According to a study published in the journal Proceedings of the National Academy of Sciences (PNAS), after studying carbon samples found in graphite embedded in zircon crystals in Western Australia, researchers believe that life on our planet may have started hundreds of millions of years earlier than previous estimates.
Elizabeth A. Bell and her co-authors explain that "evidence of life on Earth is manifestly preserved in the rock record," but the record of microfossils (so small they require a microscope to see) only goes back so far, to 3.5 billion years ago. Meanwhile, the chemofossil record (chemicals from the decomposition of a once-living organism) dates back to about 3.8 billion years ago, and the rock record to some 4 billion years ago. However, last year, zircon crystals were discovered in the Jack Hills of Western Australia that pre-date the rock record by .4 billion years, making them the oldest known minerals on the planet.
Bell and her team studied more than 10,000 Jack Hills zircon crystals and discovered the graphite and carbon samples in a "crack-free" region of the minerals. "We used the caesium ion beam to drill through the [zircon] surface and into the graphite, such that the graphite was never exposed to contaminants prior to analysis," she told Scientific American. The researchers believe that the graphite was "incorporated during crystallization of this igneous zircon." That means that if the carbons were produced by live organisms, then a "terrestrial biosphere" may have formed around 4.1 billion years ago—that's roughly 300 million years earlier than scientists previously thought.
Bell also told Scientific American that while the isotope ratio of the carbon found (which is used as an "indicator of life" in rocks) could be a result of processes that don't involve living organisms, the conditions in which the graphite was discovered would make those alternative processes "incredibly complicated, perhaps unfeasibly so."
The next step in confirming that the carbon was produced by living organisms would be to find and study more samples in the Jack Hills zircon crystals.