General Astronomy/Earth's Early Years
Around 3.9 to 4.6 billion years ago, the beginning stages of our solar system formation were set in motion. Over the course of the next 100 million years, all the planets and moons in our solar system as we know it were created, including our Earth and orbiting Moon. The early years of the Earth are referred to as the "Pre-Cambrian" era, and extend until about 500 million years ago. This era includes the Hadaen Eon, the Archean Eon, and the Proterozoic Eon.
The Hadaen Eon
The first time period that the Earth proceeded through was the Hadaen Eon, which extended from approximately 4.6 to 3.8 billion years ago. The Earth at this time was very unlike the Earth today - lacking almost all oxygen and water filled oceans, the heat from the solar system formation as well as internal gravitational energy heating coupled with collisions with leftover planetary bodies made the Earth quite uninhabitable. In fact, the Earth began as mostly molten, and the resulting heat from this material as well as the extreme heat released from the settling of the planetary crusts made any atmosphere unattainable. The Earth's atmosphere was quite unlike what we see today - it comprised mainly of dust leftover from the solar nebula and also of lighter elements such as hydrogen and helium, and later water vapor and gases. During the Archaen eon is when a substantial atmosphere begins to form from the volcanic activity on Earth.
During the Archean Era, which began 2.5 billion years ago, many of the patterns seen in the Hadaen Eon persisted - namely, the large amounts of heat being radiated due to the solar system formation, the increased tectonic activity and crustal upheaval and recycling, as well as the lack of any significant atmosphere.
However, despite the alien image presented to us, the precursors of life were already in place. For starters, by the end of the Archean Eon, continents had begun to form, as well as water deposits resulting from cooling sedimentary rock. Furthermore, gas was expelled from volcanoes and released from the planets core had established an atmosphere of sorts, consisting of carbon dioxide, nitrogen, and water. It is thought that life developed at this time, with ample fossil evidence attributing to its existence. Members of the Bacteria and Archean families have been discovered preserved in fossils from this time. It should also be noted that rock from the Archean time period is the earliest rock discovered on Earth.
The Proterozoic Eon is the last eon of the Pre-Cambrian era, as well as the last period before complex forms of life became common on Earth. It extends from 2.5 billion years ago to around 500 million years ago, and is comprised of three geological eras: Paleoproterozoic, Mesoproterozoic, and Neoproterozoic. One of the more notable changes in the Proterozoic era was the buildup up of oxygen in our environment. Due to the biological process of photosynthesis, oxygen had been produced throughout the Archean era, but had been reabsorbed by natural "sinks" in the environment. Such sinks were unoxidized sulfur and iron, which had until this era absorbed most of the oxygen produced. During this eon, the expansion of Prokaryotic life continued, but a new development was seen as well - Eukaryotic life. Both of these life forms continued to flourish, setting the stage for the even more advanced life forms that were soon to evolve.
Cattermole, Peter. Building Planet Earth. London: Cambridge UP, 2000.
"Earth (Planet)." Microsoft Encarta. 3 Mar. 2007. 1 Mar. 2007 <http://encarta.msn.com/encyclopedia_761569459_5/Earth_(planet).html>.