DOI
https://doi.org/10.1007/s00239-017-9824-6

Summary
Life is a perfect example of a complex system. And it is precisely the complexity that can be studied to understand both the origin of life starting from pre-biotic chemistry and to identify possible forms of life outside the planet. One of the main problems encountered in this type of research is to quantify complexity and define a common way to compare chemical and biological units on a complexity scale. This work proposes a mathematical description of molecular complexity which allows to quantitatively evaluate the complexity of chemical structures. Combining this assessment with the measurement of information complexity results in a two-dimensional complexity space that spans the entire spectrum, from molecules to organisms. Systems showing molecular and informational complexity (biogenic units) are generally associated with complex functional mechanisms both to generate and to reproduce. By calculating the complexity of the exemplified biogenic units (ribozymes, protein enzymes, multimeric protein complexes, and an entire virus particle) and estimating the complexities of prokaryotic and eukaryotic cells as well as multicellular organisms, one can identify evolutionary stages defined in the space of complexity.

Relevance for Complex Systems Knowledge
Quantification of the complexity of chemical and biological systems across the universe is a valuable tool for to identify and prove the existence of life. This is because any kind of life represent a dissipative system and require complexity to part from an equilibrium state. For this reason, complexity may be a reliable marker for life-like processes that forms the natural basis for maintenance of structure, replication, mutation, and selection. From one side chemical complexity is a necessary prerequisite for any form of life or life-like system, (as an example: phenomena as the genetic program, the compartmentalization, metabolism, the capability to regenerate and adapt, or the ability to evolve are intimately connected with chemical complexity). From the other the individuation of new systems characterized by the maintenance and perpetuation of complex chemistry can be indication of the presence of a living system (e.g. the individuation of extraterrestrial forms of life). Based on the proposed method different classes of compounds can be quantitatively correlated and compared on one universal based on their chemical complexity.

Point of Strength
This manuscript, for the first time, apply the concept of chemical complexity to large biomolecules demonstrating that this approach, in combination with the orthogonal measure of information complexity “can generate a universal complexity scale that allows correlating and quantitatively comparing prebiotic and life-like systems in the Universe.”
In order to combine the molecular with sequence information complexity the author introduced the concept of biogenetic unit that con be associated to individual molecules, multimeric complexes, single cells, organisms, or even societies. Smaller biogenic units may be nested in larger ones reflecting the entire continuous spectrum from prebiotic units to life.
The approach proposed by this work to compare the complexity of biogenic units is the first example of an approach that can be applied independently of a shared definition of "life"