<human>:  When did life first appear on Earth?
<bot>: The first life on Earth probably evolved about 4 billion years ago. This is a very difficult question to answer, but here are a few ideas. The first life on Earth probably evolved from simple single-celled organisms, which probably appeared about 3.8 billion years ago. These early single-celled organisms probably developed into more complex multi-cellular organisms, which eventually gave rise to plants and animals. Some scientists think that life first appeared on Earth around 3.5 billion years ago, but there is still much debate about this.