The earliest fossils are found in the Archean Eon. This eon spans from about 4.0 billion to 2.5 billion years ago. While life likely originated earlier, the Archean marks the period when we start finding tangible evidence of ancient organisms in the form of fossils.

These early fossils are primarily microbial in nature, like:

* Stromatolites: Layered rock formations created by microbial mats.

* Microfossils: Fossilized remnants of single-celled organisms.

While the Archean eon is known for its early life forms, it's important to note that the search for the very first life on Earth is ongoing and new discoveries are constantly being made.