A schematic illustrating habitability of physical spaces as a function of physical and chemical conditions. The diagram is a simplified depiction of niche space (here three dimensions are shown – in reality, the space is an n-dimensional hypervolume) and the extent to which these niche conditions are occupied in physical spaces. The habitable space for life is enclosed within a much larger (red) space that is uninhabitable (also see Figure 4). The blue object represents habitable space that is inhabited. At the edge of this habitable and inhabited set of physical and chemical conditions is a putative yellow ‘rind’ of habitat space at the edges of life which could be inhabited by life with appropriate evolutionary innovations. Within the habitable and inhabited space there are grey shapes depicting localized physical spaces that are habitable, but uninhabited (uninhabited habitats). The purpose of the schematic is to illustrate that uninhabited, but habitable spaces exist both within physical spaces that are known to be permissive for microbial growth and potentially at the limits of life at extremes. Life, especially using technology, can establish itself in conditions away from the blue spheroid with appropriate expenditure of energy (e.g., a self-sustaining space settlement), shown as isolated blue objects in the red space. — FEMS Microbiology Reviews
Although a large fraction of Earth’s volume and most places beyond the planet lack life because physical and chemical conditions are too extreme, intriguing scientific questions are raised in many environments within or at the edges of life’s niche space in which active life is absent.
This review explores the environments in which active microorganisms do not occur. Within the known niche space for life, uninhabited, but habitable physical spaces potentially offer opportunities for hypothesis testing, such as using them as negative control environments to investigate the influence of life on planetary processes.
At the physico-chemical limits of life, questions such as whether spaces devoid of actively metabolising or reproducing life constitute uninhabitable space or space containing vacant niches that could be occupied with appropriate adaptation are raised.
We do not know the extent to which evolution has allowed life to occupy all niche space within its biochemical potential. The case of habitable extraterrestrial environments and the scientific and ethical questions that they raise is discussed.
Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻
