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In 1992, researchers found evidence of what may have been the earliest life on Earth at the time - a tiny 3.5 billion-year-old imprint on Australian rock.
Since then, however, scientists have been debating whether these marks really represent ancient microbes, and even if they do exist, they are really that old.
now, a comprehensive analysis of these microfossils suggests that they do represent ancient microbes.
these microbes are so complex that life on our planet must have been born 500 million years ago.
, a geological biologist at Curtin University in Perth, Australia, who was not involved in the study, said the new study showed that these early microbes were surprisingly complex enough to use photosynthesis and use other chemical processes to get energy. Alison Olcott Marshall, a geological biologist at the University of Kansas in Lawrence, U.S.,
who was also not involved in the study, stressed that the study "could lead to a series of new studies on these rocks as other researchers look for data that supports or refutes this new hypothesis."
In the new study, William Schopf, a palaeontologist at the University of California, Los Angeles, teamed up with the discoverers of the Australian microbial fossils, as well as John Valley, a geoscientist at the University of Wisconsin-Madison.
Valley is an expert in an analytical technique called secondary ion mass spectrometry (SIMS), which can determine the proportion of different forms of carbon in a single sample, which is key to determining whether it is organic carbon.
Schopf spent four months working under a microscope and eventually found a sheet of fossilized rock that was large enough for SIMS analysis; the sample contained 11 microbial fossils whose diversity of shape and size suggested they represented five microbes.
Schopf also provided samples of rocks, none of which contained any hypothetical fossils for comparison.
researchers found several different carbon ratios in the sample.
studies have shown that the carbon content of the two microfossils is the same as that of modern bacteria, which use light to make carbon compounds to power their activities -- a primitive photosynthesis that does not contain oxygen.
the carbon ratio of the other two microfossils is the same as that of anobacteria, which rely on methane as their energy source, which plays a key role in the development of multicellular organisms.
the carbon ratio of the last microfossil suggests that the methane produced by the microbe is part of its metabolism.
Schopf, Valley and their colleagues report the findings in the December 18 issue of the Proceedings of the National Academy of Sciences.
Schopf says the fact that there are so many different carbon ratios reinforces the hypothesis that these are real fossils.
he says any inorganic process that could produce imprinting will leave only a uniform carbon-ratio signature. 'The fact that microbes have been so diverse at this point in Earth's history suggests that life on our planet may be back 4 billion years, ' said
Schopf.
other researchers have found signs of life for such a long time, these findings are more controversial than Schopf's results.
" the new findings further confirm the idea that these microstructures are biological.
" Rasmussen agrees.
he feared the microfossils might be very badly preserved.
olcott Marshall, who argues that the rock imprint is not a fossil at all, but a product of geological processes.
she said that "the errors made by this analytical technique are so great" that the data is not enough to indicate that there are different types of microbes in the rocks.
, but SIMS experts praised the work. "It's a very careful and well-thought-out experiment,"
. "They've worked hard to make sure everything is properly calibrated," said Lara Gamble, a chemist at the University of Washington in Seattle who wasn'
who was not involved in the study.
" Rasmussen hopes that future studies will lead to further analysis of more microfossils. "Given that we're looking at some of the oldest signs of life, it's worth it,"
.
," he said, "it is essential that we hone our skills in identifying the ancient life characteristics of Life on Earth as we look to Mars and beyond."
"