A rocky start: How we got our oxygen

They are the earliest evidence of life on Earth.

- Gowri S

Stromatolites date to a time before oxygen. These stony structures hold the fossilised remains of cyanobacteria, a sort of ancient ancestor to us all; a single-celled organism that produced much of the oxygen in our air.

Billions of years ago, cyanobacteria grew in colonies, forming sticky, gelatinous mats in shallow water.

As cyanobacteria used sunlight for photosynthesis, they consumed dissolved carbon dioxide from the marine water (converting it into sugars for energy) and released oxygen as a byproduct. This biological activity altered the local water chemistry, specifically increasing the pH and causing the precipitation of calcium carbonate (limestone).

Cyanobacteria, incidentally, are still around in large numbers.

This means one can study the rocky formations for clues to the chemical makeup of marine environments and tidal flats going back billions of years.

Going back, in fact, to a time when Earth's atmosphere was made up largely of methane and carbon-dioxide; when the earliest supercontinents still lay submerged beneath gigantic oceans; when the skies were likely orange from volcanic activity; and when only single-celled organisms like bacteria and archaea existed.

What secrets do they reveal here at home? Take a look.