LJ wrote:
If the beginning of life is a good possibililty, then wouldn't biological explanations of the features seen in ALH84001 be more simple than the combination of non-biological processes that would be required to produce the same features?

Alex writes:
In my opinion, no, because you have not discounted that the counteraguing processes are not also "good possibilit[ies]." Though it is almost canonical among scientists that Occam's Razor can be (and has been) wielded to suit a specific argument, one must bear in mind that the issue under debate here is the original McKay et al. [1996] (and all progeny papers) assertion of "four lines of evidence," WHICH TAKEN TOGETHER, argue that biology is the simplest explanation to account for the "bio-type" features they found in ALH84001. In other words, McKay et al. have wielded Occam's Razor to strengthen their claims, which is perfectly valid. For those who argue otherwise, the job is to weaken the lines of evidence, which by extension weakens the McKay et al. position. The key point, in my opinion, is that the McKay et al. interpretation relies on their lines of evidence operating *together*. If other workers are able to demonstrate non-unique (e.g., non-biological causes for similar observations, possibility of terrestrial contamination, etc.), then the combinative force of McKay et al. loses strength. In this instance, Occam's Razor is turned (and wielded) against those making the original claim.

That said, I do not want to leave the impression that I do not respect the work of McKay, Thomas-Kperta, Gibson, etc. I believe that it was entirely legitimate to publish their 1996 paper in Science, though the intervening debate has certainly left me skeptical of their claims. However, as Steve Squyres, PI for the 2003 MER/Athena Mission, once said, "Four maybes [the McKay et al. lines of evidence] do not equal 'yes.'" At any rate, the ensuing back-and-forth of the ALH84001 debate is a good example of exactly how Science (not just the journal ;-) should work.

Alex R. Blackwell
University of Hawaii