Earth history

Earth history encompasses the existence of planet Earth, beginning when it reached roughly its present size 4.5 billion years ago, and extending into the indefinite future. It is far from synonymous with the history of Gaia, which was born at most 3.8 billion years ago and, according to present understanding, will probably last no more than one billion years into the future.

Geologists divide Earth history into four large segments called eons.

The Hadean Eon
The Hadean began when Earth attained its present size, having accumulated mass through accretion from all the debris in the primordial protoplanetary disk that was orbiting somewhere around 150 million kilometers (90 million miles) from the Sun. However, throughout the Hadean, killer asteroids and comets continued to bombard the planet's surface fairly frequently, frequently liquefying it and ensuring that any life that may have arisen was quickly wiped out again (or possibly ejected into space). This period ended with a particularly intense series of impacts known as the Late Heavy Bombardment about 4 billion years ago.

The Archaean Eon
The Archaean began with the end of the Late Heavy Bombardment around 3.8 billion years ago. Biologists believe the origin of life occurred around this time. The Archaean gives its name to a taxonomic domain consisting of organisms even more primitive than bacteria, known as archaeans. Bacteria originated later in this period, with photosynthesis evolving 3.5 billion years ago. However, throughout the remainder of the Archaean, oxygen produced by photosynthetic bacteria was quickly absorbed near where it was produced.

The Proterozoic Eon
The Proterozoic began when photosynthetic cyanobacteria triggered the oxygen crisis about 2.5 billion years ago, by allowing large amounts of their oxygen waste to accumulate in the atmosphere. This led to a mass extinction of anaerobic bacteria, but also contributed to several evolutionary advancements. Protists appeared 2 billion years ago, representing the first eukaryotic cells which went on to form the building blocks of multicellular organisms. Those organisms themselves appeared at roughly the same time as sexual reproduction, 1.2 billion years ago. After a period of extreme cold called the Cryogenian, 850-650 million years ago, which may have been related to the conversion of greenhouse gases into oxygen, these innovations finally led to two massive adaptive radiations which began the next eon of history.

The Phanerozoic Eon
The first major diversification of multicellular organisms, known as the Avalon Explosion, occurred about 575 million years ago. This was followed by the better-known Cambrian Explosion, which was discovered by paleontologists first, and led them to mark the boundary of the Phanerozoic at 542 million years ago. This is the eon that contains the present and will probably extend until the death of Gaia.