Beta‐oxidation of fatty acids and detoxification of reactive oxygen species are generally accepted as being fundamental functions of peroxisomes. Additionally, these pathways might have been the driving force favoring the selection of this compartment during eukaryotic evolution. Here we performed phylogenetic analyses of enzymes involved in beta‐oxidation of fatty acids in Bacteria, Eukaryota, and Archaea. These imply an alpha‐proteobacterial origin for three out of four enzymes. By integrating the enzymes' history into the contrasting models on the origin of eukaryotic cells, (...) we conclude that peroxisomes most likely evolved non‐symbiotically and subsequent to the acquisition of mitochondria in an archaeal host cell. (shrink)

As free‐living organisms, alpha‐proteobacteria produce reactive oxygen species (ROS) that diffuse into the surroundings; once constrained inside the archaeal ancestor of eukaryotes, however, ROS production presented evolutionary pressures – especially because the alpha‐proteobacterial symbiont made more ROS, from a variety of substrates. I previously proposed that ratios of electrons coming from FADH2 and NADH (F/N ratios) correlate with ROS production levels during respiration, glucose breakdown having a much lower F/N ratio than longer fatty acid (FA) breakdown. Evidently, higher endogenous ROS (...) formation did not hinder eukaryotic evolution, so how were its disadvantages mitigated? I propose that the resulting selection pressures favoured the evolution of a variety of eukaryotic ‘innovations’: peroxisomes for FA breakdown, carnitine shuttles, the linkage of beta‐oxidation to antioxidant properties, uncoupling proteins (UCPs) and using mitochondrial uncoupling during beta‐oxidation to reduce ROS. Recently observed relationships between peroxisomes and mitochondria further support the model. (shrink)