I was going to write about the discovery of the oldest fossil cells but Jerry Coyne beat me to the punch [Newly found: the world’s oldest fossils!]. The new fossil bacteria are thought to be 3.4 billion years old and they were discovered in Australia only a few kilometers from the site where the so-called "cyanobacteria" fossils were discovered almost twenty years ago. Those fossils were reported to be even older (3.5 billion years) but the discovery has been completely discredited. The "fossils" aren't fossils [Did Life Arise 3.5 Billion Years Ago?]. That makes this discovery the oldest known cells (Wacey, 2011). Interestingly, the senior author on this paper is Martin Brasier and he was one of the scientists who challenged the earlier result of William Schopf. Read all about it on Jerry Coyne's
The fossils are associated with a sulfur-rich mineral called pyrite. This mineral is produced by modern sulfate-reducing bacteria and it's reasonable to assume that the primitive bacteria detected in these ancient rocks also carried out sulfate reduction. That's not surprising since there wasn't much oxygen in the deep ocean 3.5 billion years ago.
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