The fossil record indicates that the first organisms were single-celled microorganisms, likely similar to bacteria and archaea.

Here's a breakdown:

* Earliest evidence: The oldest known fossils are approximately 3.5 billion years old and are found in rocks in Greenland. These fossils are of stromatolites, layered structures formed by microbial mats.

* Simple lifeforms: These early organisms were prokaryotic, meaning they lacked a nucleus and other membrane-bound organelles. They likely obtained energy through processes like photosynthesis or chemosynthesis.

* Evolution of complexity: Over millions of years, these single-celled organisms diversified and evolved, eventually giving rise to more complex life forms like eukaryotes (organisms with a nucleus).

The fossil record, though incomplete, provides strong evidence for the evolution of life on Earth and highlights the incredible diversity of life that has existed over billions of years.