The early years of Earth are a murky mess, so we don't really know how or when life first arose. With each new fossil discovery, the timeline for life is pushed back earlier and earlier, but the really old ones are usually up for debate. That was the case for microscopic fossils found in Western Australia decades ago, which appeared to show evidence of life dating back almost 3.5 billion years, but those findings have since been contested. New analysis shows that the microfossils are indeed biological, and the find may have implications for the likelihood of extraterrestrial life.

The title of Oldest Fossil is a controversial one. This new study, from scientists at UCLA and the University of Wisconsin-Madison, lays claim to it with these microfossils dating back 3.465 billion years. That's pretty old, but it's just pipped by fossils found in an ancient hot spring in the Pilbara region of northwestern Australia, which are 3.48 billion years old. Both are beaten out by stromatolites in Greenland that have been dated to 3.7 billion years, while a batch of Canadian fossils may have celebrated a staggering 4.3 billion birthdays.

But because it's so difficult to peer that far back in time, all of these finds need to be taken with a grain of salt. The fossils at the heart of this study were first described in 1993, but it was later debated whether or not they were biological or if the microscopic structures could have formed through other processes. On closer analysis, the team now claims to have confirmed that the fossils are indeed biological, identified different taxa that they belong to and described the possible physiological behaviors of the organisms.

The team analyzed the samples using a secondary ion mass spectrometer (SIMS). This instrument can identify the different carbon signatures of organisms by weighing up the ratio of carbon-12 to carbon-13 isotopes. By comparing those ratios to a standard figure, as well as a sample of neighboring rock that contained no fossils, the team was able to confirm that the fossils were indeed of biological origin.

"The differences in carbon isotope ratios correlate with their shapes," says John Valley, co-lead researcher on the study. "If they're not biological there is no reason for such a correlation. Their C-13-to-C-12 ratios are characteristic of biology and metabolic function."

Based on this finding, the team identified 11 specimens of organisms that belonged to five different taxa, and determined how they might have lived. Among them were phototropic bacteria that produce energy from sunlight, Archaea that produce methane, and gammaproteobacteria that consume methane. At that time in Earth's history, methane is believed to have been the main constituent of the Earth's atmosphere.

Carbon isotope analysis revealed that this 3.5 billion-year-old micro-organism likely consumed methane(Credit: J. William Schopf/UCLA Center for the Study of Evolution and the Origin of Life)

Discovering life this diverse at this early stage has important implications for the hunt for extraterrestrial life. If life can evolve so quickly in Earth's history, the team reasons, then it should be able to do so on other planets just as easily.

"By 3.465 billion years ago, life was already diverse on Earth; that's clear – primitive photosynthesizers, methane producers, methane users," says J. William Schopf, lead author of the study. "These are the first data that show the very diverse organisms at that time in Earth's history, and our previous research has shown that there were sulfur users 3.4 billion years ago as well. If the conditions are right, it looks like life in the universe should be widespread."

NASA is currently planning to collect a sample of Martian soil and return it to Earth to study, and the team suggests that the SIMS instrument will probably be used to check if similar microfossils exist on the Red Planet.

The research was published in the journal PNAS.

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