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(Let's not even discuss Year Zero, seeing as this jockeying for Year One position occurred before the concept of zero had even been invented.) If we wanted to allow for commerce, trade, and simple communication across cultures to develop, we needed to be living in the same year. The Byzantine Empire started its first year in what was considered the year of creation (our 5509 B. The Church of Alexandria began its Year One in what is now 284 A. And two, when most people see it, they think it stands for Christian Era and Before Christian Era, so it doesn't really solve the problem people wanted to solve.” As the world continued to “shrink” due to the establishment of trade routes and expansion of population and as once-insular communities started opening up and exploring, a single Year One would have inevitably dominated.
The higher the uranium-to-lead ratio of a rock, the more the Pb-206/Pb-204 and Pb-207/Pb-204 values will change with time.
This value is derived from several different lines of evidence.
Unfortunately, the age cannot be computed directly from material that is solely from the Earth.
The oldest rocks which have been found so far (on the Earth) date to about 3.8 to 3.9 billion years ago (by several radiometric dating methods).
Some of these rocks are sedimentary, and include minerals which are themselves as old as 4.1 to 4.2 billion years.