The first organisms to appear on Earth were single-celled prokaryotes, specifically bacteria. These simple life forms emerged about 3.5 billion years ago.

Here's a breakdown:

* Prokaryotes: These are single-celled organisms that lack a nucleus and other membrane-bound organelles.

* Bacteria: A large and diverse group of prokaryotes.

While there's still ongoing research, the earliest evidence of life on Earth comes from fossilized stromatolites, which are layered rock formations created by ancient microbial mats.

It's important to note that these early prokaryotes were likely very different from the bacteria we see today. They were likely extremophiles, able to survive in harsh environments like the early Earth, which was very hot and lacked an oxygen-rich atmosphere.