The Earth has existed for around 4.5 billion years, and much has happened in its history to shape and create the Earth we know today. But what exactly happened up until the origin of life?
4.5 billion years ago, the Earth was formed, along with the Sun. The Earth was probably created by aggregating of dust and rocks, forming one of four terrestrial, rocky planets in our Solar System. During this period of early Earth, the surface was incredibly hot, not even quite cooled to the point that the rock at the surface was completely solid. The Earth took a while to cool down, because it was continuously bombarded by other rocks in space, which kept temperatures high and prevented surface rocks on Earth from solidifying and any water from remaining in liquid form. These bombardments also released gases that accumulated into an atmosphere, mainly consisting of carbon dioxide and carbon monoxide.
Once this bombardment stopped, Earth’s surface temperature cooled down, the rocks at the surface solidified, and this allowed liquid water to remain at the surface, eventually forming oceans, around 4.4 billion years ago. After this, life may have formed between 4.4 and 3.9 billion years ago, but any traces of that life would have been wiped out by the Period of Late Heavy Bombardment, a period around 3.9 billion years ago when asteroids began to collide with Earth (and most of the inner planets) continuously. This also most likely evaporated the liquid water on Earth. After this period ended, Earth solidified again and accumulated oceans, setting the stage for the origin of life.
Geological time scales; found at http://www.geosociety.org/science/timescale/2012timescl-550.gif
Looking at the above official geological timeline, the time we are talking about is in the Hadean Eon, the earliest time period in Earth’s history. Life begins in the Archaean eon, so named because the original life on Earth came in the form of single-celled organisms called Archaea.
In the next post, we’ll talk about the origin of life, why Earth was the “best” planet in our Solar System to develop life, and Fermi Paradox: why haven’t we found extraterrestrial life?
History of Life, 5th edition, Richard Cowen
Microcosmos: Four Billion Years of Microbial Evolution, Lynne Margulis and Dorion Sagan
Smithsonian Natural History Museum, http://paleobiology.si.edu/geotime/main/htmlversion/archean3.html