The theory of symbiogenesis assumes that the most probable explanation for improbably complex structures (living or otherwise) lies in the association of less complicated parts. Sentences are easier to construct by combining words than by combining letters. Sentences then combine into paragraphs, paragraphs combine into chapters, and, eventually, chapters combine to form a book--highly improbable, but vastly more probable than the chance of arriving at a book by searching the space of possible combinations at the level of letters or words. It was apparent to Merezhkovsky and Kozo-Polyansky that life represents the culmination of a succession of coalitions between simpler organisms, ultimately descended from not-quite-living component parts. Eukaryotic cells are riddled with evidence of symbiotic origins, a view that has been restored to respectability by Lynn Margulis in recent years. But microbiologists arrived too late to witness the symbiotic formation of living cells.

Barricelli enlarged on the theory of cellular symbiogenesis, formulating a more general theory of "symbioorganisms," defined as any "self-reproducing structure constructed by symbiotic association of several self-reproducing entities of any kind."[11] Extending the concept beyond familiar (terrestrial) and unfamiliar (extraterrestrial) chemistries in which populations of self-reproducing molecules might develop by autocatalytic means, Barricelli applied the same logic to self-reproducing patterns of any nature in space or time--such as might be represented by a subset of the 40,960 bits of information, shifting from microsecond to microsecond within the memory of the new machine at the IAS. "The distinction between an evolution experiment performed by numbers in a computer or by nucleotides in a chemical laboratory is a rather subtle one," he observed. [12]

Barricelli saw the IAS computer as a means of introducing self-reproducing structures into an empty universe and observing the results. "The Darwinian idea that evolution takes place by random hereditary changes and selection has from the beginning been handicapped by the fact that no proper test had been found to decide whether such evolution was possible and how it would develop under controlled conditions," he reported in a review of the experiments performed at the IAS. "A test using living organisms in rapid evolution (viruses or bacteria) would have the serious drawback that the causes of adaptation or evolution would be difficult to state unequivocally, and Lamarckian or other kinds of interpretation would be difficult to exclude. However if, instead of using living organisms, one could experiment with entities which, without any doubt could evolve exclusively by 'mutations' and selection, then and only then would a successful evolution experiment give conclusive evidence; the better if the environmental factors also are under control as for example if they are limited to some sort of competition between the entities used." [13]

After forty-three years, Barricelli's experiments appear as archaic as Galileo's first attempt at a telescope--less powerful than half a pair of cheap binoculars--although Galileo's salary was doubled by the Venetian Senate in 1609 as a reward. The two Italians compensated for their primitive instruments with vision that was clear. Barricelli tailored his universe to fit within the limited storage capacity of the IAS computer's forty Williams tubes: a total of one two-hundredth of a megabyte, in the units we use today. Operating systems and programming languages did not yet exist. "People had to essentially program their problems in 'absolute'," James Pomerene explained, recalling early programming at the IAS, when every single instruction had to be hand-coded to refer to an absolute memory address. "In other words, you had to come to terms with the machine and the machine had to come to terms with you."[14]

Working directly in binary machine instruction code, Barricelli constructed a cyclical universe of 512 cells, each cell occupied by a number (or the absence of a number) encoded by 8 bits. Simple rules that Barricelli referred to as "norms" governed the propagation of numbers (or "genes"), a new generation appearing as if by metamorphosis after the execution of a certain number of cycles by the central arithmetic unit of the machine. These reproduction laws were configured "to make possible the reproduction of a gene only when other different genes are present, thus necessitating symbiosis between different genes."[15] The laws were concise, ordaining only that each number shift to a new location (in the next generation) determined by the location and value of certain genes in the current generation. Genes depended on each other for survival, and cooperation (or parasitism) was rewarded with success. A secondary level of norms (the "mutation rules") governed what to do when two or more different genes collided in one location, the character of these rules proving to have a marked effect on the evolution of the gene universe as a whole. Barricelli played God, on a very small scale.

The empty universe was inoculated with random numbers generated by drawing playing cards from a shuffled deck. Robust and self-reproducing numerical coalitions (patterns loosely interpreted as "organisms") managed to evolve. "We have created a class of numbers which are able to reproduce and to undergo hereditary changes," Barricelli announced. "The conditions for an evolution process according to the principle of Darwin's theory would appear to be present. The numbers which have the greatest survival in the environment... will survive. The other numbers will be eliminated little by little. A process of adaptation to the environmental conditions, that is, a process of Darwinian evolution, will take place."[16] Over thousands of generations, Barricelli observed a succession of "biophenomena," such as successful crossing between parent symbioorganisms and cooperative self-repair of damage when digits were removed at random from an individual organism's genes.

The experiments were plagued by problems associated with more familiar forms of life: parasites, natural disasters, and stagnation when there were no environmental challenges or surviving competitors against which organisms could exercise their ability to evolve. To control the parasites that infested the initial series of experiments in 1953, Barricelli instituted modified shift norms that prevented parasitic organisms (especially single-gened parasites) from reproducing more than once per generation, thereby closing a loophole through which they had managed to overwhelm more complex organisms and bring evolution to a halt. "Deprived of the advantage of a more rapid reproduction, the most primitive parasites can hardly compete with the more evolved and better organized species... and what in other conditions could be a dangerous one-gene parasite may in this region develop into a harmless or useful symbiotic gene." [17]

Barricelli discovered that evolutionary progress was achieved not so much through chance mutation as through sex. Gene transfers and crossing between numerical organisms were strongly associated with both adaptive and competitive success. "The majority of the new varieties which have shown the ability to expand are a result of crossing-phenomena and not of mutations, although mutations (especially injurious mutations) have been much more frequent than hereditary changes by crossing in the experiments performed."[18] Echoing the question that Samuel Butler had asked seventy years earlier in Luck, or Cunning? Barricelli concluded that "mutation and selection alone, however, proved insufficient to explain evolutionary phenomena."[19] He credited symbiogenesis with accelerating the evolutionary process and saw "sexual reproduction [as] the result of an adaptive improvement of the original ability of the genes to change host organisms and recombine."[20]