<human>:  When did life first appear on Earth?
<bot>: Life first appeared about 4 billion years ago. The first organisms were prokaryotes, single-celled organisms that lacked a nucleus and organelles. These early organisms were simple and they had only a few different kinds of cells. The first multicellular organisms appeared about 1.8 billion years ago. These organisms had a central body and multiple specialized cells. These multicellular organisms are called eukaryotes, and they include plants, animals, and fungi.