Are we alone? What is the origin of life? These are some of the most profound questions of humankind, and have captured the imagination of scientists and lay people alike for centuries. Astrobiology it is a field that tackles these questions in earnest and offers for the first time tangible goals and astounding discoveries that were never before thought possible. Recent discoveries in this field are rapidly changing our view of the potential for life elsewhere in the universe. New advancements in remote detection technologies have enabled us to identify hundreds of planetary systems, and the search for Earth-like planets continues at a breathtaking pace. Of perhaps even greater significance is that we have found life to survive under conditions previously thought impossible. These include microbes flourishing under extremes of pressure on the ocean floor, at temperatures above boiling and below freezing, and at the intense radiation conditions found in outer space. As a result of these fascinating and almost daily new discoveries, our concept of the boundaries of life has been altered forever.

Both the title and concept of the theoretical article written by Dr. Rhawn Joseph (Joseph, 2009) is thus highly intriguing. While interesting as far as a speculative article goes, the argument put forward by the author can be criticized in relation to the assessment of the facts and the failure to address issues which could call his conclusions into question. It is particularly pertinent to point out these issues so readers are aware of all of the facts and arguments in this field.

In agreement with Dr. Joseph, there certainly is the possibility (and indeed plausibility based on experimental data) of panspermia as a mechanism of transmitting life between planetary bodies (Arrenhius, 1908). Dr. Joseph is correct in his assessment on the survivability of microorganisms in the hostile conditions of space, and the ability of some microorganisms to survive the high impact and velocity that would be experienced upon ejection from a host planetary body and re-entry/impact on a receiving planetary body. As Joseph rightly points out, a number of studies have shown that microorganisms can survive the extreme conditions of space (Nicholson et al. 2000, Horneck et al. 2001, Horneck et al. 2002), which include exposure to extreme vacuum, desiccation, cold temperatures, ultraviolet and ionizing radiation, as well as bombardment by heavy high-energy particles (Mastrapa et al. 2001). The fascinating ability of various Deinococcus species to withstand atomic radiation and X-rays by rebuilding their entire genomes (Lovett 2006), is a testament to how life on Earth is adapted to survive in extreme conditions. By extrapolation, this broadens the number of planetary bodies that although inhospitable to more complex and ‘advanced’ life forms such as ourselves, could potentially harbor viable microbial life. Further experiments on a range of microorganisms including Bacillus subtilis spores and Deinococcus radiodurans cells have shown the potential for life to survive the high energy acceleration/deceleration associated with ejection into space and the landing on a surface planet (Horneck et al. 2001, Mastrapa et al. 2001, Burchella et al. 2001). The fact that microorganisms only need a few meters of surface material to sufficiently protect them from the hazards of space (Horneck et al. 2002), suggests that surviving and interplanetary journey is indeed a possibility worthy of consideration.

However I think the author is blurring the line somewhat between the concept of delivering life from another source (the potential distribution of life throughout the universe), and using panspermia as an explanation for the origin of life on Earth (and indeed put forward as the only viable explanation by Joseph (2009). Most significantly, the empirical evidence for microfossils and/or past microbial life in meteorites originating outside our solar system is controversial and has been disputed, and cannot be considered established fact. The author cites the discovery of microfossils in the Martian meteorite ALH84001 (McKay et al. 1996). However many in the scientific community have raised legitimate questions which have led many to conclude the evidence presented is not biological in origin (Anders 1996, Shearer and Papike, 1996, Bell 1996, Becker et al 1997, Bradley et al. 1997). Although the study by McKay et al. (1996) was rigorously conducted, all lines of evidence put forward suggesting the potential of microfossils of biological origin have been disputed, and many believe the evidence has been refuted. Potential issues of contamination and the fact that many of the ‘biological artefacts’ have now been shown to be abiotic clearly calls into question the conclusion of the original paper by McKay et al. (1996), and thus the claims by Joseph (2009) in the current paper.

In addition to ALH48001, the evidence cited by Joseph (2009) which he argues proves the existence of microbial microfossils in other meteorites has also been disputed. As one example, the ‘discovery of ‘extraterrestrial biogenic hydrocarbons’ in the Orgueil meteorite has since been refuted and invalidated (McCall 2006). As to the other 14 meteors and the work of Dr. Hoover (2006) of NASA, these findings are also not beyond dispute. Neglecting to mention or dismissing these controversies (Joseph 2009) results in a biased representation of the facts. As the late Carl Sagan once said ‘Extraordinary claims require extraordinary evidence’, and thus while significant doubt remains over any claim of past or present evidence of life in meteorites remains, this impacts significantly on the evidence cited by Joseph (2009). While there is certainly the possibility of finding evidence of past or present life on Martian rocks (or other meteorites) to suggest that conclusive evidence has been found is simply incorrect.

The author also makes a very bold claim that ‘life on Earth must have also originated from life’; this is not an established fact as it is well understood that there is as yet no one unifying theory that can explain the origin of life. While using the concept of the continuance of life on Earth as a basis for trying to understand the origin of life is a reasonable one, the obvious questions of what started it all off must surely be considered. Where then did the first ‘life’ come from if Joseph’s claim is correct? Claiming that the abiogenic origin of life theory (the so-called organic soup) is based on ‘super natural forces’ and based on religious beliefs is simply incorrect. The groundbreaking work of Miller and Urey in the 1950s was based on rational thinking and sound scientific ideas, and clearly demonstrated the potential for the building blocks of life to be created under simple chemical conditions that may have represented those on early Earth (Miller 1953, Miller and Urey, 1959).

Since then many scientists have conducted variations on the Miller-Urey experiment with both different mixes and gases (methane, ammonia, carbon dioxide) and different energy sources (electrical charges and ultraviolet light) and showed a great variety of organic compounds produced (Oró et al. 1961, Oró 1965, Lascano and Bada, 2004, Johnson et al. 2008). It has in fact been shown that 22 amino acids, several complex lipids and sugars, as well as all five of the chemical bases used in DNA and RNA can be produced in purely abiotic conditions. Understanding what may have happened next in the origin of life (i.e., going from the building blocks of life to ‘life’ itself) is certainly a challenge, however the rational hypotheses put forward by the likes of Carl Woese and the ‘RNA World’ (Woese 1968) do not deserve to be dismissed so lightly by the author (Joseph 2009).

In conclusion the idea put forward that Life on Earth came from other planets (Joseph 2009) is certainly thought provoking and one that should stimulate healthy debate on the issue. However, the manner in which some of the facts have been interpreted and presented (particularly the evidence put forward regarding meteors and microfossils) results in a somewhat biased argument and proposal. To state that "the only reasonable explanation is that the first living creatures to appear on this planet were produced by other living things which arrived on Earth" (Joseph 2009), is a very big of leap and one which is not fully supported by rigorously tested empirical data. If in the end, one day Dr Joseph is proven correct, then it will be a monumental discovery that could impact on areas such as theology, ethics, and other philosophical issues that may ultimately define who we are.

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