Did respiration evolve before photosynthesis?

Animals today stay alive by breathing in oxygen-rich air through a process known as oxygenic respiration, which consumes oxygen (O2) and releases carbon dioxide (CO2) as a byproduct. Most plants, on the other hand, convert sunlight and carbon dioxide into energy through a process known as photosynthesis, which consumes CO2 and releases O2 into the atmosphere. Because photosynthesis is a source of oxygen, it seems intuitive that photosynthesis evolved first: once enough O2 was in the air, then respiration would be able to arise in the newly oxygen-enriched atmosphere. However, some biologists have argued since the 1970’s that respiration in fact evolved first. There are many reasons that this might be the case, and new measurements of bacterial respiration at very low levels of O2 have revived this “early-respiration” hypothesis.

In a recent paper written by myself and my two graduate advisers, we argue that small quantities of O2 could have reached the surface of early Earth through transport by atmospheric dynamics. This transport would primarily occur in the Wintertime hemisphere, where a “polar Winter vortex” develops near the polar region, because the lack of sunlight in Winter would allow for greatest amount of O2 to accumulate. Our calculations show that enough dissolved O2 could have accumulated in polar Winter waters to allow early forms of marine life (i.e. microbial life) to develop and use respiration–without needing to wait for photosynthesis to oxygenate the atmosphere. Although our model calculations cannot prove that respiration did in fact evolve first, they least demonstrate a proof-of-concept that the “early-respiration” hypothesis is in fact viable.

Our paper is titled “Availability of O2 and H2O2 on pre-photosynthetic Earth” and appears in the May issue of the journal Astrobiology.