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It is a fundamental principle of evolutionary theory that Nature never anticipates. A gene pool cannot plan for changes in the environment, for the possible future emergence of new predators or any other contingency; the existential threats any gene pool ever encountered existed in the here-and-now. In this posting I propose that bilateral symmetry was originally selected, not for its future mobility-enhancing potential, but because it functioned as a cancer-preventing device in the earliest Bilaterians.
Those familiar with my published theory know it asserts that all multicells can be divided into two mega-groups: those constructed of somatic cells which all contained functioning cancer triggers (oncogenes) and those that were incapable of dying of cancer. As my peer-reviewed 1984 Letter in the Journal of Theoretical Biology stated succinctly, "This theory states that oncogenes, thus defined, have been present in every cell of every specimen of every species of the Bilateria that ever existed, and that they have existed nowhere else in Nature."
My 1983 Letter asserted that evolutionarily significant lethal cancer did occur in developing Bilaterians in numbers sufficient to create selection pressure for the accumulation of anti-cancer mechanisms and, because cancer begins with replication error, that all those defenses " ... enhanced the ability of the genomes to create organisms in which the genetic program is expressed with great fidelity in all somatic cells." (1)
My 1983 Letter asserted that evolutionarily significant lethal cancer did occur in developing Bilaterians in numbers sufficient to create selection pressure for the accumulation of anti-cancer mechanisms and, because cancer begins with replication error, that all those defenses " ... enhanced the ability of the genomes to create organisms in which the genetic program is expressed with great fidelity in all somatic cells." (1)
I am convinced that one of the earliest anti-cancer adaptations was rejection of the radial symmetry found in other multicells (including the likely ancestors of Bilaterians) and selection of symmetry along a longitudinal axis, a radical departure that enabled gene pools to produce sufficient numbers of animals free of cancer during development.
The anti-cancer benefits of bilateral symmetry.
1. Minimization of the number of somatic cells.Because cancer is the result of imperfect mitosis a simple anti-cancer tactic available to the earliest Bilaterian gene pools was to minimize mitoses by producing the smallest animals possible, those with the fewest number of oncogene-equipped somatic cells. Although it is impossible to know anything about the anatomy of the earliest Bilaterians, extant animals displaying characteristics that would minimize the risk of lethal cancer are the nematodes, one species of which, Caenorhabditis briggsae, is perhaps one of its most successful practitioners: in that species each animal consists of fewer than one thousand somatic cells. (2) A few moments reflection will convince the reader that strict limitation of mitosis to the degree displayed in nematodes and other small animals would not be possible in multicells committed to radial symmetry; their growth would always entail an increase in the circumference of the soma and each increase would require an increasing number of fresh mitoses.
2. Enablement of mirror-imaging and eutely. Gene pools functioning under the imperative that they must include in every somatic cells a cancer-triggering mechanism were impelled to exercise precise control over construction of every animal. Just as the avoidance of lethal errors in mitosis were more likely if the gene pool produced only small multicells, so would errors be minimized if the gene pools employed fully the tools of uniformity: gene pools that "learned" to repeat—in countless numbers—precisely identical sequences of developmental events were less likely to commit fatal errors. Within the individual specimens a form of repetition, the arrangement of somatic cells along a common axis with each side meticulously emulating the other—mirror-imaging—was adopted. Within species phenotypic uniformity among specimens was adopted for the same reason. In some species of extant Bilaterians (mites and nematodes) uniformity is so extreme that all cells in all specimens are situated identically, a phenomenon known as eutely.
3. The emergence of burrowing ability. Although lateral symmetry eventually enabled prodigious feats of mobility, according to the fossil record (3) one of its earliest and simplest manifestations enabled Bilaterians to burrow into the sea bottom where they were shielded from UV and other extra-terrestrial carcinogenic radiation.
Alternative explanations.
I am not aware of any other published explanations of Bilateria origin that propose evolutionarily significant events exclusive to Bilaterians, events that did not also occur in other multicells. The origin of the Bilateria—which apparently occurred only once—is viewed as an event requiring no unique mechanistic explanation. One would think that such a momentous occurrence would have elicited several serious theoretical proposals.
3. The emergence of burrowing ability. Although lateral symmetry eventually enabled prodigious feats of mobility, according to the fossil record (3) one of its earliest and simplest manifestations enabled Bilaterians to burrow into the sea bottom where they were shielded from UV and other extra-terrestrial carcinogenic radiation.
Alternative explanations.
I am not aware of any other published explanations of Bilateria origin that propose evolutionarily significant events exclusive to Bilaterians, events that did not also occur in other multicells. The origin of the Bilateria—which apparently occurred only once—is viewed as an event requiring no unique mechanistic explanation. One would think that such a momentous occurrence would have elicited several serious theoretical proposals.
NOTES
(1) In this posting I present a comprehensive fact-based argument in favor of my conviction that without the occurrence of lethal juvenile cancer meticulous control over development would not have emerged and Bilaterians would not exist.
(2) Grzimek, B., Grzimek's Animal Life Encyclopedia, Van Nostrand Reinhold, 1984.
(3) Valentine, James W., "The Evolution of Multicellular Plants and Animals," Scientific American, September, 1978.
Comments and questions to the author are welcomed here.
At this site you will find links to additional material including my original Letters to the Journal of Theoretical Biology and the 1992 Nature review of my book.
Copyright © 2014 by James Graham
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