Tuesday, January 4, 2011
The ten most important questions for astrobiology: Number 7
Is the Earth unusually suitable for complex life?
Now, I've deliberately phrased this question in this particular way in order to better illustrate some of the factors involved in addressing it. The words 'unusually' and 'complex', and even 'suitable', can all trigger lengthy debate. In and of itself the question should be straightforward - a simple rephrase would be 'are there many planets like the Earth in the universe?', but that leaves open the meaning of 'like the Earth', which generally ends up translated as 'with creatures like us running around'.
A few years back the debate over this kind of question got pretty heated with the book 'Rare Earth' by Ward and Brownlee. It tried to make the case that well, yes, the Earth is unusually suited for complex life, and that most other planets out there in the cosmos might be ok for microbes, but not the likes of us. A fair amount of the argument is based around a list of critical requirements - things like plate tectonics, the Moon, the right elemental mix and so on - get one of these wrong and, the authors contended, multi-cellular, walking talking life just doesn't have a chance at ever occurring. The simplest criticism of this type of reasoning is that there needs to be an explicit, quantifiable, connection between these phenomena and the rather horribly unknown biochemical probability space for apes like us showing up - and such connections are thin on the ground. Jim Kasting gave an excellent and thorough rejoinder to Rare Earth in a 2001 review that you can find amongst his papers. In essence Kasting made a convincing case that almost every negative phenomena invoked by Ward and Brownlee could be found to be, if not a positive for complex life, certainly not a showstopper, and vice-versa.
Intriguingly some of these arguments also to parallel discussions of the anthropic principle, although the fundamental requirements for complex life on a planet are much harder to pin down than, for example, the requirement for large, resonant, nuclear cross-sections in order to produce carbon in the universe. One suspects that there may not be anything quite as cut or dry as that in determining whether 'simple' life transitions to 'complex' life, especially when evidence here on Earth points towards microbial life having made multiple experiments with multi-cellular organization - in an on again, off again fashion.
It is also a question that bumps up against philosophical/statistical concepts of a priori and a posteriori statements. A genuine a priori statement is one that is without question (all sheep are born), while an a posteriori statement relies on interpretation of observation (most sheep have four legs). The hypothesis that the Earth is indeed unusually suited for complex life is both a bit of a priori (we are here, therefore Earth is suitable for complex life) and bit of a posteriori (the Earth is the only example of a planet suitable for complex life, and therefore may be unusual). The alternative hypothesis, that Earth is not unusually suited to complex life, suffers from the same problem, all of which boils down to having a sample size of one. Even if a Rare Earth type argument could point to a smoking gun - something utterly, undeniably critical for complex life that was also utterly, undeniably going to be unusual, it would always be questionable because of the sample size of one.
It would be wonderful to be able to answer this question with experiment. Imagine we could build hundreds of copies of the Earth, but alter something in each case. In dramatic fashion we might demand that the collisional formation of the Moon never occurred, or that terrestrial volcanism shut down after a mere billion years or so. Among the (slightly) more subtle variants we might add or remove one or two mass extinction events during the planet's history, or tweak ocean acidity during the pre-Cambrian by a pH or so, or even just remove a single seemingly unimportant species. We might find that complex multi-cellular life is acutely sensitive to us changing anything, but we might also find that convergent evolution is more than just for biochemical structures, and that complex life literally fights tooth and claw for existence once the wheels are set in motion. Obviously the universe may have performed this experiment for us already, the challenge is to find and examine those other worlds, and that is going to take us a long time at this point.
So, the answer to question 7? Go canvas the universe - and that's what most astrobiologists are aiming for.