The earliest fossil evidence found suggests that life has existed on Earth for 3.5 to 3.9 billion years. The surface of the planet was hypothesized to be inhospitable to all life until after the Late Heavy Bombardment, which occurred roughly 3.8 to 4.1 billion years ago.
The earliest forms of life on Earth were single-celled organisms that resembled contemporary prokaryotes, formed from organic molecules in the primordial oceans. The oldest fossils known, as of 2014, are ancestors of blue-green algae, formed into mats of organic material known as stromatolites. These sedimentary layers, found in Australia, have been successfully dated to 3.5 billion years ago. Older deposits found in western Greenland may date to 3.7 billion years ago, though findings are not conclusive, according to science publication Nature Geoscience.