Stromatolites

Stromatolites are layered mounds, columns, and sheet-like sedimentary rocks. They were originally formed by the growth of layer upon layer of cyanobacteria, a single-celled photosynthesizing microbe that lives today in a wide range of environments ranging from the shallow shelf to lakes, rivers, and even soils.  Cyanobacteria are prokaryotic cells (the simplest form of modern carbon-based life) in that they lack a DNA-packaging nucleus.  Bacteria, including the photosynthetic cyanobacteria, were the only form of  life on Earth for the first 2 billion years that life existed on Earth.

SEM photo of E-Coli, a modern bacterium that lives in the human intestine	Photomicrograph of Oscillatoria, a modern blue-green alga

Drawing of columnar stromatolites like the ones shown in the picture below.  Shows internal layering and close-up of  lamina structure.  Surface is covered by a felt of cyanobacterial filaments  that trap sediment grains that are washed across the mat surface.  Abundant sediment supply produces granular laminae, low sediment supply produces layers rich in organic matter (green in drawing).

This picture shows modern stromatolites (columns and mounds) in Shark Bay (Western Australia).  Although algal mats and films are common in many modern sedimentary environments, they are rarely recognised because of continual destruction (grazing) by snails, worms, and other animals.  Today, only in places where metazoan grazing is restricted due to extreme conditions (high salinity, too hot, etc.; basically only cyanobacteria can survive) can stromatolite domes like these form in ways that are reminiscent of the Precambrian (when metazoan grazing was absent because metazoans had not evolved yet).  The Precambrian was the age of microbes, macroscopically expressed in colonial structures that we call stromatolites.

In the rock record, stromatolites can be recognized by characteristic laminar structures, such as those seen in the picture above.  This is basically the cross section of a columnar stromatolite as seen in the Shark Bay picture.

Although simple, cyanobacteria were ultimately responsible for one of the most important "global changes" that the Earth has undergone. Being photosynthetic, cyanobacteria produce oxygen as a by-product. Photosynthesis is the only major source of free oxygen gas in the atmosphere. As stromatolites became more common 2.5 billion years ago, they gradually changed the Earth's atmosphere from a carbon dioxide-rich mixture to the present-day oxygen-rich atmosphere. This major change paved the way for the next evolutionary step, the appearance of life based on the eukaryotic cell (cell with a nucleus).