Archean

## General

**The Archean** is the second of four geological eons in Earth's history, representing 4 to 2.5 billion years ago. During this time, the earth's crust had cooled down enough for continents to form and the oldest known life to begin. Life was simple throughout the Archean, represented primarily by shallow-water microbial mats called stromatolites, and the atmosphere lacked free oxygen. The Archean was preceded by the Hadean eon and followed by the Proterozoic.

## Tectonic activity

Evidence for the details of plate motion and other Precambrian tectonic activity is poorly preserved. It is generally accepted that small proto-continents existed before 4280 Ma and that most of the Earth's landmasses merged into a single supercontinent around 1130 Ma. The supercontinent known as Rodinia broke up around 750 Ma. Several ice ages have been identified, dating back to the Huronian epoch, roughly between 2400 and 2100 Ma. One of the most studied is the Sturtian-Varangian Ice Age, about 850–635 Ma, which may have brought glacial conditions as far as the equator, resulting in a "snowball earth". The atmosphere of early Earth is not well understood. Most geologists believe it was composed mostly of nitrogen, carbon dioxide, and other relatively inert gases, and lacked free oxygen. However, there is evidence that an oxygen-rich atmosphere existed since the early Archean.

## Atmosphere & oceans

The Archean atmosphere is believed to have been almost devoid of oxygen. Astronomers believe the Sun was 70 to 75 percent of today's luminosity, but temperatures on Earth appear to have been close to modern levels just 500 million years after Earth's formation (the faint young Sun paradox). The presence of liquid water is evidenced in certain highly deformed gneisses formed by metamorphism of sedimentary protoliths. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in Earth's history. Alternatively, Earth's albedo may have been lower at the time due to less land area and cloud cover.

Today it is still believed that molecular oxygen was not a significant part of the Earth's atmosphere until photosynthetic life forms evolved and began to produce it in large quantities as a by-product of their metabolism. This radical change from a chemically inert atmosphere to an oxidizing atmosphere caused an ecological crisis sometimes referred to as the oxygen catastrophe. First, the oxygen would have quickly combined with other elements in the Earth's crust, primarily iron, removing it from the atmosphere. After the supply of oxidizable surfaces was exhausted, oxygen would have accumulated in the atmosphere and the modern high-oxygen atmosphere would have developed. Evidence of this is found in older rocks containing massive formations of banded iron deposited as iron oxides.

## Life

The processes that gave rise to life on Earth are not fully understood, but there is strong evidence that life arose towards the end of the Hadean eon or the beginning of the Archean eon.

Life in the Archean was limited to simple single-celled organisms (without a nucleus) called prokaryotes. In addition to the bacterial domain, microfossils from the archaea domain have also been identified. No eukaryotic fossils are known from the earliest Archean, although they may have evolved during the Archean without leaving any. Fossil sterans, indicative of eukaryotes, have been reported from Archean strata but have been shown to originate from contamination with younger organic material. No fossil evidence of ultramicroscopic intracellular replicators such as viruses has been discovered.