A Hypothesis for How ETC May Have Evolved
A proposed link between SLP/fermentation and the evolution of ETCs:
In a previous discussion of energy metabolism, we explored substrate level phosphorylation (SLP) and fermentation reactions. While SLP and fermentaton together are perfectly good ways to harvest energy, one of the byproducts of these reactions is the acidification of the cell. It is thought that cells needed, therefore, to co-evolve mechanisms that helped remove protons accumulated from SLP and ferementation from the cytosol (interior of the cell). One solution to the "proton problem" may have been evolution of the F0F1-ATPase, a multi-subunit enzyme that translocates protons from the inside of the cell to the outside of the cell by hydrolyzing ATP (see figure below). This arrangement works as long as small reduced organic molecules are abundant and freely available and that sufficient ATP can be made through SLP to "fuel" the business of the cell and to also export the protons that accumulate. However, as these biological processes continue, the small reduced organic molecules will likely be used up and their concentration will decrease. The resulting scarcity of fuel, therefore, puts a demand on cells to find alternative mechanisms to harness energy and/or to become more efficient.
In the scheme proposed above, one potential source of "wasted ATP" is its use in the removal of protons from the cell's cytosol; organisms that could find other mechanisms to expel accumulating protons while still preserving ATP could have a selective advantage. It is hypothesized that this selective evolutionary pressure potentially led to the evolution of the first membrane-bound proteins that used red/ox reactions as their energy source (depicted in second picture) to pump out the accumulating protons. Enzymes and enzyme complexes with these properties exist today in the form of the electron transport complexes like Complex I, the NADH dehydrogenase.
Figure 1. Proposed evolution of an ATP dependent proton translocator
Figure 2. As small reduced organic molecules become limited, organisms that can find alternative mechanisms to remove protons from the cytosol may have an advantage. The evolution of a proton translocator that uses red/ox reactions rather than ATP hydrolysis could substitute for the ATPase.
Continuing with this line of logic, if organisms evolved that could now use red/ox reactions to translocate protons across the membrane they would create an electrochemical gradient, separating both charge (positive on the outside and negative on the inside; creating an electrical potential) and pH (low pH outside, higher pH inside). With excess protons on the outside of the cell membrane, and the F0F1-ATPase no longer consuming ATP to translocate protons, it is hypothesized that the electrochemical gradient could then be used to power the F0F1-ATPase "backwards" — that is, to form or produce ATP by using the energy in the charge/pH gradients set up by the red/ox pumps (as depicted below). This arrangement is called an electron transport chain (ETC).
Figure 3. The evolution of the ETC; the combination of the red/ox driven proton translocators coupled to the production of ATP by the F0F1-ATPase.
NoTE: Extended reading on the evolution of electron transport chains
If you're interested in the story of the evolution of electron transport chains, check out this more in-depth discussion of the topic at NCBI.