The origin of life is a challenging field of science because most hypotheses cannot be decisively proven or rejected based on the experimental evidence and statistics. Estimation of the probability of each scenario requires models or assumptions which are not certain themselves. In addition, this topic became a battlefield between science and the theory of intelligent design which is closely linked with religion. Most scientists believe that life originated on earth, whereas supporters of intelligent design argue that life is too complex to originate on earth during such a short time. Considering these problems, it is important to treat all alternative scenarios equally without preferences based on politics, beliefs, or funding for the research. Unfortunately, many scientists think that extra-terrestrial origin of life should not be taken seriously until the hypothesis of terrestrial origin is shown to be false. Thus, the publication of Dr. Joseph (2009a), which attempts to evaluate the possibility of various alternative hypothesis, is a welcome ground-breaker. It provides a thorough review of facts and mechanisms related to the possibility of extra-terrestrial origin of life.

Evidence on the possibility of transfer of primitive life from other planets (panspermia) comes not only from astrophysics and astrobiology, but also from genetics ( Joseph 2009b; Sharov 2006). It appears that the functional complexity of life, which can be roughly measures by the size of non-redundant functional genome, increases exponentially in evolution and thus can be used as a clock of macro-evolution (Sharov 2006). The exponential pattern of complexity increase can be explained by several mechanisms of positive feedback, including cooperation between genes (hypercycle effect), gene duplication with subsequent specialization in function, and creation of new functional niches for new genes. Because the existing functional complexity accelerates the further growth of complexity, we expect that early evolution of life was extremely slow. The regression of log-transformed functional non-redundant genome size versus the time of origin of large taxonomic groups indicates that genome complexity increased 7.8 fold per 1 billion years. The expected origin of life, estimated by extending the regression back into time, is ca. 10 billion years ago, which is much earlier than the age of the solar system. This is the first quantitative measure of the time of life origin. It is possible that primitive organisms have even slower exponential rates of complexity increase. For example, in Archaea and Eubacteria, the genome size increased only 1.9 and 2.5 fold per 1 billion years, respectively. With this smaller rate of increase, the estimated origin of life gets closer to the expected origin of the Universe.

The genome of the last universal common ancestor (LUCA) of living organisms on Earth is thought to have ca. 300 genes that included at minimum 140 superfamilies of protein domains (Mushegian 1999, Ranea et al., 2006). Each protein domain is optimized for a specific molecular function (e.g., protein-protein interaction, protein-DNA interaction, catalysis of specific reactions, unwinding of DNA, and others). Development of each protein domain requires long evolutionary times if driven by Darwin’s genetic selection. Denton et al. (2002) argued that protein folds represent “platonic forms” that emerged via self-organization, rather than genetic selection. There is no doubt that self-organization can accelerate the evolution of protein structure in the vicinity of some attractor, however it cannot accelerate an evolutionary transition from one fold to another. Any mutation that disrupts the protein fold makes it dysfunctional, and is likely to be eliminated via selection. Thus, it is unlikely that the evolution of proteins was faster than the evolution of any other structures in organisms.

The evolution of proteins is the latest chapter in the process of the origin of life. It was preceded by the RNA-world where many catalytic functions were performed by nucleic acids (Schuster 1993). And the RNA world was preceded by the coenzyme world where heredity was carried by isolated coenzyme-like molecules that combined autocatalysis (self-copying) with support of some specific functions of a larger system (Sharov 2009). It is conceivable that oil microspheres could support the existence of a coenzyme autocatalytic network on their surface. This model of life's origin is similar to the surface metabolism theory of Wächtershäuser (1988), but it describes explicitly the mechanisms of early heredity and selection without nucleic acids. Because the initial steps of biological evolution were extremely slow, the coenzyme world may have lasted for 1 billion years, followed by ca. 1-2 billion years of the RNA-world.

The most interesting idea of Joseph (2009a) is the possibility that the solar system originated from the remnants of the exploded parent star. If life was present on the planets of the parent star, then the hypothesis of Earth contamination with microbial life forms appears more plausible. First, massive ejection of microbes into space from a planet can be a result of the supernova explosion. This ejection mechanism can be more effective than bombardment of the planet with comets or asteroids. Second, there is no need to account for long inter-stellar travel of microbial spores. As a result, the density of surviving spores in the protoplanetary disk may be higher than in the case of inter-stellar travel. Finally, some planet of the parent star or fragments of a planet may survive the supernova explosion and become the core of a new planet after formation of the next-generation star. Such broken planets may harbor large quantities of surviving microbes in the deep layers. The review summarizes information about the ability of microbes to survive in harsh conditions comparable to the open space. In fact, many microbes are pre-adapted to surviving journeys in space.

But I do not share the belief of Joseph (2009a) that the extra-terrestrial origin of life has been proven. Most supporting facts still allow alternative explanations. The statement that fossilized microorganisms are present in meteorites is definitely premature. There is no proof that bacteriomorphic structures in meteorites are indeed fossilized bacteria. Rare organic molecules in meteorites may result from contamination or artifact. I agree with Joseph (2009a) that “claims of contamination are not proof of contamination”. The fact that only stony meteorites have bacteriomorphic structures is interesting and increases the possibility that these structures represent extra-terrestrial bacteria. However, it is still not sufficient to consider the extra-terrestrial origin of life as a solid fact. We definitely need more data before we come to conclusions. Especially interesting will be exploration of other planes of the solar system, as well as satellites of Jupiter and Saturn.

Joseph (2009a) uses the same erroneous logic that is used by his opponents. From the true statement that “no one has ever demonstrated that life can be produced from non-life” Joseph (2009) comes to a conclusion that “only life can produce life”, which is not necessary true. Similarly, the true statement that “no one has proven that living organisms can survive long journey in open space” does not imply that “life originated on earth”. It is time to honestly recognize that we don’t know the final answer despite the mounting evidence in favor of extra-terrestrial origin of life. Thus, Earth contamination with microbes coming from other planets should be viewed as a hypothesis. Based on the rate of increase of the genome complexity and facts summarized by Joseph (2009a), we can say that the hypothesis of extraterrestrial origin of life is more plausible that the alternative hypothesis of terrestrial origin. But it is still a hypothesis.

References

Denton, M. J., Marshall, C.J., Legge, M. (2002). The protein folds as platonic forms: new support for the pre-Darwinian conception of evolution by natural law. J Theor Biol., 223, 263-5.

Mushegian, A. (1999). The minimal genome concept. Curr. Opin. Genet. Dev., 9, 709-714.

Ranea, J. A., Sillero A., Thornton, J.M., Orengo, C.A. (2006). Protein superfamily evolution and the last universal common ancestor (LUCA). J. Mol. Evol., 63, 513–525.

Schuster, P. (1993). RNA based evolutionary optimization. Orig. Life Evol. Biosphere, 23, 373-391.

Joseph, R. (2009a). Life on Earth came from other planets. Journal of Cosmology, 1, 1-56.

Joseph, R. (2009b). The evolution of life from other planets. Journal of Cosmology, 1, in press

Sharov, A. A. (2006). Genome increase as a clock for the origin and evolution of life. Biol Direct. 1, 17.

Sharov, A. A. (2009). Coenzyme autocatalytic network on the surface of oil microspheres as a model for the origin of life. Int. J. Mol. Sci. 10(4), 1838-52.

Wächtershäuser, G. (1988). Before enzymes and templates: theory of surface metabolism. Microbiol. Rev., 52, 452-484.