we learned that while the Phanerozoic—the whole age of fossils—is roughly 500 million years long, the Earth is about 4550 million years old (4550 Ma). Almost all of Earth's history is Precambrian.
Two Kinds of Earth History
Phanerozoic history is made for museums, with halls of fossils representing the geologic periods—the days of the trilobites, the amphibians, the dinosaurs and so on, with Man at the end the most interesting of all. It's not as obvious, but during the Phanerozoic Earth itself went through significant changes as well—global climate, the composition of the air, the chemistry of the sea and the arrangement of the continents. It can be disconcerting to learn how strange the world has been at times during the Phanerozoic.
Precambrian history is much stranger, and strange in ways that museums can't show. It is a story of an Earth that is quite alien, that only very slowly became friendly to living things like ourselves. Compared to the age of fossils, the Precambrian's story is poorly connected and full of gaps and ambiguity. And there are no dinosaurs or other animated characters, just the dimly pictured face of the planet itself.
Precambrian time has three large divisions: the Hadean, Archean and Proterozoic Eons.
The Hadean Eon (4550–3800 Ma)
The Hadean Eon is not formally recognized, but it essentially marks the era before we have any rocks. The oldest rocks on Earth are just short of 4 billion years old. What we know about this time is mostly based on modeling and conjecture. It basically covers the formation of the planet in the original cloud of gas and dust that the sun was born in.
The most plausible story is that Earth was born a hot body as smaller rocky bodies slammed together (the other theory, growing less tenable in my opinion, is that Earth assembled in a cold state). The iron core separated out and the mantle was a magma ocean. A large body, roughly Mars's size, collided with the proto-Earth and splashed enough mantle material into orbit to assemble the Moon. All of this happened well before 4000 Ma.
A few microscopic zircon crystals, recycled from earlier rocks, have been dated from before 4000 Ma. It is argued that these indicate the presence of crustal rocks that formed continents, with corresponding oceans. The oldest rocks, from about 3980 Ma, show signs that living things were already processing carbon at this time.
The Archean Eon (3800–2500 Ma)
The Archean Earth had air that would kill us: nitrogen, methane and carbon dioxide. The Archean sky must have been red. Fossil microbes—cyanobacteria—are known from rocks around 3500 Ma. These are anaerobic organisms that live not by oxidizing their food but by reducing it, snatching carbon and hydrogen ions from their surroundings and leaving oxygen as a waste product.
Rocks remaining from that time are small continental bodies of tonalite, a granitoid rock, surrounded by belts of greenstone, or altered basaltic rocks. What these rocks signify, and whether they participated in plate tectonics like today's, are disputed questions. The mantle was several hundred degrees hotter than today, producing magmas not found in today's volcanoes. Bodies of Archean rocks form today's cratons, the cold, stiff hearts of the continents. Canada has the largest area of Archean rocks on Earth, but all of today's continents have Archean cores.
Sedimentary rocks from shallow Archean seas are preserved, containing sediments that could not exist today because there was no oxygen in the air. Any oxygen produced by the cyanobacteria was snapped up by dissolved iron, which came out of solution and piled up in widespread areas of banded iron formation.
