Los Angeles: Life on Earth may have started at least 4.1 billion years ago - 300 million years earlier than previously documented, a new study suggests.
The discovery by University of California - Los Angeles (UCLA) researchers indicates that life may have begun shortly after the planet formed 4.54 billion years ago.
"Twenty years ago, this would have been heretical; finding evidence of life 3.8 billion years ago was shocking," said Mark Harrison, co-author of the research and a professor of geochemistry at UCLA.
"Life on Earth may have started almost instantaneously. With the right ingredients, life seems to form very quickly," added Harrison.
The new research suggests that life existed prior to the massive bombardment of the inner solar system that formed the Moon's large craters 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 Patrick Boehnke, a co-author of the research and a graduate student in Harrison's laboratory.
"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," Harrison said.
The researchers, led by Elizabeth Bell - a postdoctoral scholar in Harrison's laboratory - studied more than 10,000 zircons originally formed from molten rocks, or magmas, from Western Australia.
Zircons can capture and preserve their immediate environment, meaning they can serve as time capsules.
The scientists identified 656 zircons containing dark specks that could be revealing and closely analysed 79 of them with Raman spectroscopy, a technique that shows the molecular and chemical structure of ancient microorganisms in three dimensions.
Bell and Boehnke, who have pioneered chemical and mineralogical tests to determine the condition of ancient zircons, were searching for carbon, the key component for life. One of the 79 zircons contained graphite - pure carbon - in two locations. The researchers are "very confident" that their zircon represents 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," Harrison said.
The graphite is older than the zircon containing it, the researchers said. They know the zircon is 4.1 billion years old, based on its ratio of uranium to lead; they don't know how much older the graphite is. The carbon contained in the zircon has a characteristic signature - a specific ratio of carbon-12 to carbon-13 - that indicates the presence of photosynthetic life.
The research is published in the journal Proceedings of the National Academy of Sciences.