Do microbes have an impact on evolution?
Microbes have had a significant influence on evolution and have therefore also helped to shape our life today. Several billion years ago, the Earth's atmosphere contained only a very low proportion of oxygen (O2). Existing life forms were therefore adapted to oxygen-free or oxygen-poor (anaerobic) conditions. When the Earth was about half as old as it is today, there was a drastic increase in oxygen in the atmosphere. This is due to the activity of the ancestors of today's cyanobacteria, which carried out a then novel form of photosynthesis. This metabolism, which also occurs in a similar form in plants today, released large quantities of oxygen. Oxygen is very reactive and therefore deadly for living organisms without appropriate adaptation. Therefore, the increase in atmospheric oxygen led to a mass extinction of oxygen-intolerant (anaerobic) organisms. However, this mass extinction opened up habitats in which aerobic organisms (which require oxygen to live) could develop. Microbes thus made a decisive contribution to the development of oxygen respiration and the evolution of higher organisms, including us humans.
The rapid increase in oxygen concentration by several orders of magnitude around 2.4 billion years ago is also known as the "great oxygen catastrophe". Oxygen can take on highly reactive forms: If, for example, a single oxygen atom is formed instead of the normal form O2, it immediately looks for new partners. Such reactive oxygen species can destroy vital cell structures. As a result, living organisms at that time were forced to adapt to an oxygen-rich environment or to move into oxygen-free niches. Life forms that were unable to do this became extinct (see also here).
The switch to an oxygen-dependent metabolism represents a pioneering achievement in the evolution of energy metabolism, as oxygen respiration has evolutionary advantages over earlier forms of respiration. The utilisation of nutrients by means of oxygen respiration can be compared with the combustion of petrol in a conventional combustion engine: in this case, the petrol (nutrient) is completely burnt with oxygen to produce carbon dioxide (CO2) and water to generate energy. While the release of energy in the engine is explosive, living organisms have found a way to divide the breakdown into many small individual steps and store a large proportion of the energy released (oxidative phosphorylation). Living organisms that carry out other types of respiration (e.g. iron or nitrate respiration) can only utilise a small proportion of the energy absorbed with food. The evolutionary advantage of the faster and more efficient provision of energy through oxygen respiration has led to the fact that today all highly complex life forms such as animals, fungi and plants (eukaryotes) breathe oxygen.
By the way, cyanobacteria do not always produce oxygen!
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© Text Paul Bolay, UFZ Leipzig, Department Solar Materials/VAAM, paul.bolay[at]ufz.de
© Figure Mahir Bozan, UFZ Leipzig, mahir.bozan[at]ufz.de, use according CC 4.0
