The Earth is estimated to be 4.5 billion years old, and the first microorganisms appeared around a billion years later. At that time, the composition of the atmosphere was very different from that of today.CO2 and N2 were in very high concentration, along with a significant amount ofH2. On the other hand, there was a total lack of O2. This gas became a major component of the atmosphere only after eons of photosynthetic activity by cyanobacteria. In fact, oxygen is a by-product of the cleavage of water during photosynthesis. The first living organisms therefore evolved in an anoxic environment.
The vital process depends mainly on the synthesis of reduced carbon compounds and their use to generate the energy necessary for life. There are two ways of conceiving the prebiotic generation of these compounds : either they were synthesized in space (or on earth by electrical phenomena), or they were synthesized, among other things, from the reduction ofCO2 coupled with the oxidation of hydrogen. These two alternatives are known respectively as the "heterotrophic" and "autotrophic" theories of the origin of life on earth.
In the laboratory, we have determined the three-dimensional structure of several iron-sulfur enzymes, purified from anaerobic microorganisms, which catalyze " primordial " reactions such asH2 oxidation orCO2 reduction. Our work supports the " autotrophic " theory, and is also consistent with the idea that these reactions took place on sulfides such as pyrite (FeS2) or pyrrhotite (FeS). These points will be discussed during the seminar.
