The Earth is old. Some stars that rage today were not even clouds of gas when it was born. The Earth has seen the rise and fall of planets and of galaxies. Its history is a long one, stretching back over a time so long that years are almost meaningless. As far as we know, it is the only world capable of supporting complex life, or indeed any kind of life at all. With the discover of hundreds of exoplanets, this may not always be the case. The possibility of extra-terrestrial life, even just bacteria, is one of the greatest concepts in all of science.
By studying the effects of radioactive decay, there is a general consensus among scientists that the Earth is approximately 4.6 billion years old. This makes it not much younger than the sun itself. Some asteroids are older; Mars is almost certainly older, although Venus might not be. Since dealing with billions of years at a time is cumbersome, geologists have invented a scale which is easier for a human mind to visualize. Scientists break down time as follows:
• Supereon: this is the largest unit of geologic time. The only supereon is the Precambrian, which stretches roughly 4 billion years. Most of earth’s history falls into this group. However, as you shall see, most interesting events in the time line of life occur later.
• Eon: there are 4 of these. Each is half a billion years or more in duration.
• Eras measure several hundred million years. There are 12 in total.
• Period: no defined length or number.
• Epoch: tens of millions of years.
• Age: millions of years.
Smaller units than these exist, such as a millennium, or one thousand year period. However, these units are too small to define any meaningful length of geologic time. The planet is capable of vast changes, but these changes occur over many years. Usually, smaller units of time are applied to human history where changes occur more rapidly.
Human civilization exists wholly within the Holocene epoch of the Quarternary period. This period began about 12,000 years ago. Civilization itself is probably only 10,000 years old. However, humanity as a species stretches back over two million years.
There are various sub-divisions which can exist within any one unit. For example, the terms early, mid, and late are often used. These terms are capitalized when a formal division is recognized. For example, "Mid-Jurrasic’ is an officially recognized time period. It is also the time period when Dinosaurs flourished. Due to the enormous amounts of study concerning the Dinosaurs, it helps to have more finely divided time periods. Other times, such as the Pre-Cambrian, are spread out enough where an official breakdown is not necessary.
Please note: all dates are as accurate as possible, but still approximate. GYA means billion years ago, MYA a million years ago, and KYA a thousand years ago.
The rest of this essay can be found here. D.29 Geologic time