Abstract EANA2025-133

All known life is on Earth, but there may be life elsewhere. We do not need a definition of life to make sense of this proposition. The question of whether there is life beyond Earth is intelligible insofar as biology is taken to be about life understood as a general phenomenon, that may or may not exist in particular places at particular times. Universal biology seeks universal principles about life, that are not only true of Earth-like life. Physical laws, assumed to be universal, constrain the ways life can be. Therefore, progress can be made by hypothesizing universal laws about life from known regularities, and assessing whether they can be explained by physical principles.

Given the second law of thermodynamics, the activity of living things and their complex structure can only be maintained by an influx of free energy from the environment, which organisms use to do work and maintain their highly ordered state, while simultaneously increasing the entropy of the environment. Organisms are therefore necessarily open systems with a boundary that separates them from the outside, allowing them to maintain a thermodynamic disequilibrium with the environment.

Living processes involve endergonic reactions that are forced to run thermodynamically ‘uphill’ by being coupled with thermodynamic favourable reactions. This is achieved by molecular machines whose functioning depends on random thermal motion – a feature of viscous liquids at the nanoscale. For this reason, life processes must be chemical. Living systems also use a restricted set of molecules from the vast array of compounds that can be formed from the same elements. This is partly because biomolecules are produced in chemical cycles, which generate specific molecules. Furthermore, living things require molecules that can play certain roles, including: carrying out activities; energy transduction and storage; information storage and transmission, and structural elements. Polymers composed of interchangeable monomers are an excellent, possibly universal solution for making molecular machines that can do work, and for storing information.

Neither replication nor Darwinian evolution are exclusive to living systems. Molecular replication can occur in prebiotic chemical systems, generating populations of molecules which can undergo natural selection for faster replication and/or increased stability. The fact that life requires a boundary means that living things form populations of separate, individual things which, if they reproduce, also evolve by natural selection.

There are finite ways to obtain free energy, and only two main ways of obtaining materials: synthesizing them from simple abiotic compounds (autotrophy) or consuming preexisting complex molecules (heterotrophy). The ways in which organisms obtain free energy and materials determines their possible interactions with the abiotic environment and with other organisms, giving rise to broad ecological categories such as primary producers, consumers, parasites, etc., which are also expected to be universal.

Although the lack of examples of life beyond Earth may seem like an insurmountable epistemic limitation to our understanding of life as a general phenomenon, the universality of physics allows to hypothesise some universal principles about life.