The “Wow! signal” of the terrestrial genetic code Icarus Volume 224, Issue 1, May 2013, Pages 228–242 Vladimir I. shCherbaka, E-mail the corresponding author, Maxim A. Makukovb, Corresponding author contact information, E-mail the corresponding author a Department of Mathematics, al-Farabi Kazakh National University, al-Farabi Avenue 71, 050038 Almaty, Kazakhstan b Fesenkov Astrophysical Institute, Observatory 23, 050020 Almaty, Kazakhstan dx.doi.org/10.1016/j.icarus.2013.02.017, How to Cite or Link Using DOI Permissions & Reprints Abstract It has been repeatedly proposed to expand the scope for SETI, and one of the suggested alternatives to radio is the biological media. Genomic DNA is already used on Earth to store non-biological information. Though smaller in capacity, but stronger in noise immunity is the genetic code. The code is a flexible mapping between codons and amino acids, and this flexibility allows modifying the code artificially. But once fixed, the code might stay unchanged over cosmological timescales; in fact, it is the most durable construct known. Therefore it represents an exceptionally reliable storage for an intelligent signature, if that conforms to biological and thermodynamic requirements. As the actual scenario for the origin of terrestrial life is far from being settled, the proposal that it might have been seeded intentionally cannot be ruled out. A statistically strong intelligent-like “signal” in the genetic code is then a testable consequence of such scenario. Here we show that the terrestrial code displays a thorough precision-type orderliness matching the criteria to be considered an informational signal. Simple arrangements of the code reveal an ensemble of arithmetical and ideographical patterns of the same symbolic language. Accurate and systematic, these underlying patterns appear as a product of precision logic and nontrivial computing rather than of stochastic processes (the null hypothesis that they are due to chance coupled with presumable evolutionary pathways is rejected with P-value < 10–13). The patterns are profound to the extent that the code mapping itself is uniquely deduced from their algebraic representation. The signal displays readily recognizable hallmarks of artificiality, among which are the symbol of zero, the privileged decimal syntax and semantical symmetries. Besides, extraction of the signal involves logically straightforward but abstract operations, making the patterns essentially irreducible to any natural origin. Plausible ways of embedding the signal into the code and possible interpretation of its content are discussed. Overall, while the code is nearly optimized biologically, its limited capacity is used extremely efficiently to pass non-biological information. Highlights ► The SETI hypothesis of an intelligent signal in the genetic code is tested. ► The code is shown to possess an ensemble of same-style precision-type patterns. ► The patterns are shown to match the criteria of an intelligent signal. Keywords Astrobiology; Search for extraterrestrial life 1. Introduction Recent biotech achievements make it possible to employ genomic DNA as data storage more durable than any media currently used (Bancroft et al., 2001, Yachie et al., 2008 and Ailenberg and Rotstein, 2009). Perhaps the most direct application for that was proposed even before the advent of synthetic biology. Considering alternative informational channels for SETI, Marx (1979) noted that genomes of living cells may provide a good instance for that. He also noted that even more durable is the genetic code. Exposed to strong negative selection, the code stays unchanged for billions of years, except for rare cases of minor variations (Knight et al., 2001) and context-dependent expansions (Yuan et al., 2010). And yet, the mapping between codons and amino acids is malleable, as they interact via modifiable molecules of tRNAs and aminoacyl-tRNA synthetases (Giegé et al., 1998 and Ibba and Söll, 2000; see also Appendix A). This ability to reassign codons, thought to underlie the evolution of the code to multilevel optimization (Bollenbach et al., 2007), also allows to modify the code artificially (McClain and Foss, 1988, Budisa, 2006 and Chin, 2012). It is possible, at least in principle, to arrange a mapping that both conforms to functional requirements and harbors a small message or a signature, allowed by 384 bits of informational capacity of the code. Once genome is appropriately rewritten (Gibson et al., 2010), the new code with a signature will stay frozen in the cell and its progeny, which might then be delivered through space and time to putative recipients. Being energy-efficient (Rose and Wright, 2004) and self-replicating, the biological channel is also free from problems peculiar to radio signals: there is no need to rely on time of arrival, frequency and direction. Thus, due to these restrictions the origin of the famous “Wow!” signal received in 1977 remains uncertain (Ehman, 2011). The biological channel has been given serious considerations for its merits in SETI, though with the focus on genomes (Yokoo and Oshima, 1979, Freitas, 1983, Nakamura, 1986, Davies, 2010 and Davies, 2012). Meanwhile, it has been proposed to secure terrestrial life by seeding exoplanets with living cells (Mautner, 2000 and Tepfer, 2008), and that seems to be a matter of time. The biological channel suggests itself in this enterprise. To avoid anthropocentric bias, it might be admitted that terrestrial life is not the starting point in the series of cosmic colonization (Crick and Orgel, 1973 and Crick, 1981). If so, it is natural to expect a statistically strong intelligent-like “signal” in the terrestrial genetic code (Marx, 1979). Such possibility is incited further by the fact that how the code came to be apparently non-random and nearly optimized still remains disputable and highly speculative (for reviews of traditional models on evolution of the code see Knight et al., 1999, Gusev and Schulze-Makuch, 2004, Di Giulio, 2005 and Koonin and Novozhilov, 2009). The only way to extract a signal, if any, from the code is to arrange its elements – codons, amino acids and syntactic signs – by their parameters using some straightforward logic. These arrangements are then analyzed for patterns or grammar-like structures of some sort. The choice of arrangements and parameters should exclude arbitrariness. For example, only those parameters should be considered which do not depend on systems of physical units. However, even in this case a priori it is unknown exactly what kind of patterns one might expect. So there is a risk of false positives, as with a data set like the genetic code it is easy to find various patterns of one kind or another. Nonetheless, the task might be somewhat alleviated. First, it is possible to predict some general aspects of a putative signal and its “language”, especially if one takes advantage of active SETI experience. For example, it is generally accepted that numerical language of arithmetic is the same for the entire universe (Freudenthal, 1960 and Minsky, 1985). Besides, symbols and grammar of this language, such as positional numeral systems with zero conception, are hallmarks of intelligence. Thus, interstellar messages sent from the Earth usually began with natural sequence of numbers in binary or decimal notation. To reinforce the artificiality, a symbol of zero was placed in the abstract position preceding the sequence. Those messages also included symbols of arithmetical operations, Egyptian triangle, DNA and other notions of human consciousness (The Staff at the National Astronomy and Ionosphere Center, 1975, Sagan et al., 1978 and Dumas and Dutil, 2004). Second, to minimize the risk of false positives one can impose requirements as restrictive as possible on a putative signal. For example, it is reasonable to expect that a genuinely intelligent message would represent not just a collection of patterns of various sorts, but patterns of the same “linguistic style”. In this case, if a potential pattern is noticed, further search might be narrowed down to the same sort of patterns. Another stringent requirement might be that patterns should involve each element of the code in each arrangement, whereas the entire signal should occupy most, if not all, of the code’s informational capacity. By and large, given the nature of the task, specifics of the strategy are defined en route. Following these lines, we show that the terrestrial code harbors an ensemble of precision-type patterns matching the requirements mentioned above. Simple systematization of the code reveals a strong informational signal comprising arithmetical and ideographical components. Remarkably, independent patterns of the signal are all expressed in a common symbolic language. We show that the signal is statistically significant, employs informational capacity of the code entirely, and is untraceable to natural origin. The models of emergence of primordial life with original signal-free genetic code are beyond the scope of this paper; whatever it was, the earlier state of the code is erased by palimpsest of the signal.
Posted on: Sat, 03 Aug 2013 15:20:00 +0000
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