The bacteria are living unicellular and are prokaryotes. This means that its genetic material, a double-stranded circular DNA molecule, is free in the cytoplasm, not enclosed within a nucleus.
Given that microfossils and stromatolites (fossil colonies of bacteria mixed with minerals) have been found in sediments from various geological eras, and even in sedimentary rocks over 3.5 billion years old, bacteria are said to have existed from very remote times.
So much so that they have existed for a long period of Earth’s history when there were not even other life forms yet. In fact, the bacteria introduced very significant evolutionary events.
- See also: Viruses (biology)
Today it is usually distinguished into two large groups:
- Bacteria: are represented by those that predominate in today’s natural environment, with the presence of different oxygen levels and various metabolisms.
- The Archaea: evolutionarily represent a previous category, with metabolisms specially adapted to extreme environmental situations, such as lack of oxygen (remember that, according to rigorous studies, there was no oxygen on the planet until the vegetables, the great oxygen liberators, appeared), or very salty or very acidic environments with high temperatures.
The great evolutionary success of bacteria is largely attributed to their surprising metabolic versatility. It can be affirmed that all the possible mechanisms for obtaining matter and energy exist distributed among the various classes of bacteria.