Posted: Jul 09, 2018 2:21 am
Macdoc wrote:You'll see how doctors of medicine were graduated from Harvard medical school never having seen a patient or done an autopsy, mired in the four humours nonsense
Wait a minute ... these doctors were operating in this mode two decades after the emergence of Koch's Postulates? Duh.
Though of course, we now know that Koch's Postulates are difficult at best to apply to viruses, and would have been impossible to apply thereto in 1918, but with modern techniques, you can still give it a shot. Even though attempts to produce better criteria, such as the Bradford Hill Criteria, have been made subsequently, Koch's Postulates are still useful in the right circumstances.
Meanwhile, back to microbes on Enceladus ... while admitting that using the one data point of Earth isn't the most rigorous way forward, the problem is that at the moment, it's the only avenue open to us, until we alight upon other data points involving demonstrable living organisms found elsewhere. It's because we're interested in finding those data points, because said data points will illuminate our understanding of numerous questions, that effort is being expended in this direction. Said effort is lent weight by the fact that biologically important molecules are not only found on Earth - a good number of them have been detected spectroscopically residing within interstellar gas clouds. Furthermore, experiments replicating conditions in those clouds, have demonstrated that other important biological molecules can be generated within, using the existing molecular feedstock, courtesy of ultraviolet photolysis. Stars, of course, generate a lot of ultraviolet light, and that ultraviolet light can have long-range effects. Then of course we have the chemical analyses of various meteorite samples, which have demonstrated the presence of a wealth of biologically important molecules residing therein.
At the moment, we're constrained by the fact that it takes years for our space probes to reach even relatively close objects such as Enceladus, and that we have to expend much diligent effort planning those missions beforehand. Heading off to somewhere like Gliese 581 and scanning its planets for life forms isn't an option at the moment, because that system is 20 light years away, and our current best space probes will take about a quarter of a million years to get there.
However, the single data point of Earth has still provided us with much useful information on what to look for. Such as, for example, the fact that here on Earth, carbon uptake by living organisms is isotopically skewed in favour of 12C, courtesy of the operation of photosynthesis. Photosynthesis isn't likely to be a driving force for any life forms on Enceladus, buried under tens of kilometres of ice, and therefore residing in total darkness. But, any other isotopically skewing biochemistry that arises there, immediately provides scientists with the appropriate instruments with a huge clue as to what's going on. For those interested in the technical details, there's an entire book devoted to isotope fractionation in biological systems, and the chemical reactions where this takes place to a measurable extent.