Identifying molecules as biosignatures with assembly theory and mass spectrometry

Abstract

The search for alien life is hard because we do not know what signatures are unique to life. We show why complex molecules found in high abundance are universal biosignatures and demonstrate the first intrinsic experimentally tractable measure of molecular complexity, called the molecular assembly index (MA). To do this we calculate the complexity of several million molecules and validate that their complexity can be experimentally determined by mass spectrometry. This approach allows us to identify molecular biosignatures from a set of diverse samples from around the world, outer space, and the laboratory, demonstrating it is possible to build a life detection experiment based on MA that could be deployed to extraterrestrial locations, and used as a complexity scale to quantify constraints needed to direct prebiotically plausible processes in the laboratory. Such an approach is vital for finding life elsewhere in the universe or creating de-novo life in the lab.

A Drive towards Thermodynamic Efficiency for Dissipative Structures in Chemical Reaction Networks

“…Given the large and high-dimensional, yet highly structured (in the sense that not every chemical reaction is allowed) nature of chemical space, leading to intriguing—and only partly understood—structures and dynamics in the most complex chemical reaction networks we know, namely those in biochemistry (e.g., [ 17 , 56 , 57 ]), it seems reasonable to assume that similar dissipative hierarchies and associated pressures towards thermodynamic efficiency might have emerged in biological systems. This may be the case, as existing biogeochemical networks feature a universal scaling law, which is not a product of the underlying chemical space alone [ 56 ].…”

The Grayness of the Origin of Life

“…It should be noted that metrics that assign a continuum of values to characteristics of life inherently deal with grayness. For example, measures of how assembled a molecule is or of biological autonomy, individuality, and agency have all been developed recently and are relevant for assessing the degree to which a system possesses a particular biological property [53][54][55]132,133]. By considering multiple independent biosignature metrics, it is possible to create a theoretical framework in which the probability of a sample being of biological origin can be assessed.…”

“…Randomly formed strings, while having high entropic information, are unlikely to contain evolutionarily useful information. Therefore, the ability to store useful information is determined by the length of the encoding string coupled with maintenance of non-random sequences selected by evolution over time (e.g., see [53,54] for a recent discussion of related ideas). This shift from low useful information content to high is a grayness at the origin of life.…”

Planetary Mass Spectrometry for Agnostic Life Detection in the Solar System

“…"Molecular assembly (MA)" index which is computationally derived to express the number of types of operations needed to generate a compound, is a novel mathematical method that can agnostically determine whether a certain compound requires information processing systems (and hence life) to produce. In contrast to previous approaches, the MA index has provided the first intrinsic experimentally tractable measure of chemical complexity using MS (Marshall et al, 2021) (Figure 3A).…”

“…It is these principles that could be applied to the search for non-Terran biochemical systems using MS approaches. Mass spectrometers that reveal molecular complexity ("molecular assembly" index) (Marshall et al, 2021), chemical structures, and patterns will play an important role in the future of agnostic biosignatures detection on other worlds.…”