A new study claims to have found evidence that life existed on our planet at least 4.1 billion years ago – about 300 million years earlier than prior estimates.
According to the study, published in the journal Proceedings of the National Academy of Sciences, life existed on Earth prior to the massive bombardment of the inner Solar System that formed large craters on the Moon about 3.9 billion years ago.
“If all life on Earth died during this bombardment, which some scientists have argued, then life must have restarted quickly,” said study co-author Patrick Boehnke, a graduate student at the University of California, Los Angeles.
Researchers had long believed the planet was dry and desolate during that time period.
“The early Earth certainly wasn’t a hellish, dry, boiling planet; we see absolutely no evidence for that. The planet was probably much more like it is today than previously thought,” said co-author Prof. Mark Harrison, also of the University of California, Los Angeles.
Prof. Harrison, Boehnke and their colleagues studied more than 10,000 zircons (durable minerals related to the synthetic cubic zirconium used for imitation diamonds) originally formed from molten rocks at Jack Hills, Western Australia.
The researchers identified 656 Jack Hills zircons containing dark specks that could be revealing and closely analyzed 79 of them with Raman spectroscopy, a technique that shows the molecular and chemical structure of ancient microorganisms in three dimensions.
They were searching for carbon, the key component for life. One of the 79 zircons contained graphite – pure carbon – in two locations.
The scientists are very confident that their zircon contains inclusions of 4.1 billion-year-old graphite.
“There is no better case of a primary inclusion in a mineral ever documented, and nobody has offered a plausible alternative explanation for graphite of non-biological origin into a zircon,” Prof. Harrison said.
“The graphite is older than the zircon containing it,” he said.
“Its complete encasement in crack-free, undisturbed zircon demonstrates that it is not contamination from more recent geologic processes.”
According to the scientists, the zircon is 4.1 billion years old, based on its ratio of uranium to lead. However, they don’t know how much older the graphite is.
The graphite has a characteristic signature – a specific ratio of carbon-12 to carbon-13 – that indicates the presence of photosynthetic life.
“On Earth, simple life appears to have formed quickly, but it likely took many millions of years for very simple life to evolve the ability to photosynthesize,” Prof. Harrison said.
Elizabeth A. Bell et al. Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon. PNAS, published online October 19, 2015; doi: 10.1073/pnas.1517557112
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