Fossils Give Clue to Smell of Ancient Earth

Once life evolved to consume food, instead of harnessing energy from the sun or other chemical sources, picky eaters and gourmands soon followed. Now fossil evidence shows just what was on the menu about 1900 million years ago, and what was the favorite dish.

Like bones with bite marks leftover in a fire, a microfossil assemblage found near Lake Superior in Canada showed that tiny ancient microbes (about 1 micron in size) tended to munch on the larger sheaths (though still really small at 5 microns) of the cyanobacterium-like organism Gunflintia as opposed to the cysts of another bacterium (Huroniospora).

The Gunflintia fossils, found in their namesake Gunflint chert, showed not only perforated sheaths, but also pyritization where iron sulfide (pyrite or ‘fool’s gold’) replaced the biomaterial of the original sheath — a visible marker of a waste product from heterotrophic sulfate-reducing bacteria.

To our modern-day olfactory senses however the ancient Earth would have been a stinky place.

“In fact we’ve all experienced modern bacteria feeding in this way as that’s where that ‘rotten egg’ whiff of hydrogen sulfide comes from in a blocked drain. So, rather surprisingly, we can say that life on earth 1,900 million years ago would have smelled a lot like rotten eggs,’ said Martin Brasier of Oxford University’s Department of Earth Sciences, an author of the study published in this week’s Proceedings of the National Academy of Sciences (PNAS).

‘Whilst there is chemical evidence suggesting that this mode of feeding dates back 3,500 million years, in this study for the first time we identify how it was happening and ‘who was eating who,’ Brasier added.

Whereas plants and some bacteria that are autotrophs can obtain their energy from sunlight, heterotrophs must obtain their energy from the food they eat.

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