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Evolution: Update on whether 3.5B-year-old "fossils" are really that

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Hotly debated 3.5 billion-year-old microbe fossils get another look

Ars Technica said:
The title of “oldest evidence of life” has been provisionally claimed by a growing and confusing crowd of discoveries recently. At least until the last few years, the crown rested comfortably on a 3.47 billion-year-old rock from Western Australia called the Apex Chert. First described in the early 1990s, this rock contained a variety of microscopic structures that looked for all the world like the fossilized remains of microbial life.

Like other finds in this category, the Apex Chert has seen its fair share of controversy as researchers skeptically poked and prodded. Just two years ago, we covered a study that concluded these microfossils were simply clever lookalikes created by minerals crystallizing near a hydrothermal vent. In that version of events, some carbon (which may or may not have come from living things) stuck to vaguely microbe-shaped mineral crystals.

A recent study led by William Schopf—who discovered the Apex Chert in the first place—brings newer tools to bear on the question. And the researchers believe the results show that these microfossils are not impostors.

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The isotope signatures can actually hint at what these organisms would have been like. Two of them are within the range of photosynthetic, single-celled life.

The other three would match up with an interesting pair: methane-producing archaea and methane-consuming bacteria. That would be pretty cool, as the existence of these two types of life have been guessed at from carbon isotope measurements of very old rocks but never pinned to specific microbial fossils. Their presence would hint at the diversity of life, even in the early days. Then again, recent studies have claimed to find evidence of life from 3.7 or even 3.95 billion years ago—and that would make 3.47 billion-year-old lifeforms comparative spring chickens.

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SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions at PNAS.

The researchers did it with carbon-isotope fractions -- organisms tend to prefer less C-13 than common nonbiological processes, because C-13 is heavier than the more common C-12. They discovered evidence of five species: two photosynthesizers, one methanogen, and two methane consumers. This suggests some diversification and ecological complexity.

The highest-level split in present-day organisms is between the Bacteria or Eubacteria, and the Archaea, both prokaryotes. Eukaryotes are some odd hybrid of the two, and also latecomers, around 2 billion years old. This research claims to have found representatives of both, the Bacteria (photosynthesizers) and the Archaea (methanogens). So their split happened before 3.5 billion years ago, and the origin of life is even older.