which era lasted the longest
Best Answer: Geological Period that lasted the longest: The Precambrian Era. The Precambrian Era is the longest geological era beginning 4.5 billion years ago and lasting approximately 4 billion years. The Precambrian era is generally separated into two eons the first of which being the Archaean Eon. spanning from around 4.5 billion years ago to 2.5 billion years ago, during this time air temperatures lowered and the water vapour condensed forming the first primeval oceans.