How will we find life beyond Earth? Can we use a molecule's complexity to distinguish life from non-life?

A common way to search for extraterrestrial life is to look for signs of complex organic molecules on other moons and planets. One trouble with this approach, though, is that lots of complex molecules can arise from inorganic processes. To be sure that complexity indicates life, we also need to distinguish forms of complexity that could only be produced by information-rich processes – things that must be alive.

On today’s show, we talk with astrobiologist Sara Walker about this idea in relation to a new theory, called assembly theory, that she and colleagues are currently developing. Assembly theory characterizes the complexity of objects, including molecules, by how many steps are required to make them – the more steps, the higher the object’s complexity index. This perspective reorients our attention from the traits of objects that make them complex to the historical sequence of events that must have occurred to create them. Sara proposes that this idea provides natural ways to think about a large set of interesting processes, including how information is manifest and used in living systems, the creative roles of natural selection in evolution, and the ever present problem of understanding levels of selection.

This was Sara’s second appearance on the show, check out her first episode here.

Cover art by Keating Shahmehri