2.8: Mapping the history of life on earth

Assuming that life arose spontaneously, we can look at the fossil record to better understand how life diversified and it's impact on the Earth. Consider 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”, the “primeval atom”, or the “cosmic egg”. This model is based on an idea originally proposed by the priest, physicist, and astronomer Georges Lemaître (1894-1966).^[67] The Big Bang model arose from efforts to answer the question where are the fuzzy nebulae (patches of light in the night sky) located? are they within or outside of our galaxy? Answering this question required a way to determine how far these nebulae were from Earth. Edwin Hubble (1889-1953) and his co-workers were the first to provide compelling evidence that nebulae were actually galaxies in their own right, each very much like our own Milky Way galaxy. Each is composed of many billions of stars. This was surprising. It made that the Earth, sitting on the edge of the Milky Way seem less important – a change in cosmological perspective similar to that associated with the idea that the Sun, rather than the Earth, was the center of the solar system.

Hubble and colleagues combined two types of observations to measure the movement of galaxies with respect to the Earth. The first allowed them to estimate the distance from the Earth to a galaxy. The second used Doppler shift. For an object moving toward an observer, the observed wavelength of emitted light will be shortened, that is, shifted toward the blue end of the spectrum. When an object is moving away from the observer the wavelength will be lengthened, shifted to the red end of the spectrum. Based on their observations, Hubble concluded the further a galaxy appears to be from Earth, the greater that shift is toward the red. 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.^[68] The "Big Bang" model concludes that the Universe is ~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. The age of the Earth and the other planets in the solar system has been estimated as ~4.5 to 5 x 10⁹ years. You can get some perspective on deep time using the “Here is Today” website.

After its formation, the Earth was bombarded by extraterrestrial materials. This bombardment began to subside around ~3.9 billion years ago, reaching its current level by ~3.5 billion years ago.^[69] It is not clear whether life arose multiple times and was repeatedly destroyed early on or if the origin of life was a one-time event, taking hundreds of millions of years before it succeeded, after which it has survived and expanded to the present day.

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