Science Literature

It has often been noted that Darwinian mechanisms could not operate prior to the first presumed replicating cell. Without replication, there can be no selection - or can there? This is the conundrum addressed by Nowak and Ohtsuki in their attempt to "formulate a general mathematical theory for the origin of evolution". These authors recognise that the origin of the first replicating cell must have had a sophisticated precursor, because otherwise the jump in complexity would have been too great.

"We have proposed a mathematical theory for studying the origin of evolution. Our aim was to formulate the simplest possible population dynamics that can produce information and complexity. We began with a "binary soup" where activated monomers form random polymers (binary strings) of any length. Selection emerges in prelife, if some sequences grow faster than others. Replication marks the transition from prelife to life, from prevolution to evolution. Prelife allows a continuous origin of life."

A prebiotic chemistry is proposed with activated monomers denoted by 0 or 1 (Source here)

As in all simulations, the model has to involve simplifications. First, the authors propose a primordial soup with monomers as active ingredients. These entities have the potential for generating "unlimited information".

"Let us consider a prebiotic chemistry that produces activated monomers denoted by 0* and 1*. These chemicals can either become deactivated into 0 and 1 or attach to the end of binary strings. We assume, for simplicity, that all sequences grow in one direction. [. . .] Each sequence is produced by a particular lineage that contains all of its precursors. In this way, we can define a prebiotic chemistry that can produce any binary string and thereby generate, in principle, unlimited information and diversity. We call such a system prelife and the associated dynamics prevolution."

It is significant that the authors draw attention to "unlimited information" potential, rather than random sequence generation. But biological information requires much more than a specified bit string! Numerous people have wrestled with the thought that the genetic code needs to be translated if it is to have any biological function. Base sequences are no different from junk if there is no machinery to turn the code into something biologically meaningful - and all the machinery that we are aware of is extraordinarily complex. For more on this, go here. Even allowing that sequences could have emerged containing information in code form, we have not progressed conceptually beyond the typing monkey scenario.

The bit sequences are considered to be formed at a quantifiable speed, and it is also premised that the tempo of decay is quantifiable. "Selection emerges in prelife, if different reactions occur at different rates." In this model, the concept of selection is based on the longevity of sequences that spontaneously form in the soup. This use of the word "selection" is confusing, because there is no sense in which one sequence competes against other sequences and demonstrates fitness in the primordial ecology. The authors claim that the information carriers "compete for resources" but this is not convincing. Resources are relevant to the growth of the sequences, but not to the time before they decay. There is nothing about the longer-lasting sequences that can be related to environmental selection.

"Traditionally, one thinks of natural selection as choosing between different replicators. Natural selection arises if one type reproduces faster than another type, thereby changing the relative abundances of these two types in the population. Natural selection can lead to competitive exclusion or coexistence. In the present theory, however, we encounter natural selection before replication. Different information carriers compete for resources and thereby gain different abundances in the population. Natural selection occurs within prelife and between life and prelife. In our theory, natural selection is not a consequence of replication, but instead natural selection leads to replication. There is "selection for replication" if replicating sequences have a higher abundance than nonreplicating sequences of similar length. We observe that prelife selection is blunt: Typically small differences in growth rates result in small differences in abundance."

There are many more issues of simplification that could be explored. However, enough has been said above to justify the conclusion that this paper does not deserve any more attention until the model is validated. This the authors have avoided doing. They make no attempt to justify their assumptions by reference to current thinking about abiogenesis. They say nothing about the importance of testing the model and validating the findings. This is a great weakness of many research programmes based on simulation, where the authors live in virtual reality and think that because they can model it, there is some substance in their work. One could say exactly the same about simulations of evolution: they may have a use in getting across theoretical concepts but they have no established link with the real world.

The media reporting of this paper suggests hype is in the air. Here is the subtitle from Scientific American: "How did self-replicating molecules come to dominate the early Earth? Using the mathematics of evolutionary dynamics, Martin A. Nowak can explain the change from no life to life". The reader is being invited to conclude that Nowak has answers supported by maths, not that he has an entirely tentative hypothetical model, full of unsubstantialted assumptions. Another example comes from the pen of Roger Highfield. He emphasises the suggestions emerging from the research, as though it provides a foundation on which to build. It would be more realistic to portray the proposed scenario as virtual reality, urgently needing a heavy dose of scientific realism:

"An analysis suggests that the soup of chemicals on the early Earth was naturally evolving towards creating the first life, a discovery that suggests alien life should be common. A mathematical analysis of how simple chemicals crossed the threshold between dead and living suggests that natural selection that gave us the vast diversity of life on the planet, from bacteria to tigers, was at work in the primordial Earth too."

According to Nowak: "Mathematics is the proper language of evolution. I don't know what the 'ultimate understanding' of biology will look like, but one thing is clear: it's all about getting the equations right." Getting the equations right is exactly what this blog is about. The scientific approach is to model real-world systems and test the ability of the model to replicate reality. Nowak's danger is giving the impression he has made a contribution to scientific knowledge without making any attempt to validate his model.

Prevolutionary dynamics and the origin of evolution
Martin A. Nowak and Hisashi Ohtsuki
Proceedings of the National Academy of Sciences, Published online before print September 12, 2008, doi: 10.1073/pnas.0806714105

Abstract: Life is that which replicates and evolves. The origin of life is also the origin of evolution. A fundamental question is when do chemical kinetics become evolutionary dynamics? Here, we formulate a general mathematical theory for the origin of evolution. All known life on earth is based on biological polymers, which act as information carriers and catalysts. Therefore, any theory for the origin of life must address the emergence of such a system. We describe prelife as an alphabet of active monomers that form random polymers. Prelife is a generative system that can produce information. Prevolutionary dynamics have selection and mutation, but no replication. Life marches in with the ability of replication: Polymers act as templates for their own reproduction. Prelife is a scaffold that builds life. Yet, there is competition between life and prelife. There is a phase transition: If the effective replication rate exceeds a critical value, then life outcompetes prelife. Replication is not a prerequisite for selection, but instead, there can be selection for replication. Mutation leads to an error threshold between life and prelife.

See also:

Coppedge, D.F. The Prevolution of Evolution: Life Marches In, Creation-Evolution Headlines, 09/17/2008

Highfield, R. Soup of chemicals on primordial Earth was naturally evolving toward life, The Daily Telegraph, 17/09/2008

Wax, H. Using Math to Explain How Life on Earth Began, Scientific American, October 2008.