The Microbial World
The earliest microscopic fossil remains of probable prokaryotic organisms have been found in rocks dating back 3,500 million years. The most diverse and abundant of these are single-celled bacteria, representing one of the most ubiquitous forms of life on the planet. By around two billion years ago, eukaryotes - larger and more complicated organisms with a true nucleus and complex internal cell structures - had evolved. These organisms were the first to develop complex multicellularity and became the first individual life forms to leave fossils visible to the naked eye (macrofossils).
© Royal Ontario Museum. Photo: Jean-Bernard Caron
Some of these early eukaryote macrofossils resemble simple algal seaweeds, but are difficult to classify in living groups; others can be identified as different kinds of primitive algae or as fungi. These fossils remain extremely rare, and the first 3,500 million years of life on Earth was largely dominated by single-celled microbes lacking a nucleus.
© Cambridge University. Photos: Nick Butterfield.
Ample evidence of ancient microbial life has been left behind as layered rock structures called stromatolites. These formed in watery environments where the sticky biofilms secreted by microscopic filamentous cyanobacteria trapped and cemented grains of sand or mud in alternating layers. Modern stromatolites are usually found in sheltered, shallow marine environments where high salt levels in the water keep bacteria-grazing animals at bay. Over time, new layers of sediment are added, sometimes forming metre-high domes, with the cyanobacteria living only on the uppermost, youngest surface. Because they were strongly bound together by bacterial biofilms, ancient stromatolites preserve well in the rock record.
© Royal Ontario Museum. Photo: Desmond Collins (left) and Jean-Bernard Caron (right).
Fossilized stromatolites are found in many locations around the world, indicating the bacteria that built them were widespread and abundant from 2,500 million years ago until the end of the Ediacaran Period (542 million years ago). The rapid decline in stromatolite diversity and abundance during and after the Cambrian Period was probably due to grazing activities by newly-evolved animals and changes in environmental conditions.
© Royal Ontario Museum. Photos: Jean-Bernard Caron.
The prokaryotic cyanobacteria - particularly those forming stromatolites - were largely responsible for the first free oxygen in Earth's atmosphere, which began to accumulate around 2,500 million years ago. Cyanobacteria release oxygen as a by-product of photosynthesis, the same process plants use to convert carbon dioxide into sugars that can be used for energy. Eukaryotic organisms require oxygen, so life could not become large and complex until the oceans and atmosphere contained enough oxygen to support their evolution.