Assuming that life arose spontaneously on early Earth, we can now look at what we know about the history of Earth and the fossil record to better understand the appearance and diversification of life. This is probably best done by starting with what we know about where the Universe and Earth came from. The current scientific model for the origin of the universe is known as the Big Bang. It arose from efforts to answer the question of whether the fuzzy nebulae identified by astronomers were located within or outside of our galaxy. This required some way to determine how far these nebulae were from Earth. Edwin Hubble (1889-1953) and his co-workers were the first to realize that nebulae were in fact galaxies in their own right, each very much like our own Milky Way and each is composed of many billions of stars. This was a surprising result, since it made Earth, sitting on the edge of one among many, many galaxies seem less important. It is a change in cosmological perspective similar to that associated with the idea that the sun, rather than Earth, was the center of the solar system (and the Universe).
To measure the movement of galaxies with respect to Earth, Hubble and colleagues used the Doppler shift, which is the effect on the wavelength of sound or light of an object’s velocity relative to an observer. In the case of light emitted from an object moving toward the observer, the wavelength will be shortened, that is, shifted to the blue end of the spectrum. Light emitted from an object moving away from the observer will be lengthened, that is, shifted to the red end of the spectrum. Based on the observed Doppler shifts in the wavelengths of light coming from stars in galaxies and the observation that the further a galaxy appears to be from Earth, the greater that shift is toward the red, Hubble concluded that galaxies, outside of our local group, were all moving away from one another. Running time backward, he concluded that at one point in the past, all of the matter and energy in the universe must have been concentrated in a single point. A prediction of this Big Bang model is that the Universe is estimated to be ~13.8 +/- 0.2 billion (109) years old. This is a length of time well beyond human comprehension; it is sometimes referred to as deep time – you can get some perspective on deep time using the Here is Today website (http://hereistoday.com). Other types of data have been used to arrive at an estimated age of Earth and the other planets in the solar system as ~4.5 x 109 years.
After Earth first formed, it was bombarded by extraterrestrial materials, including comets and asteroids. This bombardment began to subside around ~3.9 billion years ago and reached its current level by ~3.5 billion years ago.45 It is not clear whether life arose multiple times and was repeatedly destroyed during the early history of Earth (4.5 to 3.6 billion years ago) or if the origin of life was a one-time event, taking hundreds of millions of years before it succeeded, which then managed to survive and expand around 3.8 to 3.5 billion years ago.