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Pushing back the origin of photosynthesis

More than a decade ago the oldest sedimentary rocks in the world at Isua in West Greenland hit the headlines, and not for the first time. Inclusions of graphite in crystals of the mineral apatite from the Isua supracrustals had yielded carbon isotopes unusually deficient in 13C relative to 12C, which is often regarded as a sign that life was involved in the carbon cycle at the time. The Isua rocks have been reliably dated at around 3.8 billion years (Ga) so that added over 400 Ma to the time at which life was present on Earth. Sedimentary rocks formed at 3.4 Ga contain the first tangible signs in the form of stromatolites thought to have been secreted by biofilms of blue-green bacteria which are oxygen-generating photosynthesisers. Sadly, limestones at Isua, indeed all the putative sedimentary rocks there were metamorphosed and deformed plastically so that such features, if they were ever present, had been obliterated. Apatite was thought to be so strong and resistant to heating that carbon within its crystals would have preserved original isotopic ‘signatures'. Detailed studies to test this hypothesis refuted the early age for life, which reverted back to around 3.4 Ga. But Isua presents too good an opportunity for its geochemical secrets to be left uninvestigated.


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