The oldest fossil evidence belongs to stromatolites. These are layered structures formed by the activity of cyanobacteria, also known as blue-green algae.

Here's why:

* Ancient age: Stromatolite fossils have been found dating back to 3.5 billion years ago (bya), making them some of the oldest known fossils on Earth.

* Key role in early Earth: Cyanobacteria were crucial in changing the early Earth's atmosphere by producing oxygen through photosynthesis. This paved the way for the evolution of more complex life forms.

* Unique formation: Stromatolites are formed by layers of cyanobacteria and sediment, creating distinctive rock formations.

While there are other very old fossils, like the microfossils of bacteria found in 3.7 bya rocks, the evidence surrounding those is more debated. Stromatolites, however, provide a clearer and more widely accepted example of early life on Earth.