The oldest known fossils are stromatolites, which are layered structures formed by the activity of cyanobacteria, also known as blue-green algae. These fossils date back to approximately 3.7 billion years ago (bya), found in Greenland.

However, some scientists argue that microfossils found in Western Australia, dating back to 4.1 bya, could be even older. These are much smaller structures, and their interpretation as fossilized life is still debated.

It's important to note that the search for the oldest fossils is constantly evolving, and new discoveries are being made all the time. As technology advances, we may find even older evidence of life on Earth in the future.