The Archean Eon Lasted from 4.0 BYA to 2.5 BYA. In that time the earth formed its crust and its plate tectonics along with the first photosynthesizing organism.
In the beginning of the eon, the earth had cooled down enough to solidify the crust and condense the water vapor enough to rain and form the first oceans. When the oceans developed, so did the first continents. The continents were formed by the melting rocks within the oceanic crust. When minerals, such as silica, were melted, they floated to the top of the oceans, due to their light density. The earths atmosphere at the time was composed of the basic elements of living organisms, which are nitrogen, hydrogen, carbon and low levels of oxygen.
In the middle to late part of this eon, unicellular organisms began to develop and by the end of the eon we had our first photosynthesizing organism, which created oxygen for the atmosphere. Recent information suggests that earth's life formed around hydrothermal vents, which are found in the deep sea.
Some of the first organisms on earth are called "Archaea", which are tiny bacteria that live in harsh environments such as hot springs or deep sea vents.
The oldest evidence of life may be around 3.5 billion years old and they look like present day stromatolites. Stromatolites are created by microorganisms that pile thin layers of sediment on top of each other so they can get closer to sunlight for photosynthesis. Since this evidence that is dated around 3.5 billion years ago was possibly produced inorganically (by non-living processes), scientists can't say whether or not it counts as the first living organism. The first concrete evidence of living organisms came around 2.7 billion years ago, which also came in the form of stromatolites.
Stromatolites also gave important information about the environment. The oceans had large amounts of dissolved iron. The iron mixed with oxygen, causing a type of rust which was mined for iron ore. Once the iron in the ocean was precipitated out as iron oxide, the photosynthetic microorganisms (like archaea bacteria) were let out into the atmosphere. Since photosynthetic microorganisms produce oxygen, this caused a rich influx of oxygen in the atmosphere during the Proterozoic Eon.