Marcel, a mechanical chessplayer... his exquisite 19th-century brainwork --- the human art it took to build which has been flat lost, lost as the dodo bird ... But where inside Marcel is the midget Grandmaster, the little Johann Allgeier? where's the pantograph, and the magnets? Nowhere. Marcel really is a mechanical chessplayer. No fakery inside to give him any touch of humanity at all.
--- Thomas Pynchon, Gravity's Rainbow .
Ideally, the science of biology should embrace all forms of life. However in practice, it has been restricted to the study of a single instance of life, life on earth. Life on earth is very diverse, but it is presumably all part of a single phylogeny. Because biology is based on a sample size of one, we can not know what features of life are peculiar to earth, and what features are general, characteristic of all life. A truly comparative natural biology would require inter-planetary travel, which is light years away. The ideal experimental evolutionary biology would involve creation of multiple planetary systems, some essentially identical, others varying by a parameter of interest, and observing them for billions of years.
A practical alternative to an inter-planetary or mythical biology is to create synthetic life in a computer. The objective is not necessarily to create life forms that would serve as models for the study of natural life, but rather to create radically different life forms, based on a completely different physics and chemistry, and let these life forms evolve their own phylogeny, leading to whatever forms are natural to their unique physical basis. These truly independent instances of life may then serve as a basis for comparison, to gain some insight into what is general and what is peculiar in biology. Those aspects of life that prove to be general enough to occur in both natural and synthetic systems can then be studied more easily in the synthetic system. ``Evolution in a bottle'' provides a valuable tool for the experimental study of evolution and ecology.
The intent of this work is to synthesize rather than simulate life. This approach starts with hand crafted organisms already capable of replication and open-ended evolution, and aims to generate increasing diversity and complexity in a parallel to the Cambrian explosion. To state such a goal leads to semantic problems, because life must be defined in a way that does not restrict it to carbon based forms. It is unlikely that there could be general agreement on such a definition, or even on the proposition that life need not be carbon based. Therefore, I will simply state my conception of life in its most general sense. I would consider a system to be living if it is self-replicating, and capable of open-ended evolution. Synthetic life should self-replicate, and evolve structures or processes that were not designed-in or pre-conceived by the creator ([5,30]).
Core Wars programs, computer viruses, and worms ([6,9,10,11,13,14,32,33]) are capable of self-replication, but fortunately, not evolution. It is unlikely that such programs will ever become fully living, because they are not likely to be able to evolve.
Most evolutionary simulations are not open-ended. Their potential is limited by the structure of the model, which generally endows each individual with a genome consisting of a set of pre-defined genes, each of which may exist in a pre-defined set of allelic forms ([1,7,8,12,20,29]). The object being evolved is generally a data structure representing the genome, which the simulator program mutates and/or recombines, selects, and replicates according to criteria designed into the simulator. The data structures do not contain the mechanism for replication, they are simply copied by the simulator if they survive the selection phase.
Self-replication is critical to synthetic life because without it, the mechanisms of selection must also be pre-determined by the simulator. Such artificial selection can never be as creative as natural selection. The organisms are not free to invent their own fitness functions. Freely evolving creatures will discover means of mutual exploitation and associated implicit fitness functions that we would never think of. Simulations constrained to evolve with pre-defined genes, alleles and fitness functions are dead ended, not alive.
The approach presented here does not have such constraints. Although the model is limited to the evolution of creatures based on sequences of machine instructions, this may have a potential comparable to evolution based on sequences of organic molecules. Sets of machine instructions similar to those used in the Tierra Simulator have been shown to be capable of ``universal computation'' ([2,24,26]). This suggests that evolving machine codes should be able to generate any level of complexity.
Other examples of the synthetic approach to life can be seen in the work of [3,16,21,22,31]. A characteristic these efforts generally have in common is that they parallel the origin of life event by attempting to create prebiotic conditions from which life may emerge spontaneously and evolve in an open ended fashion.
While the origin of life is generally recognized as an event of the first order, there is another event in the history of life that is less well known but of comparable significance: the origin of biological diversity and macroscopic multicellular life during the Cambrian explosion 600 million years ago. This event involved a riotous diversification of life forms. Dozens of phyla appeared suddenly, many existing only fleetingly, as diverse and sometimes bizarre ways of life were explored in a relative ecological void ([18,27]).
The work presented here aims to parallel the second major event in the history of life, the origin of diversity. Rather than attempting to create prebiotic conditions from which life may emerge, this approach involves engineering over the early history of life to design complex evolvable organisms, and then attempting to create the conditions that will set off a spontaneous evolutionary process of increasing diversity and complexity of organisms. This work represents a first step in this direction, creating an artificial world which may roughly parallel the RNA world of self-replicating molecules (still falling far short of the Cambrian explosion).
The approach has generated rapidly diversifying communities of self-replicating organisms exhibiting open-ended evolution by natural selection. From a single rudimentary ancestral creature containing only the code for self-replication, interactions such as parasitism, immunity, hyper-parasitism, sociality and cheating have emerged spontaneously. This paper presents a methodology and some first results.
Apart from its value as a tool for the study or teaching of ecology and evolution, synthetic life may have commercial applications. Evolution of machine code provides a new approach to the design and optimization of computer programs. In an analogy to genetic engineering, pieces of application code may be inserted into the genomes of digital organisms, and then evolved to new functionality or greater efficiency.