Earth's Archean Period

The Archean Period marks the beginning of Earth's proper geologic history. It is during this period from 3.8 billion to 2.5 billion years ago that the earliest documented rock formations were created on Earth's crust. Also during this early Eon in Earth's history, continents and oceans began to form, and life had begun the long saga of evolution in the primordial stew of elements of Earth's oceans.

Although much cooler than the supposed fiery hell of the Hadean Period, the beginning of the Archaen Period saw a heat flow on Earth that was three times higher than it is today. Much of this had to do with the fact that earth was still accreting, or forming, and the molten iron core was pumping out considerable heat. Furthermore, radioactive decay as well as a preponderance of volcanic activity on the surface helped to contribute to this stifling heat. At this time, tectonic and volcanic activity might have been too great for continents to form. It wasn't until the end of the Archean Period that protocontinental masses began to form actual continents. About 70% of the continental masses we know today were formed during the late Archean Period. During the Archean, asteroids were becoming less common in the solar system and Earth was not being bombarded as much. This helped to reduce the heat on the surface and allow for cooling of the young planet Earth.

At this point in Earth's history, it seems unlikely that there was any free gaseous oxygen in the atmosphere. In fact, it was a very toxic mix of methane, ammonia, and other gases inhospitable for life on the surface. However, liquid water must have been on the planet due to the presence of sedimentary rocks and the fact that the sun's diameter was smaller than its present-day size. This would allow for temperatures cool enough for water to be present without evaporating off the hot surface of the Earth. Giant cloud formations would have dumped huge quantities of rain onto the cooling surface, thus forming giant oceans.

The Archean is most important for the early traces of life that take place during the period. In the liquid oceans that now covered Earth, the first single-celled life forms were beginning to take shape. These bacteria might have used methane or hydrogen in their metabolism due to the absence of free oxygen either in the atmosphere or dissolved in the ocean water. We know this from the presence of bacterial microfossils that date back to roughly 3.5 billion years. Also, photosynthetic bacteria might have begun to colonize certain rocks on Earth's surface out of the oceans themselves. Although the earliest fossil records we have of life forms date back to 3.5 million years, it is quite possible that single-celled organisms had begun to colonize the planet before that time. All subsequent life on Earth can be traced back to these single-celled bacteria living in the oceans and rocks of our young and barren planet.

At the end of the Archean Period, the first photosynthetic algae began to appear on Earth. Their success would have severe consequences on Earth's atmosphere and allow aerobic (oxygen using) organisms to begin their long road to success on our planet.