Abstract
An enduring mystery in biology is how a physical entity simple enough to have arisen spontaneously could have evolved into the complex life seen on Earth today. Cairns-Smith has proposed that life might have originated in clays which stored genomes consisting of an arrangement of crystal monomers that was replicated during growth. While a clay genome is simple enough to have conceivably arisen spontaneously, it is not obvious how it might have produced more complex forms as a result of evolution. Here, we examine this possibility in the tile assembly model, a generalized model of crystal growth that has been used to study the self-assembly of DNA tiles. We describe hypothetical crystals for which evolution of complex crystal sequences is driven by the scarceness of resources required for growth. We show how, under certain circumstances, crystal growth that performs computation can predict which resources are abundant. In such cases, crystals executing programs that make these predictions most accurately will grow fastest. Since crystals can perform universal computation, the complexity of computation that can be used to optimize growth is unbounded. To the extent that lessons derived from the tile assembly model might be applicable to mineral crystals, our results suggest that resource scarcity could conceivably have provided the evolutionary pressures necessary to produce complex clay genomes that sense and respond to changes in their environment.
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Notes
In growth that proceeds downward, rather than upward, the tiles labelled resource tiles function as measurement tiles, and vice versa. Thus, both assemblies that can predict the resource tile types that are available from the measurement tiles, and those that can predict the measurement tile types that will be available from the current concentration of resource tiles will be selected for. Note, however, that gate types may be non-deterministic during downward growth, which could result in crystal growth stalling when a tile is incorporated that creates a binding site that matches no gate tile’s outputs. Therefore, consideration of downward growth rates is necessary for a complete evaluation of a crystal’s fitness; but we neglect it here to simplify the presentation.
This estimate assumes that since Δt ≫ s, at any particular time the resource and measurement tile concentrations are uncorrelated. So half the time, the resource and measurement tiles are compatible, and the crystal grows at a rate of D + 1 seconds per zig-zag; and half the time the resource and measurement tiles are not compatible, and the crystal essentially doesn’t grow.
Note that counters whose natural period is slightly less than the period of the environment will easily remain synchronized with the environment, even when there is variation in the arrival times of the tiles being added.
While zig-zag assemblies copy layers on both growth fronts, for the purposes of our argument, we will assume that computation on zig-zag assemblies proceeds in only one direction (upward). While this is not necessarily so for the tile sets we describe, growth can be restricted to one direction by using a transformed tile set (Winfree 2006) that uses extra tiles to prevent growth in the wrong direction.
In cases where a complete shape is necessary for selective advantage, it is also possible to use a construction where crystals can grow forward as well as backward, so that the parent crystal can grow back the piece of the shape that was lost during splitting (Winfree 2006).
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Acknowledgments
We thank Andrew Turberfield, Paul Rothemund, Robert Barish, Ho-Lin Chen, and Ashish Goel for insightful conversations and suggestions. This work was supported by NASA Grant No. NNG06GA50G.
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Schulman, R., Winfree, E. How crystals that sense and respond to their environments could evolve. Nat Comput 7, 219–237 (2008). https://doi.org/10.1007/s11047-007-9046-8
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DOI: https://doi.org/10.1007/s11047-007-9046-8