Assessing the relationship between hypoxia and life on Earth, and implications for the search for habitable exoplanets

Atmospheric oxygen at the level of present-day Earth (21%) is generally regarded as permissive for the development and survival of complex multicellular life, such as animals. The hypothesis that oxic conditions are conducive for multicellular life has subsequently been applied to the search for life elsewhere in the universe, with planets exhibiting atmospheric oxygen of a similar level to Earth being considered as candidates for habitability. However, an opposing view states that in fact hypoxia (1–5% oxygen) is a prerequisite for tissue renewal, and therefore also for complex multicellularity. Hypoxia is present in many facets of life, such as during early mammalian embryogenesis as well as in niches like bone marrow, where populations of undifferentiated stem cells are maintained. This view implies that oxic conditions are indeed challenging for the evolution of complex multicellularity, and furthermore questions the association between oxic conditions and the potential for multicellular life on exoplanets. Here, I review, test and discuss the association between hypoxic conditions and the evolution of complex multicellular life. Through an extensive study of the available literature, I review how hypoxia and cellular hypoxia-response machineries play a vital role in many stages of metazoan development, regulating the transcription of multiple genes and maintaining cell stemness during mammalian embryogenesis. I perform experiments to mimic the development of an hypoxic sedimentary niche on an exoplanet, and test how sediment grain size alters the depth of the hypoxic zone within these sediments. The results demonstrate that, in the absence of microorganisms, the hypoxic niche expands to greater depths within coarser sediments than in finer sediments. Where microorganisms were present within this sediment, the depth of the hypoxic niche was significantly reduced. Microbial respiration rapidly turned the sediment anoxic. Therefore, I propose that the absence of microorganisms within this type of sediment is advantageous for the maintenance and expansion of the hypoxic niche; however this absence may hinder the development of larger forms of life within these sediments. I also evaluate the likelihood of hypoxic conditions on rocky exoplanets orbiting white dwarfs. Utilising data from extrasolar white dwarfs polluted by external material (that likely represents planets that formerly orbited the stars), oxygen fugacities are used as proxies for the oxidation state of these planetary atmospheres. These values are com-pared to oxygen fugacities of early and present-day Earth and Mars in order to observe the similarities between their atmospheric oxygen levels. It is concluded that one of the studied white dwarfs may have developed an hypoxic atmosphere during its lifetime, and thus that the former planet orbiting the star had the potential to hold complex multicellular life.