American Scientist: The Origin of Life

In the May-June 2009 issue of American Scientist, is a featured article entitled “Origin of Life”  [1]. In summary:

The authors investigate the metabolic evidence, reaction thermodynamics and chemical logic of the primordial autocatalytic cycle and conclude not only that it can explain the origin of life, but also that the appearance of life was very likely an inevitable side effect of the laws of thermodynamics.

 

The authors offer the following analogy of their proposal [bold added]:

Consider the requirements of the U.S. Interstate highway system. The system includes an enormously complex network of roads; major infrastructure devoted to extracting oil from the Earth, refining oil into gasoline and distributing gasoline along the highways, a major industry devoted to producing automobiles; and so on. If we wanted to explain this system in all of its complexity, we would not ask whether cars led to roads or roads led to cars, nor would we suspect that the entire system had been created from scratch as a giant public works project. It would be more productive to consider the state of transport in preindustrial America and ask how the primitive foot trails that must certainly have existed had developed into wagon roads, then paved roads and so on. By following this evolutionary line of argument, we would eventually account for the present system in all its complexity without needing recourse to highly improbable chance events.

In the same way, we argue, the current complexity of life should be understood as the result of a multistep process, beginning with the catalytic chemistry of small molecules acting in simple networks—networks still preserved in the depths of metabolism—elaborating these reaction sequences through processes of simple chemical selection, and only later taking on the aspects of cellularization and organismal individuality that make possible the Darwinian selection that biologists see today. Our task as origin-of-life researchers is to look at the modern highways and see what they reveal about the original foot trails.

 

The theme here is that the relevant pre-life chemistry necessary for creating life (“the foot trails”) was, comparatively speaking, quite simple. The authors then go on to argue that the origin-of-life process was further facilitated by the necessities of thermodynamics, which favored the formation of particular chemical cycles:

[W]e consider a primordial reductive citric acid cycle the most likely route from geochemistry to life—the rivulet that formed at the top of the energy hill, through which the pond of energy began its thermodynamic escape. We then ask how, from this simple beginning, could the complexity we see in the modern cell arise.

 

Given that their discussion is of pre-cellular chemistry, so that the discussion centers on subsets of chemical flows freely embedded in some larger context of sinks and sources, it seems somewhat problematic to invoke thermodynamics for these inherently open subsystems; however, set that aside. What seems more interesting to me is that if all the origin-of-life problem required was some comparatively simple chemistry, and a thermodynamically favorable environment, then….why have we not yet created life in the lab? Certainly, we ought to be able to construct the appropriate vessels, “soups”, and thermodynamic conditions to facilitate the formation of life; indeed, as the authors state, life should be “inevitable”.

One could perhaps appeal to some statistical argument about the frequency of such creation, or to some pesky technical issue, as to why this has not yet been done. However, this strikes me as somewhat weak; it would seem reasonable to expect that there could be produced at least a compelling proto-life example, based on these premises. But where is it?

Another possibility is that the authors’ premises are wrong to begin with, and that the origin-of-life process did not go from simple chemistry to complex to cellularization. Indeed, there is no a priori reason to expect that the origin-of-life problem is solvable specifically by reductionist means. As such, life could indeed still be “inevitable” — it may simply be that the conditions for its arising required a much more complex set of chemical conditions and environmental forcings than typically envisaged. This would not mean that the origin-of-life problem is inscrutable or unsolvable, far from it; however,  it would mean that the problem is perhaps much more tightly coupled to the organizational complexes of pre-biotic chemistry and conditions than to the particular chemical cycles which became codified in the organisms themselves. By this account, the details of the codified chemical cycles are best thought of as residue; it is the organization of the system which is key.

In terms of the author’s analogy of highways and foot trails: the task would not be to look at the modern highways and see what they reveal about the original foot trails, but instead to look at the modern highways and see what they reveal about the nexus of conditions that caused the particular organization of roadways which resulted.

 

References

[1] Trefil, J., Morowitz, H., Smith, E. “Origin of Life“. American Scientist. 97(3):206. DOI:10.1511/2009.78.206.

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3 Responses to American Scientist: The Origin of Life

  1. Tim,

    Your comments are in-line with the premise of the book Into the Cool by Eric Schneider and Dorion Sagan.

    They claim that life is a solution to nature’s tendency to want to dissipate gradients.

    It could be the case that the spontaneous organization resulting the the manifestation we recognize as life happens on a time-scale too slow for us to notice. It could be that our attention span has thus far been too short to notice any such organization taking place in the lab. Perhaps by recreating the appropriate “nexus of conditions” (gradients and substances that could be organized), any such spontaneous organization could be sped up considerably. There is no guarantee, though, that nature will cooperate in a timeframe convenient to the lifetime of the experimentalist.

    Jeff

  2. Tim Gwinn says:

    Hi Jeff,

    I’m a little surprised if my comments are in line with that book, but I have not read it. As I stated, “it seems somewhat problematic to invoke thermodynamics for these inherently open subsystems”. That is, contrary to the article’s authors, I do not think that thermodynamics or gradients – though certainly present – are a primary causal force the relevant properties in the origin-of-life problem. I tend to agree with Rosen that “the problems associated with open systems are dynamical problems and not thermodynamic ones” [Essays p. 252].

  3. Tim Gwinn says:

    To try to clarify my previous comment, there are two points I tried to make:
    1) I am dubious that thermodynamics is the appropriate paradigm for understanding and modelling these kinds of open systems.
    2) The larger issue is that focusing on the thermodynamics is like focusing on the details of the chemistry: it can lead one to miss the organization – the relational properties – which is separate from both of those details.