What is the thought to be one of the earliest forms of life on Earth

single-celled micro-organisms

Explanation:

The first living things on Earth, single-celled micro-organisms or microbes lacking a cell nucleus or cell membrane known as prokaryotes, seem to have first appeared on Earth almost four billion years ago, just a few hundred million years after the formation of the Earth itself.

some of the earliest life forms on earth were lizards and fish