Charles Q. Choi, Astrobiology Magazine ContributorDate: 24 March 2011 Time: 01:36 PM ET
Structures thought of as the oldest known fossils of microbes might actually be microscopic mineral formations not associated with life, suggesting that astrobiologists must be careful calling alien objects "life" when scientists have trouble telling what is or was alive on Earth.
More than 20 years ago, microscopic structures uncovered in the roughly 3.5-billion-year-old Apex Chert formation in western Australia were described as the oldest microbial fossils. These structures were interpreted as cyanobacteria, once known as blue-green algae, embedded in a silica-loaded rock formed in a shallow marine setting. These structures were all detected in slices of rock just 300 microns thick, or roughly three times the diameter of a human hair.
However, the interpretation of the structures has always been controversial, and it is still hotly debated among scientists searching for Earth’s earliest evidence for life. Specimens from the site apparently displayed branching structures that some researchers said were inconsistent with life, while others dismissed such branching as artifacts from photo software
Analysis of the structures themselves suggested they were carbon-based, and therefore associated with the organic chemistry of life, but some contended they were a type of carbon known as graphite, while others said they were kerogen, a mixture of organic compounds.
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