Review | Contrasting theories of life: Historical context, current theories. In search of an ideal theory

Throughout the last century, multiple models have been developed to explain life. Varela-Maturana’s autopoiesis, Dyson-Kauffman’s autocatalytic sets, Ganti’s chemoton, von Neumann’s universal constructor, Eigen’s hypercycle, Ashby’s homeostat, Prigogine’s dissipative structures, Rosen’s (M, R)-systems, all of them created for the same purpose: to characterize what seems indefinable. The phenomenon of life is one of the most fascinating to me. That is why, over the last ten years I have dedicated myself to disseminating many of the theories mentioned above. To my surprise, I was able to find multiple similarities among these diverse ways of portraying biological entities.

Many of the seminal papers containing such theories are contemporaneous with each other. For example, in 1958 Robert Rosen introduced what is now the canonical model of relational biology: (M,R)-systems. The idea is very simple, a living organism is such that all enzymes, all catalytic processes used within that system, are synthesized by the organization itself, without recourse to any external agent. This property is called closure to efficient causation. Similarly, in 1972 Humberto Maturana and his student Francisto Varela, published what many consider the culmination of the Kantian (and more recently “Kauffmian”) idea that the parts of a living organism reciprocally produce one another. Autopoiesis, a word derived from the Greek αὐτo- (auto-) ‘self’ and ποίησις (poiesis) ‘creation, production’, is in the authors’ words:

“a machine organized (defined as a unity) as a network of processes of production (transformation and destruction) of components which: (i) through their interactions and transformations continuously regenerate and realize the network of processes (relations) that produced them; and (ii) constitute it (the machine) as a concrete unity in space in which they (the components) exist by specifying the topological domain of its realization as such a network.”

Poetic, huh? But wait a minute. Isn’t that exactly what Robert Rosen referred to? The differentiating factor between Rosen’s (M, R)-systems and Varela-Maturana’s autopoiesis is that Rosen did not include (ii) as a necessary condition for life. All autopoietic systems are (M, R)-systems, but not all (M, R)-systems are autopoietic. Although both concepts are very similar, Maturana-Varela’s book makes no reference to Robert Rosen’s work. There is a legend, taken from the paper that I will review today, which says that “Rosen and Varela once participated in the same meeting, but when they met they found that they had nothing to say to one another. In Rosen’s case it may be that he preferred to be self contained”. This last conjecture is not far-fetched at all, as it is well known that Rosen almost never had co-authors on his publications.

Contrasting theories of life: Historical context, current theories. In search of an ideal theory is a paper written by Athel Cornish-Bowden and María Luz Cárdenas, both prominent researchers in the field of biochemistry, particularly known for their work on metabolic control analysis. They often collaborate on publications exploring the fundamental aspects of life. In this lovely piece, they compare all of the major current theories of life, exploring the historical aspects behind their development. Following a dialectical narrative, at the end of the paper we are offered what could be considered the necessary ingredients for building a successful theory of life. But before we get to that point, let’s start from the beginning.

The enigma of life is nothing new. That is why in the first chapter the authors devote themselves to scrutinizing some of the many definitions of life that have been proposed throughout history. One thing I found very striking was that the chronological order used by the authors is non-linear. For their part, the writers follow a taxonomic and evolutionary approach to the concept of life, contrasting primitive notions that were formalized up to a couple of centuries later, and at the same time glimpsing the relationship between different definitions. As if that were not enough, they also analyze other interesting aspects, from outdated characterizations of life to whether it even makes sense to define life.

We thus arrive at the second part of this remarkable publication, which is fully devoted to examining the most important theories of life developed in the latter half of the last century. Each of these theoretical frameworks is placed in its historical context, allowing us to better appreciate the convergence of these ideas and their eventual success. At the same time, the authors highlight the disadvantages of each theory, leaving us to conclude that just as there are many similarities between the different models, there are also multiple conceptual gaps present in all of them.

This is how we reach the third part of this work, which I find the most interesting and engaging. First, the authors introduce a list of six properties that, according to the models described in the two previous chapters, characterize living beings. They also enumerate five features that any set of ideas claiming to be a theory of life must include. From there, the authors analyze the criteria each framework uses to define life. This analysis culminates in Table 5, which I believe is one of the most brilliant contributions of the work.

Table 5 from Contrasting theories of life: Historical context, current theories. In search of an ideal theory

Towards the end of the article, the fourth section is devoted to pointing out the biological omissions on what could be cataloged as the most relevant life theories of the last century. Here the authors’ message is very clear: all the theories, although with interesting ideas, omit metabolic regulation, which is the process that controls metabolic pathways to ensure that organisms can generate energy and maintain homeostasis. In addition to this property, the authors also explore other interesting points when developing a model that characterizes living beings, like controlled growth. I was particularly struck by the fact that they have brought the concept of causality to the table, showing that biological relativity and the absence of hierarchy in biological entities are quite related to each other.

At the end of their work the authors point to two directions possible to take: the first consists of what has become everyday scientific work, taking a theory and trying to extend it. However, the second path seeks to unify the different frameworks described in the paper as a single puzzle. Although the second way seems more rocky, for me it is encouraging to come across this magnificent perspective article, where we can appreciate the convergence of great minds who have dedicated their life to the study of theoretical biology. As there are so many similarities in those different frameworks for describing life and we have identified the current shortcomings, it no longer seems we are in a labyrinth. Who knows? Perhaps we are not so far from deciphering the mystery of life.