Behaviour and the Origin of Organisms
In this paper, Egbert and collaborators describe a new type of behaviour, that they call “viability-based behaviours”, that would have allowed entities which preceded Darwinian evolution to increase the likelihood of their survival. These ante-organisms displaying that behaviour might have, according to the authors, played a key role not only in the origin of life, but also in the transition from evolving chemistry to structured organisms.
Egbert et al. begin their investigation by describing how origin of life research has, for decades, focused on understanding how “information molecules” have emerged and evolved to finally give rise to life as we know it today. They suggest that some abiotic systems—that they call ante-organisms—display organism-like behaviour and might have played a key role prior to Darwinian evolution (Figure 1).
Specifically, these ante-organisms would have regulated their own environmental conditions in a specific way. While we sometimes speak of early organisms as “modifying their environment”, the ante-organism’s behaviour would be generated solely as a response to its own viability.
This claim has profound implications: Egbert and collaborators first proceed to describe the various hypothetical environments where life could have emerged—deep sea vents, hot springs, extra-terrestrial locations, etc.—and then proceed to list the requirements imposed onto these environments:
- Support basic chemical evolution
- Favor the emergence of complexity (avoiding fast-replicating parasites, excessive mutation rates, etc.)
- Favor the emergence of functional complexity, absent in evolving chemistry but found in organisms
- Provide materials and energy
- Persist in time
Now, given the fact that ante-organisms would have behaved in a way as to support their own persistence, these constraints can visibly be relaxed. This means that the constraints imposed upon the emergence of life would have been less strict than previously thought.
One example the authors give to illustrate their point is the self-reinforcing “behaviour” of glacial ice: it has a high albedo, thus reflects light efficiently, which in return lowers the temperature and favors the persistence of ice. We however have to be somewhat cautious here about the terminology: the term “behaviour” is usually used when there’s agency (i.e., biological), whereas in this context we’re only describing physico-chemical interactions obviously. And it is this self-sustaining behaviour of the ante-organisms that might have helped in the transition from non-life to life.
An additional remark mentioned by Egbert and colleagues is that, in addition to favoring the origin of life per se, this sort of embryonic “metabolism” could have been a key element in the evolution towards organisms from chemistry. It remains unclear how organisms with fully structured metabolisms would have evolved from basic (evolving) chemistry, and these ante-organisms with chemistry-based analogs of metabolisms may have played a key role in the process.
The self-preserving behaviours of ante-organisms
Egbert and colleagues then proceed further by describing four examples of entities that could be designated as “ante-organisms” (Figure 2) that move/reconfigure themselves and the environment so as to increase the likelihood of their persistence.
A. Motile oil droplets. A reaction taking place at the interface between the droplet and the aqueous solution causes an (asymmetrical) change in surface tension, which in return causes material to move along the droplet’s interface to equilibrate the tension (Marangoni flow). When that flow is strong enough, it drives a convective flow that moves the droplet through the environment. As environmental factors modulate the efficacy of the Marangoni instability (pH, etc.) the droplet will thus “respond” (taxis) to these conditions sustaining the dissipative structure.
Here’s an example of what this can look like:
B. Ramified charge-transportation networks. These are tree- or wire-like structures, formed with collections of ball bearings are partially submerged in oil, that reorganize when high voltage gradients are applied to them. They move towards the direction that will increase the dissipation of energy, which is, Egbert and colleagues tell us, another example of self-preserving behaviour.
C. Bénard convection cells. Bénard cells are patterns of convective motion appearing in a fluid when there is a thermal differential. Since they also tend to move towards regions with greater thermal differential, we can classify them as displaying self-preserving behaviour as well.
D. Reaction-diffusion spots. One last example of ante-organism are dissipative structures that form in the 2D Gray-Scott reaction-diffusion computational model \((2V+U\to 3V\)). These structures will move in the direction of the gradient of \(U\) as spots grow more quickly in these regions. In other words, it “acts to satisfy its own needs”.
Importantly, all of these systems changes its surrounding environment (pH, concentration of \(U\), etc.) in order to respond to their “metabolic” process of self-preservation. This is the reason we can speak of them as “regulating the environment” rather than just simply “moving around”. In other words, they all share the same basic form: far-from-equilibrium dissipative structures whose “metabolism” is spatially distributed, and behave in response to system viability.
The benefits of viability-based behaviour
So why exactly are these “ante-organisms” that display “viability-based behaviour” useful in an origin-of-life context? Egbert and colleagues proceed to describe five evolutionary advantages characterizing them.
1. They adapt to environmental change, which improves robustness
The first one of these advantages is that viability-based behaviour can prolong survival. In the example of the oil droplet mentioned earlier, we saw that it moved towards regions that facilitated its motion-driving chemistry. Likewise, the reaction-diffusion spots can move away from toxins that inhibit their autocatalytic reactions. These are all examples of behaviour that boosts the dissipative structure’s metabolic processes and make them more likely to persist.
In the context of the origin of life, this also means that instead of searching for these “Goldilocks” environments that would perfectly bring together all the required conditions, we could very well be satisfied with environments that combine variations in pH, redox state, temperature, etc. as the ante-organisms could have had the ability to navigate such environments instead of being completely passive to them.
2. They adapt to internal changes, which increases robustness and evolvability
Somewhat a corollary of the previous affirmation, not only do these ante-organisms adapt to environmental change, but they also be shown adapt to internal changes. This means that if instead of the environment changing it’s their internal state that changes, the ante-organisms can behave in a way as to accommodate these changes as well.
3. They utilize multiple diverse environments
A third advantage of viability-based behaviour is the ability to exploit various environments dynamically. Let’s take the example of an autocatalytic system that uses two resources, \(R_1\) and \(R_2\), to fulfill its needs and persist. Let’s further imagine that these resources are spatially distributed: that would thus make it impossible for some entity to survive without motility. The ante-organisms, however, have that capacity to behave, to move in the environment. It is this ability that would therefore allow them to survive in environments to accommodate changing (internal) needs.
One interesting corollary of this idea is that the cycling phenomena that have been hypothesized to have played a role in the origin of life—for example, these “wet-dry cycles” (see e.g. (Damer & Deamer, 2020))—could be replaced by some species of ante-organisms that behave in such a way that their environment changes periodically. In this way, we have one less constraint to place on the environment.
4. They offer opportunities for greater organismic functional diversity
Another advantage that can be put forward by considering ante-organisms is the ability to facilitate the evolution of greater functional diversity. While in usual chemical replication systems, fitness is thought to be solely dependant on the rate of catalysis, thereby the underlying chemical architecture, in ante-organisms fitness could be thought to rely even more so on their capacity to behave as to sustain their persistance. Instead of the chemical structure being the only determinant of fitness and survival rates, ante-organisms’ fitness would likely depend on their ability to navigate/combine environments, compensate for transformations in their own metabolic operation, and so on. In return, ante-organisms that are better able to do these things would increase their fitness and survival rate, making it possible to evolve furthermore towards structures that perform even better at these tasks.
5. Their viability-based behaviour is simply implemented
One final advantage of viability-based behaviour, Egbert and colleagues tell us, is that it is simple to implement, and thus seems to be a common occurrence whenever a dissipative structure is placed in an environment contributing asymmetrically to its “metabolic” processes. There’s no need for sensors, or any complicated apparatus—the ante-organism only needs to respond to its internal needs.
To be clear: Egbert and colleagues aren’t suggesting that Bénard cells or charge-transportation networks played any role in the origin of life. What they’re describing is rather a shared architecture that characterizes several far-from-equilibrium structures, and allow them to respond dynamically to their environment. And it is this structure which could be at the origin of biological behaviour.
Moreover, the authors also mentioned that ante-organisms could be the missing piece in an explanation of how structured metabolisms evolved. Considering that a transition from “evolving chemistry” to proper metabolisms has yet to be explained, they suggest that the origin of life might have involved some type of abiological entity performing viability-based behaviours, whereby these ante-organisms became more evolvable over time.
That being said, even if it turns out that the evolution of metabolisms took another path, the existence of ante-organisms is a promising hypothesis put forth by Egbert and colleagues as to possible precursor entities of biological evolution.