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"We must, however, acknowledge, as it seems to me that man with all his noble qualities … still bears in his bodily frame the indelible stamp of his lowly origin."

- Charles Darwin, The Descent of Man (1871).

The origin of human life on Earth continues to remain a partially solved mystery that has yet to be completely understood. Historically, it was thought that an all powerful God possessed the ability to create matter at will and thus molded the universe, including Earth and all its inhabitants. This creationism theory not only formed the basis for many religions, but was also trusted and accepted without any true supporting evidence for centuries. It was not until the Scientific Revolution in the mid 16th century when the authority of the church was truly challenged. People became scientists; thought processes changed and people began to acknowledge the sensory and empirical evidence that lay before them. Perhaps the most acknowledged event since the beginning of the revolution was the publication of the book On the Origin of Species by Means of Natural Selection by Charles Darwin in 185. Based upon scientific evidence, Darwin's theory, which argued that species were not created in their present form but had evolved from an ancestral species through natural selection, was a complete antagonist to the teachings of the church (Campbell et al. 1). Since then, this evolutionary theory has exploded and continual research in this field strives to determine the original origin of life on Earth. Panspermia is one such suggestion that has recently gained support from these extensive investigations.

Panspermia encompasses the idea that life did not originate from Earth itself but was "seeded" onto the planet through cosmic elements or by other intelligent life (Gribbin 1). It suggests that the appearance of then foreign organic compounds, amino acids or even generic material initiated the development of life on ancient Earth (Raulin-Cerceau et al. 18, "Panspermia 000" 1). In contrast, the theory of an Earth based formation for life contains many little intricacies, problems and uncertainties that make this theory much more complex when compared to panspermia. Thus, application of Occams Razor, which states that in the case of two theories with similar predictions, the simpler one is more plausible, favours externally assisted panspermia over an internal formation of life on Terra by itself. Regardless, the theory behind panspermia has been supposed by many and has included acknowledged scientists such as William Thomson [Lord Kelvin], Svante Arrhenius and Francis Crick. (Hoyle and Wickramasighe 1, Gribbin 1, "Panspermia 000" 1).

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Anaxagoras, a Greek philosopher who lived in the 5th century BC, can be credited as the first person to present the idea of panspermia "the seeds of plant and animal life are inherent in the cosmos, and they take root whenever the conditions become favourable." (Hoyle and Wickramasinghe 1). However, this novel idea did not hold well in an Aristotelian society that believed in the concept of spontaneous generation; life can be created from non-living matter. ("Panspermia 000" 1). It was not until the invention of the microscope and Louis Pasteur's experiments in the mid 1th century that provided clear scientific evidence against spontaneous generation. Through his investigations on the souring of milk and the fermentation of wine, Pasteur demonstrated that life must have been derived from another pre-existing form of life, and concluded that life, indeed, was not a spontaneous process ("Panspermia 000" 1). This opened the door for many new interpretations of panspermia and in 1865, the German physician Hermann E. Richter proposed an extraterrestrial sowing of the Earth. He put forward a theory that life developed at a foreign location, migrated to and implanted the Earth. William Thomson (Lord Kelvin), in 1871, noted that the environment in the interior of meteorites were drastically different from what was experienced on the surface ("Panspermia 000" 1). He then furthered Richter's theory by suggesting that organisms could be contained within meteorites which fell onto the Earth (Raulin-Cerceau et al. 18). Further support for panspermia arose in 107 when the Swedish chemist Svante Arrhenius suggested it was possible that Earth was seeded by "germs" that originated from outside our solar system (Raulin-Cerceau et al. 18). He observed that microorganisms contained "unearthly properties" that were nearly impossible to explain through a natural selective method on Earth ("Panspermia 000" 1). He postulated that microscopic forms of life could have been ejected from a foreign planet, carried and dispersed by the radioactive pressures of a star through space and "seed" itself onto an alien planet (Horneck et al. 001). These theories are now known as lithopanspermia and radiopanspermia respectively (Raulin-Cerceau et al. 18).

The concept of radiopanspermia came under fire in 14 when French agronomist Paul Becquerel conducted several studies on the effects of ultraviolet damage on spores and bacteria at low temperatures and in a vacuum, as he intended to duplicate the conditions that these possible microorganisms would experience in their travel through outer space (Raulin-Cerceau et al. 18, "Panspermia 000" 1). The results completely disagreed with what Arrhenius had proposed. The microorganisms did not survive the experimental conditions and thus Becquerel concluded that this type of travel was not possible between the stars; the interplanetary medium was found to be completely sterilizing (Raulin-Cerceau et al. 18). It has since been discovered that UV radiation at approximately 1 a.u. [5 W/m] destroys unprotected bacteria within minutes. Horneck et al. (001) further demonstrated that exposure to UV radiation (solar photons with 160 nm 0 nm) resulted in the production of photoproducts in microbial DNA. In addition, they confirmed that combinational action of solar UV and the space vacuum was more detrimental than either factor alone (Horneck et al. 001). Thus, this illustrated the necessity for adequate shielding of the microbes, a carbonaceous coating of a few microns thick is required, for interplanetary travel (Horneck et al. 001, Mastrapa et al. 001). Furthermore, scientific advances since the 10s have also established that UV light exposure may not necessarily kill microorganisms, but rather, inactivate them through a protection mechanism that can safeguard their genetic material. Once the radiation has been removed, these microorganisms can fully reactivate themselves ("Panspermia 000" 1). This may yet provide to be another strategy for interplanetary travel of microorganisms that are not in a completely UV shielded environment. Deinococcus radiodurans and Bacillus subtilis are two such types of bacteria that proven to be resistant to UV radiation and are currently used as experimental organisms to simulated space environments. Nonetheless, the experiments that resulted from Becquerel's work in the early 10s have verified the need for UV protection while traveling in space, in a meteorite or comet for example, and therefore lead to an overall favouring of the lithopanspermia theory. While this is true, much of Arrhenius's theory regarding panspermia is still under consideration.Examination of Earth's geological record has shown the presence of fossilized microorganisms which dated back to .8 billion years ago, with clear evidence of photosynthetic life found within the Isua sediments (Sorrell 17, Hoyle and Wickramasinghe 1). It was likely that after planetary formation, the surface of the Earth underwent intense meteoritic bombardment for the first 600 to 700 million years based on the impact cratering records found on both moon and Earth (Hoyle and Wickramasinghe 1). Thus, it was possible that contained within these bombarding elements was foreign organic material necessary to "seed" life on Earth. Comets and meteorite impacts remain one of the most predominant vehicles suspected of hosting microorganisms.

Hoyle and Wickramasinghe (1) noted the atomic similarity between that of a comet and the composition of living material. Furthermore, they observed the presence of organic matter, a liquid water interior, and a shell of ice which would not only provide a nominal cultural medium for microbial growth, especially for autotrophic anaerobic bacteria, but could also provide a preservation mechanism for its inhabitants via its frozen condition. In addition, it has been discovered from observing Halley's Comet in 186 that comets do eject organic particles at a rate of a million, or more, tons per day. An infrared emission spectrum from the ejected dust revealed to match those of bacteria and mass spectrometry analysis also determined a complex organic composition from the ejected dust ("Panspermia 000" 1). This phenomenon has also been confirmed in both the Hyakutake Comet and the Hale-Bopp Comet. Thus, it was possible that comets which passed through the inner regions of our solar system billion of years ago may have released organic material which contributed to the development of life on Earth.

Similarly, it was also possible for a comet to collide with Earth and thus lead to the expulsion of any contained microorganisms. However, any contained life form must exceptionally resilient to extreme conditions. Mastrapa et al. (001) identified that meteorically based organisms that landed on the Earth must not only have the ability to not only endure the UV radiation and vacuum of space, but also extreme and sudden changes in acceleration, also known as jerk, shock pressure and heating during atmospheric entry and impact with the planetary surface. Assuming the survival of these organisms, the result would be their massive release as it has been estimated that a single comet/meteorite contains at least a few billion microorganisms ("Panspermia 000" 1). One such example of a meteorite impacting the Earth is the Martian Meteorite ALH84001 discovered in the Allan Hills of Antarctica in 16. This 1. kg meteorite was thought to have originated from the surface of Mars that was released from an asteroid or comet impact 15 million years. Analysis of the meteorite revealed the presence of .6 billion year old carbonate globules and an abundance of polycyclic aromatic hydrocarbons, which can be interpreted as the presence of fossilized Martian bacteria (Raulin-Cerceau et al. 18, "Panspermia 000" 1). Perhaps the most compelling evidence was the presence of elongated structure extremely similar to some nanobacteria found on Earth associated with crystalline magnetite which form only through biological processes ("Panspermia 000" 1). Studies conducted on the alignment of the magnetic minerals also revealed that the temperature of the meteorite did not exceed 40°C after its departure from the Martian surface (Mastrapa et al. 001). This suggested that any microorganisms contained within the meteorite may not have experienced an extreme change in temperature during its transfer from Mars to Earth and thus may have contributed to the viability of any contained microbes. Based on this information, it is possible that other meteoritic impacts may have contained organic material, survived atmospheric entry and released their contents upon impact with the planetary surface. This type of release would undoubtedly contribute to the "seeding" of the Earth.

An alternative possibility behind lithopanspermia involves the transportation or creation of organic molecules to the Earth rather than the delivery of microorganisms. Sorrel (17) suggested that comets or asteroids that struck the Earth's surface during the "bombardment" era may have either directly synthesized or deposited complex organic molecules upon impact. It is known that complex organic molecules can be synthesized in outer space through abiotic reactions and the compounds acetic acid, glycine, ethyl cyanide and acetone have been identified in the Sagittarious B cloud (Sorell 17, Campbell et al. 1). It has also been suggested that any unshielded microorganisms exposed to UV radiation will first become inactivated, followed by subsequent degradation in the vacuum of space to liberate free organic molecules or polymers ("Panspermia 000" 1). It was suggested that these molecules were incorporated into meteorites or comets which, when impacted with the Earth, "seeded" the planetary surface with organic molecules. Furthermore, it was demonstrated that, when mixed with water, these organic molecules from the meteorite were able to form vesicles (Campbell et al. 1). It is possible that these [then] alien organic molecules interacted with the natural Terran ones to contribute to the development of life on Earth.

Finally, there is the prospect that higher ordered alien life forms came to Earth and, more or less, implanted "us" here on this planet. However, there is no real scientific background or equivocal proof behind this claim since we have not encountered any authentic extraterrestrials outside of the movie theater. In fact, this statement raises more questions as opposed to answering them "Who came here? When did they come? Why did they Come? Etc…" Yet, this theory cannot be ruled out simply on this basis. The universe is extraordinarily large and it is more than likely that other forms of intelligent life, besides humans, inhabit it. If this is true, then a similar origin of life question arises are they panspermic as well?

With these advances in the theory of panspermia, the notion of Earth being "seeded" by alien molecules or microorganisms billions of years ago does not seem as impossible as it once thought to be. More specifically, the lithopanspermia theory is currently favoured due to the supporting physical and chemical evidence for such an event[s] to occur. It is possible that the modes of lithopanspermia presented in this paper do not act independently and that is it was perhaps the conjugated effects that has given rise to the large phylogenetic diversity witnessed on the Earth today. Nonetheless, the panspermia theory has gained support from many well respected scientists over the centuries but has also sparked the interest, and imagination, of many others. The birth of SETI, NASA and the IAU [International Astronomical Union] within the past century has not only been true testament to the field of astronomy and the search for extraterrestrial life but has also given the human race a standpoint of the known universe that can be used to gain a perspective of ourselves as a race. We will continue searching not only the cosmos for new stars, planets and life but also internally, to analyze our current and past forms to grasp a certain understanding about ourselves and the environment around us. We must also recognize that panspermia is not confined to our home planet but has, will or is currently occurring across the universe. Furthermore, that this theory is spontaneous and continuous as one of the main principles behind panspermia, and all its branches, is the concept of life originating from other life. Thus, it is perfectly plausible that the arrival of extraterrestrial microorganisms several billion years ago may have indeed been the "indelible stamp of his [man's] lowly origin" on Earth.

Works Cited

Campbell, Neil A., Reece, Jane B. and Mitchell G. Lawrence. 1. Biology (5th Ed). Menlo ParkBenjamin/Cummings.

Gribbin, John. 1. Panspermia Revisited. 1-. http//xxx.lanl.gov/abs/astro-ph/001

Horneck, Gerda, Rettberg, Petra, Gther, Reitz, J-rg, Wehner, Eschweiler, Ute, Strauch, Karsten, Panitz, Corinna, Starke, Verena and Christa Baumstark-Kahn. 001. Protection of Bacterial Sports in Space, a Contribution to the Discussion on Panspermia. Origins of Life and Evolution of the Biosphere. 1 57-547.

Hoyle, F. and N.C. Wickramasinghe. 1. Comets A Vehicle for Panspermia. Astrophysics and Space Science. 68 -41

Mastrapa, R.M.E., Glanzberg, H., Head, J.N., Melosh, H.J. and W.L. Nicholson. 001. Survival of Bacteria Exposed to Extreme Acceleration Implications for Panspermia. Earth and Planetary Science Letters. 187 1-8.

Panspermia 000. 1. Astrophysics and Space Science. 68 1-17.

Raulin-Cerceau, Florence., Maurel, Marie-Christine and Jean Scheider. 18. From Panspermia to Bioastronomy, The Evolution of The Hypothesis Of Universal Life. Origins of Life and Evolution of the Biosphere. 8 57-61.

Sorrell, Wilfred H. 17. Interstellar Grains as Amino Acid Factories and The Origin of Life. Astrophysics and Space Science. 54 7-41

Wickramasinghe, N.C., Wainwright, M., Narlikar, J.V., Rajaratnam, P., Harris, M.J. and D. Lloyd. 00. Progress Towards the Vindication of Panspermia. Astrophysics and Space Science. 8 40-41.

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