The oldest still-living life form on Earth
See that pale orange-coloured rock? It might not look much, but without this life-form, the rest of life on Earth would never have evolved. It’s a life-form which is still alive today, 3.5 billion years after it first evolved, and was originally responsible for breathing oxygen into our atmosphere. This fossil on Westray’s shore is a mere 380 million years old. What is it?
It’s a fossilised mat of cyanobacteria called stromatolites. Stromatolites are named after mattresses ‘stroma’ and rocks ‘lithos’. They’re rock mattresses, as they grow ever-so slowly into pillow-mounds. They are living fossils as there are still thriving examples growing in shallow water in Western Australia.
As layers of sediment formed on the bottom of the shallow inland lake that was Lake Orcadie, 380 million years ago, microscopic colonies of cyanobacteria grew over it, binding the sediment together into pillow-mounds. In the intervening years they have been buried, compressed, heated, turned to rock, exposed, weathered and now lie on the shore gradually being eroded away.
Stromatolites were here 3.5 billion years ago, when the Earth itself is only 4.5 billion years old. They’ll probably still be here long after we have gone. It’s thought-provoking to see their remains scattered over the shore.