The entire history of human life is contained in the Holocene epoch.
As far back as 1669, Nicolaus Steno began observing the characteristics of the time/location relationship between layers of sedimentary rock. His basic thinking was that sedimentary rocks were laid down in a horizontal fashion, and that the rock layers get older the deeper you dig. From these principles, geologists were able to begin defining time periods during which certain natural processes occurred on earth and left evidence within certain layers of rock. These divisions of time are known as eons, eras, periods, epochs and ages.
Eon
The eon is the largest commonly used time scale on the geologic timetable. There are four in total, Hadean, Archean, Proterozoic and Phanerozoic, listed from oldest to most recent. With the age of the earth estimated at about 4 1/2 billion years, each eon comprises somewhere around 500 million years, which, in geologic terms, is a pretty close measurement.
Era
Eons are broken into eras, with a total of 10 spread among the four eons. The first eon, Hadean, has no era beneath it, due to a lack of fossil records available for study from that ancient time. The more recent eras have abundantly more physical evidence and are more easily divided into specific periods. Each era corresponds to roughly 100 million years.
Period
An era is divided into periods, though neither the Hadean nor Archean eons are marked by periods. There are 21 defined periods, with the most recent eon, Phanterozoic, consisting of more than half of the total. The easiest way to define the length of a period would be to say it is expressed in tens of millions of years, ranging from the 22 million year Neogene all the way up to the 80 million year Cretaceous and Carboniferous.
Epoch
Periods are subdivided into epochs, a span of time usually measuring 10 million years or less. There are no epochs on the timetable until we reach the most recent eon, the Phanterozoic, and the most recent era, Cenozoic. Epochs contained in the Cenozoic era are the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene. We presently live in the Holocene epoch, which began about 11,000 years ago.
Age
The smallest time increment is called an age, which has been measured only since the Paleozoic Era and includes dozens of entries, each lasting from tens of thousands to hundreds of thousands of years long. Modern time is often referred to as the "Age of Man."
Tags: most recent, million years, divided into, Hadean Archean, Holocene epoch, million year
