We can't use both. The answer is to use the one with the smaller absolute value: With this definition of. A slightly different formal definition of the limit is used when x goes to +/- infinity, since we really cannot subtract x from infinity or vice versa-remember, infinity is a concept, not a number. Formally, the expression. Means that.
During the long interval of the Precambrian Era (which includes approximately 90% of geologic time and includes three Eons: the Hadean, the Archean and the Proterozoic) the only inhabitants of the Earth were simple microscopic organisms, many of them comparable in size and complexity to modern-day bacteria.
The Paleozoic Era is the beginning of an explosion of life forms. The Cambrian Explosion marks the era with thousands of new life forms in the ancient seas. There was certainly life before the Cambrian Period. There were even many complex life forms. In late Precambrian time was a period called the Ediacaran. Ediacaran fossils are found in the Ediacaran Hills of Australia, Northwest Territories, Yukon, British Columbia, and Newfoundland in Canada.