Oldest Fossils Ever Found Dated to 3.5 Billion Years Ago

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Most organisms that die don’t end up as fossils because the process requires very particular conditions. In the more recent past, at least the fossils are large enough to see with the naked eye. When you’re looking for fossils from the dawn of life on Earth, they’re microscopic. Some potential fossils were identified in a rock more than 20 years ago that was dated to an age of about 3.5 billion years. Now, we know those are indeed fossils, making them the oldest biological remains ever found.

The 11 microfossils were discovered in a rock sample from western Australia back in 1993. The rock was radiometrically dated to 3.5 billion years old, so it’s safe to say anything embedded in within it is similarly ancient. However, the technology didn’t exist at the time to find out if the tiny squiggles and tubes (each a mere 10 micrometers across) were actually the remains of biological organisms.

A team of researchers from UCLA and the University of Wisconsin-Madison has spent the last decade developing a process to analyze the rocks to find out if it does contain the oldest fossils in the world. The technology eventually used to study the samples is known as secondary ion mass spectrometer (SIMS). UW-Madison is one of the few institutions in the world that has a SIMS lab.

SIMS looks at the ratio of carbon atoms present in fossils, but this isn’t a dating mechanism. Carbon dating is only useful for much newer objects, and the rock has already been dated by other means to more than 3.5 billion years. SIMS analyzes the ratio of carbon-13 to carbon-12 isotopes, comparing samples from the alleged fossils with those from empty sections of rock.

A microfossil found in the rock.

The varying carbon ratios in the samples show the fossils are the remains of biological lifeforms, and researchers were also able to make some educated guesses about what type of lifeforms they were. Several appear to be phototrophic bacteria that produced energy from sunlight. There were also several forms of methane-producing archaea and a methane-consuming bacteria. A whole little exosystem in one rock.

So, this pushes the origins of single-cellular life on Earth back to 3.5 billion years ago. It may even go further back than that. The team even suggests it could be possible for life to have existed as much as 800 million years before these organisms died and fossilized. That’s when Earth’s oceans formed, providing the first potential home for life.

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