Tierra is a class of model system invented by Tom Ray specifically to investigate evolution in an abstract framework (Ray, 1992). The inspiration or metaphor for the system is that of the Cambrian explosion. That is, the primitive entities are intended to be analogous to biological (rather than molecular) species; and the dynamics are supposedly ecological and evolutionary rather than chemical. However, I shall suggest that a chemical metaphor may be at least equally applicable, and perhaps illuminating.
In practice, Tierra can viewed as a development of an earlier line of systems, including Core Wars (Dewdney, 1987) and Coreworld (Rasmussen et al., 1990), with, indeed, some strong similarities to the -universes already discussed. These all involve some sort of one dimensional or linear spatial system, similar to a conventional computer memory (the ``core'') in which are embedded patterns that can effectively act as information processors or (concurrent) computer programs. More technically, each concurrently executing program is termed a separate process. Given this general structure, one can immediately envisage circumstances in which processes can interact, can create new processes, and can be destroyed. On a chemical metaphor this means taking individual processes as molecules (with their programs defining their molecular species), and their interactions as chemical reactions. The conventional possibilities already raised here for self sustaining systems thus arise afresh in this context.
Ray's experiments can be briefly summarized--in the chemical metaphor--as follows. It is possible, in the basic Tierra system, to construct individually autocatalytic molecules. These are moderately complex, and not capable of spontaneous emergence. However, if the system is manually seeded with such a molecule, it reacts rapidly to fill the reactor. Experiments are conducted under flow conditions (there is an outflow of molecules and an inflow of the ``food set''--unallocated core memory in this case). A single individually autocatalytic species can persist in the reactor for an extended period in these circumstances. However, over time, more ``efficient'' individually autocatalytic species tend to successively emerge and displace each other. Efficiency here essentially means reaction speed. More complex dynamics then ensue. ``Parasites'' emerge, which are not individually autocatalytic in isolation but have an autocatalytic pathway which can be catalysed by fully autocatalytic ``host'' species, which are already present. These parasites and their hosts thus form--in my terms--a somewhat degenerate kind of collectively autocatalytic set; degenerate because some of the species are still individually autocatalytic. At later stages Ray reports the emergence of what he terms ``sociality'', meaning entities which ``can only replicate when they occur in aggregations''. In my terms, it seems that these should be regarded as ``properly'' collectively autocatalytic, in that there are no longer any individually autocatalytic components.
In these terms, Tierra can be regarded as again independently corroborating Kauffman's results on the likely spontaneous emergence of collectively autocatalytic reaction networks--though with the addition of some significant and interesting results on the subsequent development and even elaboration of such networks. Ray himself does not make this explicit connection, but that is presumably due, in part at least, to his perspective of an ecological/evolutionary rather than molecular metaphor.
However: again, as with the previous systems, Tierra seems to lack any mechanism for spatial localization or containment of collectively autocatalytic reaction networks. Note carefully that this is true despite the fact that Ray variously refers to ``organisms'', ``creatures'' and ``individuals'' in the system; but what he means in each case is a single Tierra process--i.e., the entity which, in my terms, is analogous to a single molecule rather than a self sustaining system. In particular, although these primitive entities each have a specific, bounded, spatial extent in the Tierra core, there is no sense in which they produce or maintain this spatial boundary though their own operations. On the contrary, Tierra enforces a notion of ``memory allocation'' whereby some outside agency allocates or reserves spatial segments of core to individual processes, and this cannot be subverted by other processes. If we consider the application of my heuristic test for autopoiesis it again seems clear that multiple instances of the same reaction network, inserted into the same Tierra reactor, would not be able to maintain their separation; and thus, Tierra is not capable of supporting fully autopoietic organization.
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