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Question:

The Earth is about 4.5 billion years old. About how many years ago did life first appear?

A 3.5 billion years
Explaination

There are many arguments about when life first appeared on Earth, with some estimates placing life starting as long ago as 3.8 billion years. The generally accepted estimate is 3.5 billion years ago, and these first life forms were single-celled organisms and RNA molecules that could self-replicate.