It might not look like much, but this unassuming rock shows signs of life going about its business in the shallow seas of Earth some 1.6 billion years ago.
The little pockmarks covering the rock are not formed by some process of natural erosion, they are actually fossilized oxygen bubbles created by tiny microbes before Eukaryotes – the group containing you, me, fungi, and plants – had come into existence. Discovered in the Vindhyan basin, central India, they can help researchers build a more detailed picture of Earth long ago and what life was up to at this time. The results are published in Geobiology.
The oxygen is thought to have been produced by cyanobacteria, which would have been forming large microbial mats in coastal waters. This phylum of bacteria, which itself is thought to have evolved some 3 billion years ago, is able to photosynthesize using sunlight and carbon dioxide to produce energy, and expel oxygen as a result. The oxygen is then thought to have got trapped in the microbial mats, before being covered with sediment and fossilizing.
In much the same way it did all those billions of years ago, cyanobacteria still form these microbial mats – which build up to form structures known as stromatolites – to this day. Most notably, this occurs in Shark Bay, Western Australia, and they basically look like a load of boulders strewn across the shallow sea. However, they are in fact living structures, and have been living in the same way for billions of years.
These new fossils can help create a bigger picture of what these early environments were like. The bubbles are tiny – between just 50 and 500 microns in diameter – but can tell us a lot. While many are spherical, some have been squished, indicating they were compressed after being formed and before being preserved. There is also what appears to be filaments weaving around some of them, which the researchers think may be the remains of the ancient cyanobacteria itself.
These large microbial mats may well have been crucial in the development and diversification of life on our planet. It is thought that as the cyanobacteria photosynthesized over a period of half a billion years, it triggered what is known as the Great Oxygenation Event between 2.5 and 2.3 billion years ago. This helped pave the way for the evolution of more complex life forms, such as most of what we know today.
By understanding how these microbial mats grew and lived, it could help shed light on one of the major events in the history of the Earth.