We can assume that the abundance of chemical energy on our cooling planet favored the formation of cells that could capture free energy from these nutrients in the absence of any oxygen. For a time, we thought that the first cells would have extracted nutrient free energy by non-oxidative, fermentation pathways. And they would have been voracious feeders, quickly depleting their environmental nutrient resources. In this scenario, the evolution of autotrophic life forms saved life from an early extinction! That is because autotrophs could create organic molecules extracting free energy from inorganic molecules or from light.
An alternative scenario that is gaining traction, suggests that the first cells may have started with oxidative reactions that used something other than oxygen as a final electron acceptor. In this scenario (to be considered in more detail elsewhere), non-oxygenic ‘oxidative’ chemistries came first, followed by the evolution of anoxic fermentative chemistries, then followed by photosynthesis, and finally respiratory pathways. In either scenario, we can safely assume that photosynthesis existed before oxygenic respiration.
We also assume that oxygenic photoautotrophs that capture free energy from light would become the most abundant autotrophs, if for no other reason than sunlight is always available (at least during the day), and oxygen is abundant in the air! The early photoautotrophs were likely the ancestors of today’s cyanobacteria. In fact, a phylogenetic study of many genes including “plastid-encoded proteins, nucleus-encoded proteins of plastid origin…, as well as wide-ranging genome data from cyanobacteria” suggests a common ancestry of freshwater cyanobacteria and eukaryotic chloroplasts (Ponce-Toledo, R.I. et al., 2017, An Early-Branching Freshwater Cyanobacterium at the Origin of Plastids. Current Biology 27:386-391).
But what about the origins of respiratory metabolism and the endosymbiotic origins of mitochondria? Let’s start by asking how respiration co-opted photosynthetic electron transport reactions that captured the electrons from H2O needed to reduce CO2, turning those reactions to the task of burning sugars back to H2O and CO2. As photosynthetic organisms emerged and atmospheric oxygen increased, elevated oxygen levels would have been toxic to most living things. Still, some autotrophic cells must have had a pre- existing genetic potential to conduct detoxifying respiratory chemistry. These would have been facultative aerobes with the ability to switch from photosynthesis to respiration when oxygen levels rose. Today’s purple non-sulfur bacteria such as Rhodobacter sphaeroides are just such facultative aerobes! Perhaps we aerobes descend from the ancestors of such cells that survived and spread from localized environments where small amounts of oxygen threatened their otherwise strictly anaerobic neighbors. Is it possible that the endosymbiotic critter that became the first mitochondrion in a eukaryotic cell was not just any aerobic bacterium, but a purple photosynthetic bacterium?