Life appeared a few hundred million years after the Earth creation during a period of heavy bombardment. Most of the early geological record of this period has been erased by later events. Thus, we remain ignorant of the true historical facts concerning the origin of life on this planet. Many theories of the possible origins of life on Earth have been postulated; one of them includes the idea that life comes from other planets. Since Arrhenius first proposed the transport of living bacteria through space, panspermia has been put forward as a possible explanation of the first occurrence of life on Earth (Arrhenius 1903). However, over the ensuing years this theory had been subject to several criticisms since its formulation, including arguments such as "panspermia" cannot be experimentally tested (Lederberg 1960) or that spores may not survive long-time exposure to the hostile environment of space (Nussinov & Lysenko 1991).

With recent discoveries and advances in space science research, panspermia reemerged as a promising field of research (Raulin-Cerceau et al. 1998). Unfortunately, a fully satisfactory explanation to the interplanetary transfer of life had not been offered, until now. In the main article of this issue (Joseph 2009), Dr. Rhawn Joseph details a fascinating novel theory based on a comprehensive scientific review, which explains how life on Earth came from other planets (Joseph 2009).

To following is a brief summary of the central points of Joseph's theory. As stars grow old and become red giants, powerful solar winds blow away the atmospheres and possibly the oceans and top soils of its orbiting planets along with microbes which are deposited in a growing nebular cloud consisting of this debris. When red giants lose mass their gravitational hold on orbiting planets is impacted and these planets may be flung from the solar system along with the biomass of microbes deep beneath the soil. The nebular cloud along with remnants of these planets forms protoplanetary discs which in turn creates new solar systems. As pointed out by Joseph (2009) just one microbe had to survive within the debris which created the Earth, and this is how life on our planet began. Dr. Joseph bases his arguments on well documented facts and sometimes on controversial evidence, which may generate exciting discussions

The viable transfer from one planet to another requires microorganisms to survive the escape process from one planet, the journey through space as well as the re-entry/impact process in another planet. Evidence that this was possible began with space experiments demonstrating the survival of microorganisms during a space journey (Horneck et al. 1984; Horneck 1993). Panspermia could now be demonstrated and tested experimentally, and a variety of additional studies have now been designed and affirmative results published.

Thus, in recent years most of the major barriers against the acceptance of this theory have been demolished when it was shown that microorganisms can survive the high impact and velocity experienced during the ejection from one planet and re-entry/impact process onto another world (Stöffler et al. 2007). Not only can they survive a violent ejection and reentry but spores can return to life even after hundreds of millions of years (Vreeland et al. 2000). Indeed, many studies have shown the microbes can sustain a journey through space and stringent sterilization techniques were the keys to the acceptance of the results from the isolation and reawakening of ancient bacteria (Vreeland et al. 2000).

Yet, resistance remains. For example, the discovery of microbial fossils on meteorites is strongly controversial. Many of the initial studies on microbes and meteors were conducted decades ago when microbiological techniques were relatively primitive compared to the present. Certainly, questions about contamination and the sample quality must be taken into account even in respect to more recent discoveries.

Regardless of the current debates about the possibility of extraterrestrial life, the theory of Dr. Joseph elucidates the general physical mechanisms of the transfer of life from other habitable planets to Earth and the distribution of life in the universe. Joseph (2009) has essentially demolished the last major argument against the transfer of life between planets and solar systems.

However, delivering life from another source merely transfers the problem of life's origin to another locality. His assertion “the likelihood that life on Earth began in an organic soup is the equivalent of discovering a computer on Mars and claiming it was randomly assembled in the methane sea” (Joseph 2009), is reminiscent of Sir Fred Hoyle. Hoyle compared the random emergence of even the simplest cell to the likelihood that “a tornado sweeping through a junk-yard might assemble a Boeing 747 from the materials therein” (Hoyle 1981). Many scientists will disagree with these arguments, myself included.

Life did not originate as a simple and random specific event, but probably through cycles of repeated events in a long time history that led to self-assembling and reproducing systems that evolved. All our non-magical notions of how life emerges are predicated on the basis of the presence of organic compounds. Considering the promising advances on the prebiotic chemistry with a variety of complex organic compounds produced in abiotic conditions, it is easily comprehensible the theory that life originated gradually from prebiotic molecules. The formation of the chemical building blocks of life has been demonstrated, but the further steps leading to life remained elusive. Although the theory of abiogenesis has not been fully tested or explored experimentally, results obtained recently have been increasing our understanding about the chemistry of living systems and the chemical origin of life. Thus, our still limited knowledge on this question cannot be used to refute the theory. Of course, to believe the Earth is the center of the biological universe is a childlike egocentrism. The fact that life is found on the Earth does not mean that life necessarily started here. Earth is not a closed box disconnected from the wider Universe. Since the 70s a variety of organic molecules has been identified in the interstellar gas (Thaddeus 2006). Therefore, the possible origin of life elsewhere is worth of consideration.

In this regard, even as Joseph (2009) disputes abiogenesis, he has nevertheless expanded the time necessary for life to have formed, from a half billion years on Earth, to billions of years before the establishment of this planet. Those who have argued there was not enough time, have now been answered.

Joseph's theory is also completely compatible with those who have advanced meteors, asteroids, or comets as a source for the origin of life.

Another theory to the origin of life on Earth postulates that meteorites might have delivered organic precursors to life, since some of these precursors would have survived the impact (Pierazzo & Chyba 1999). Carbonaceous chondrite meteorites contain abundant soluble organic compounds that are similar, sometimes identical, to those found in the biosphere, such as amino acids, carboxylic acids, and sugar derivatives (Pizzarello 2006). Some of these amino acids also show L-enantiomeric excesses, and this suggest that meteors may have contributed to terrestrial homochirality by direct input of meteoritic organic material to the early Earth (Pizzarello 2007). Although speculative, like the terrestrial origin of life from the organic soup present on early Earth, this theory is based on quality results (Pizzarello 2007).

To summarize, the abiotic explanations for the origin of life on Earth have benefited from considerable advances based on a wealth of new data. However, the state of this field is too extensive to be summarized here and needs to be explored in further reviews. Be it abiogenesis, or panspermia, the origin of life remains an open question.

In conclusion, the theory of panspermia has gained increasing credibility in recent years culminating with this provocative, controversial as well as fascinating novel theory developed by Dr. Joseph, which explains how our ancient ancestors journeyed here from the stars.

References

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Horneck, G. 1993. Responses of Bacillus subtilis spores to space environment: results from experiments in space. Origins of Life and Evolution of the Biosphere, 23, 37-52.

Hoyle, F. 1981. Hoyle on Evolution. Nature, 294, 105.

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Vreeland, R. H., Rosenzweig, W.D., Powers, D.W. 2000. Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal. Nature, 407, 897-900.