Catalyzed Oscillations and Droplet Surfaces

Droplet surfaces as an uncontrolled, bounded environment for evolution. Amphiphiles as emulsifiers are chemicals that create micelle droplets. Evolution of amphiphilic oscillations can involve protoenzymes as amphiphilic catalysts because they are self-bounding. Evolution will act on droplets to adaptively design vesicles. Evolution will apply at the level droplets and vesicles as much as at that of oscillations. The resulting protocells will have properties comparable with those of life.

The Origin of Membranes

Catalysts, Enzymes and the Evolution of Membranes

6.1     Catalysts and Droplet Surfaces
6.2     Protocells
Copyright Statement

6.1 Catalysts and Droplet Surfaces

As we mentioned, first prebiotic oscillations were bounded only in "chemical space" but once catalysts became involved, such bounding became inadequate. Catalyzed oscillations could not be selected unless catalysts and reagents were prevented from diffusing apart from one another and this required bounding in normal space. However, although lipid vesicles bound modern cells, forming such vesicles is a controlled process. It is unlikely that the inside of such vesicles was the first spatially bounded environment for the evolution of oscillations. The uncontrolled environment that could have kept chemicals and catalysts together would have been the droplets or micelles found in oil/water emulsions. Moreover, we will argue, it would not have been the body of such droplets but their surface that would have been that first environment.

6.1.1    The Chemical Components of Early Catalyzed Oscillations were Emulsifiers

Bioepistemic evolution requires that evolving systems be not merely bounded, but that they be self-bounded. That is to say, an evolving system must interpret its data in such a way as to create the boundary within which its data processing occurs. If droplets or micelles are to provide that bounding and the oscillation is to be self-bounding within that droplet, then the chemistry of the catalyzed oscillation must be such as to tend to create small oil droplets or micelles. The implication that follows from these requirements is that the first catalyzed, prebiotic oscillations involved chemical components that were emulsifiers and, therefore, were amphiphiles. The chemicals involved in this oscillation would, therefore, not have been distributed around the volume of an oil droplet but around its surface.

This is an important conclusion and deserves reiteration and further development

1. Some early prebiotic oscillations involved chemicals that are, themselves, emulsifiers. These were protolipids and would have partitioned into the surface layer of any available oily droplets.
2. The surface of such a droplet is a bounded area in which the oscillation and its evolution can continue.
3. Since the protolipid is an emulsifier it tends to "creates" the droplet, giving the protolipid oscillation a self-bounding characteristic and meeting this requirement of bioepistemic evolution.
4. The inchoate catalysts, protoenzymes, were one or more of the components of another oscillation. They too were amphipaths that partitioned into the surface layer of oily droplets and they catalyzed the oscillation that produced the protolipid. They would not have needed to be present in large amount or to have been strong emulsifiers to enjoy evolutionary benefit from the bounded environment so produced.
5. The two oscillations would have coevolved synergistically, the catalyst aiding the selection of the protolipid by speeding up its equilibrium while the protolipid produced bounding for the protoenzyme as well as the protolipid.
6. Since the protoenzyme and protolipid are concentrated in the surface layer, they will tend to be more concentrated in that layer. Thus, those potential poisons that are not amphiphilic will have a reduced effect. Moreover, selection will not favour such poisons and their concentration will not rise as would that of the selected protoenzyme.

In this way, the joint evolution of two oscillators, one with emulsifier components and one with catalytic components, bounded by the surface of a droplet, would have created the first example of linked evolution. Oil droplets become host to two oscillations evolving in a state of synergistic, mutual dependence and competition but also merging into a single, unified evolving system. The process would be similar to that described by Pohorille et al. (1996). The two oscillators come to share a relationship not dissimilar to that between separate genes in organisms. These oscillations occur on the outer surface of oil droplets and are exposed to the environment; they remain subject to many physical and chemical processes and can easily accrete or lose material. The droplets within which oscillations occur will acquire something resembling a phenotype, a set of properties reflecting the chemical nature of the components of the oscillation occurring on their surface. Sometimes oil droplets will meet and fuse together and, on other occasions, physical events will cause the larger droplets to break apart. Such events will add to the selection forces acting on the oscillations and droplets will be subject to their own selection pressures. Some optimal mixture of catalytic oscillator and emulsifier oscillator can be expected to emerge as selection begins to operate at the level of a droplet rather than at that of an oscillation.

6.1.2    The Evolution of Oscillating Pathways

The bounded environment, when coupled to repeated fusion and fission event on droplets, creates avenues for the emergence of catalyzed oscillatory pathways. This could occur simply by the merger of two preexisting, catalyzed oscillations. So for example, the two oscillations Ax Û Ix and Ix Û  Bx could merge into a single, pathway Ax Û Ix Û Bx by a simple fusion of two droplets that separately contained the necessary, catalytic components. More extended pathways could be assembled just as easily. These processes of division and fusion of droplets would greatly increase the range of easily attainable variation and one can imagine a range of activities assembling into larger scale versions of the synergistic relationship alluded to earlier. Even so, all these oscillations would involve only standard chemistry, would draw energy from the sun and would use its daily rhythm as a source of data as well as energy.

6.2 Protocells

As discussed earlier, the appearance of catalyzed oscillatory pathways creates the possibility of self-oscillating reactions that will, eventually, be selected in preference to passively driven oscillations. It is at this point that the system does it own data processing and where controlled chemical reactions come to dominate. It also leads to understand why vesicles would have been selected in preference to droplets as the bounding environment for oscillations.

6.2.1    Self-Oscillatory Chemistry in Emulsions

If, each day, a wave of chemical oscillation passes down a droplet, then it is very likely that an induced, physical wave will be created at the same time. The peristalsis produced by such a wave is likely to cause the droplet to move away from the heat source. Thus, such droplets might be expected to achieve some small movement and to move so as begin to populate regions where both the ambient temperature and the amplitude of the temperature oscillations due to the day night rhythm will decline. However, self-oscillatory chemical reactions will still be possible even at a constant temperature, provided that inputs of the necessary chemical fuel are available in the external environment. Under these conditions, it is the self-oscillatory reaction that provides the replicating data source and the period of the oscillations need no longer be one day. Chemical oscillations may have other periods. Also, the opportunity would also arise for some pathways to selectively adapt into metabolic pathways and deliver a regular supply of fuel to the self-oscillating reactions.

In such ways, oscillation driven shape changes may cause droplets to divide into two or, on other occasions, may cause droplet fusion, further reinforcing the position of the droplet as units of evolutionary selection. With selection operating on the droplet, rather than the oscillation or the pathway, one has an early kind of group selection in which selection operates on the holistic properties of that group of oscillations and pathways that happen to be present within a droplet. Evolution will thus cause selection of successful droplets and populations of droplets will emerge whose fitness and reproductive success will depend upon niches of feedstocks and environment. Thus different "species" of droplets will emerge in response to the different selection pressures found in various niches.

6.2.2    The Origin of Vesicles

As mentioned before, all self-oscillating chemical reactions require inputs of chemical energy to fuel the chemical oscillation. Without that fuel, the self-oscillation will cease though, no doubt, the first self-oscillating reactions could also passively oscillate if they ran out of fuel. The fuel itself would have been hydrophilic, since the outer surface of the oil droplet would need to be bathed in it for the self-oscillating reaction to continue.

The need for such a fuel supply suggests a selective pressure that would lead to vesicle formation, based on the need to capture a supply of fuel to maintain the oscillation. Some of the amphiphilic emulsifiers produced by the selection of droplets, described above, would have possessed the properties needed to form closed vesicular bilayers in the absence of oil droplets. This propensity would have given these particular amphiphiles a selective advantage because, if they formed the closed vesicle in the presence of hydrophilic fuel supply, some of that fuel would become trapped on the inside of the vesicle. This trapped fuel supply would then have become available only to the oscillatory reactions contained in that vesicle. This fuel reserve could have sustained the oscillation during lean times.

Eventually, with enzymatic assistance, amphiphiles would become quite abundant and those that were tended to form their own bilayer would not need to find an oil droplet. This selective advantage would be further enhanced because vesicular bilayers would tend to enclosing a small volume of the medium. This would effectively store a small amount of chemical energy supply needed for self-oscillating reactions, it would be a store of fuel. So long as the fuel lasted or could be replenished, its self-oscillating reactions could continue. This would be the beginnings of vesicles feeding. They would be protocells, primitive life forms.

6.2.3    Properties of Vesicles and Properties of Life

In summary, the kinds of phenotype that can be conceived seem interesting, capable of further development and reminiscent of the properties of life. Here are some phenotypes we have suggested might emerge.

Properties of Life	Properties of Chemical Oscillations in Vesicles
Move	Exhibit surface waves that could cause movement
Breathe
Feed	Self-oscillating reactions need chemical energy inputs which produce waste products – excrete.
Grow	Oscillations
Excrete	As above
Reproduce	Divide and undergo fusion
Respond to stimulus	Movement toward lower temperature
	Differentiate into populations occupying different niches

This list in the left hand column gives the characteristics of life, as described in old school text books in biology. The list on the right gives the properties of vesicles containing self-oscillating chemical reactions. The two lists bear comparison.

This discussion has gone far enough to make a case for "move, feed, grow, reproduce and respond to stimulus," while "excrete" seems to follow. "Breathe" would be irrelevant during prebiosis as it occurred prior to the emergence of an oxygenated atmosphere. These speculations have moved us in a direction that bears comparison with life and their direction is clear but these suggestions are not intended to be final. One could devise variations on these themes and argue a case for each of them, the best choice being a matter of debate or even personal preference.

It will, therefore, be best to stop the speculation at this point and deliver the inevitable but necessary caveat. All these thoughts are speculations; they are not facts. There seems nothing wrong with the chemistry but their evidential basis as a mechanism of prebiosis is either very limited or non-existent. In some areas, these ideas do suggest avenues of experimentation and investigation that would be worth pursuing. In such areas, some observational evidence could be sought and, hopefully, investigators will follow up some of those possibilities.

The aim here is simply to construct a sequence of stages for the origin of life that avoids invoking intrinsically improbable events. The sequence shown is intended to show that chemistry and physics, combined with a high powered data input and parsimonious mechanisms can lead to evolutionary selection and variation that will progress toward lifelike entities of increasing complexity.

Copyright Statement

© John A Hewitt MA PhD (Cantab.)
The work described here was performed as an independent investigation by John A Hewitt who asserts the right to be recognized as its author and as the originator of the novel ideas presented here. The topics to which this claim applies include, but are not limited to, the application of bioepistemic evolution to the prebiotic situation, the discussion of the sun as a data and power source for prebiotic evolving systems, the recognition of sun-induced chemical oscillations as information carriers subject to evolutionary selection and to the theories for the origin of biochemical pathways and self-oscillatory, allosteric and cyclic biochemistry that result.
This study is a greatly extended version of a poster originally presented at the Royal Society meeting on conditions for the emergence of life on the early earth, London, 13 & 14 February, 2006. This internet version was made available on 6 September, 2006. Comments and criticism are solicited - see the "contact & copyright" link for contact details.