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WorldlyGlass49
The boundaries of eras are marked by great worldwide changes in Life. What this means is that the way life forms work or exist changes drastically.
millermoldwarp

The answer is: Life.

The eras are used to determine the order of the history of the earth. These divisions are determined by the great changes that have taken place in the geology, geography, climate and other features in the life in the earth. These changes are accompanied by the evolution of life or the lack of it.