Life on Earth may have started within a few hundred million years
Earth formed about 4.54 billion years ago, and a speck of carbon trapped inside one Australian zircon crystal hints that life was present by 4.1 billion years ago. The oldest fossils everyone can actually see are layered microbial mounds about 3.48 billion years old, found in the Pilbara of Western Australia.
Hunting for the first life means reading chemical fingerprints. Living cells prefer the lighter carbon-12 atom over carbon-13 because its bonds take less energy to break, so biological material ends up enriched in light carbon compared with the surrounding rock. Graphite with that signature turns up in the Jack Hills zircon and in 3.7-billion-year-old rocks of Greenland's Isua belt.
The trouble is that non-living processes can mimic the same pattern. Heat, hydrothermal chemistry and volcanism may explain light carbon at Akilia in Greenland, and hot fluids could have altered the Isua rocks, though further spectral work has made a biological origin there more widely accepted. Claims of microfossils up to 4.28 billion years old from Quebec's Nuvvuagittuq belt face a similar challenge, since mineral-rich water and chemical gardens can grow lookalike shapes.
Fossils are harder to fake but rarely survive, because heat and pressure over billions of years wipe them out. Stromatolites, layered structures built by sun-seeking microbial mats, offer the best direct record. In the Dresser Formation their laminae thicken over the crests, just as growth toward more sunlight would predict, and sulfur isotopes in barite veins also point to microbes. Supposed stromatolites at Isua, by contrast, may simply be rock deformed by tectonic stretching. Varied microfossils in the Strelley Pool Formation, 3.43 billion years old, suggest photosynthesis was already under way.
Life has since spread almost everywhere, at least 10 kilometres below the seafloor, into vents hotter than 400 degrees Celsius, and deep under Antarctic ice. In 2020 biologists reported microbes in near suspended animation beneath the South Pacific Gyre, called the deadest spot in the ocean.
Source: Earliest known life forms
