Archean: when life in the form of uni-cellular organisms first developed, the Earth’s early
atmosphere consisted entirely of volcanic gases, and there was no free oxygen.Prokaryotes
evolved, with photosynthesising bacteria (known ascyanobacteria) emerging approximately
3 billion years ago. The presence of photosynthesising cyanobacteria resulted in the re-
lease of oxygen into the atmosphere. The oxygen concentration then slowly began to rise,
becoming poisonous for many anaerobic bacteria. The rise in atmospheric oxygen concen-
trations led to the extinction of anaerobic bacteria, while stimulating the biodiversity and
development of oxygen-tolerant life forms.The early single-celled organisms (bacteria and cyanobacteria) lived together in aquatic
colonies. These colonies were formed by trapping sediments and minerals floating in water
and by producing a mucus which bound everything together. As the colony of bacteria, min-
erals and sediments grew, so did the structure that they made, and layer upon layer built up.
In order to capture as much sunlight as possible, the shape of the top of this sedimented algal
mat was curved (Figure 11.5). These curved and multi-layered structures were preserved and
exist as fossils to become what we now know are stromatolites.Figure 11.5: Wondergat, Northwest Province, the only place in South Africa where stromatolites have
been found.Figure 11.6: Stromatolites in Shark Bay.324 11.3. Life’s History