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.
For the first billion years or so, life on Earth consisted of bacteria and archaea, microscopic organisms that represent two of the three genealogical branches on the Tree of Life (Fig. 15). Both groups are prokaryotes (single-celled organisms without nuclei). Archaea were recognized as a unique domain of life in the 1970s, based on some distinctive chemical and genetic features. The order in which branches radiate from the Tree of Life shows the sequence in which organisms evolved.