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.
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11 comments:
I hope not. I long for the day when our understanding of life transcends what could be gathered on earth alone.
I have never seen this issue so comprehensively and clearly summarized before. Great post.
The connection between the anthropic principles and statistics is particularly intriguing to me. Say there are N possible universes, and n_i, i = 1, 2, 3, ... of them permit life to have arisen once, twice, three times, etc. Surely higher numbers of i require better tuning than lower numbers, and we can safely assume that N >> n_1 >> n_2 >> n_3, ...
Now, we know "our" universe is contained in the n_i, but we do not have any indication that i>1, so it seems that any tuning beyond i>1 cannot be explained by the anthropic principle and we would have to expect to be alone in the universe. A good distribution to assume for the n_i would be the Poisson distribution, with N = n_0. Then, we obtain that the chance for n>1 is n_1/N to very good approximation if n_1 << N.
It is interesting to look at the number of planets in the universe from the anthropocentric point of view. Surely, planets exist because they are a plausible source of life, but why so many? Would not one be enough? The answer (to the anthropocentrist) is that the larger the number of planets, the less tuning of other aspects of the universe, such as chemistry or geology, is required, since the chance for life in the universe is the chance of life on one planet times the number of planets. With more planets, the chemical or geological tuning requirements can be relaxed.
Even cosmological inflation can be dragged into this picture. View it as a way to increase the number of planets astronomically with a minimum of tuning of the fundamental physical laws.
I am not sure how coherent I am being, but all this thinking is convincing me more and more that the anthropocentric viewpoint (which I think has a lot of merit) leads pretty much inescapably to the conclusion that there is no other life in the universe. Having life more than once would constitute wasteful "overtuning" that is not necessary to explain what we know and extremely unlikely.
I suppose the answer to the question would then be: No, the Earth is not unusually suitable for life, it just happens to be the one planet in the universe where it arose by chance.
In the same way a lottery winner is not unusually suitable to be rich.
Very interesting argument. It reminds me a bit of Bayesian versus Frequentist statistics, the former typically requiring the assumption of a so-called prior (e.g. a model/hypothesis) and the latter attempting to effectively construct a model entirely from observations/data. I suppose a possible counterpoint would be that one could surely pick any phenomenon and apply phenomonocentric reasoning (no, I'm sure that's not a real word!). So for example, I find a single example of a type of molecule that I've never seen before. Would I then think that this was the only example of such a molecule in the universe? I know that the universe makes elements and many molecules, surely (by this argument) it would be overtuning if more than this one example existed? Or pick something else that requires a more complex layering of events to exist - the same applies. So I guess I wonder how fair it is to single out life? It seems predicated on the prior assumption that such stuff is implicitly special. Perhaps I'm missing something here, I'm intrigued by your argument.
Having life more than once would constitute wasteful "overtuning" that is not necessary to explain what we know and extremely unlikely.
Unless there is some physical need for life, then the tuning should reflect the strength and extent of that need... hmmmmm
Caleb, I have to reiterate the point that Mars and Venus don't have life because they suffer one form of runaway effect or another that sends conditions racing far away from those considered conducive to life as we know it, as in the example that I previously gave from the physics lecture on the AP:
http://abyss.uoregon.edu/%7Ejs/images/instability.gif
These astronomy notes clearly demonstrate this point:
http://www.astronomynotes.com/solarsys/s9.htm
And life contributes to the maintenance of these balance points. As Margulis noted, The ecobalances of Earth's atmosphere, hydrosphere, and lithosphere are regulated around "homeorhetic" set points that evolve towards far-from-equilibrium states that optimize entropy production. The environment is not an independent backdrop, but is heavily influenced by the presence of living organisms that lead to near-homeostatic behavior of the Earth system for very long time scales, just like the current epoch of the "flat" universe.
The resulting co-evolving dynamical process eventually leads to the convergence of equilibrium and optimal conditions." and Kleidon (2004) agreed stating: "...(homeorhetic) behavior can emerge from a state of optimum entropy production associated with the planetary albedo"; "...the resulting behavior of a biotic Earth at a state of MEP may well lead to near-homeostatic behavior of the Earth system on long time scales, as stated by the Gaia hypothesis." Staley (2002) has similarly proposed "...an alternative form of Gaia theory based on more traditional Darwinian principles... In [this] new approach, environmental regulation is a consequence of population dynamics, not Darwinian selection. The role of selection is to favor organisms that are best adapted to prevailing environmental conditions. However, the environment is not a static backdrop for evolution, but is heavily influenced by the presence of living organisms. The resulting co-evolving dynamical process eventually leads to the convergence of equilibrium and optimal conditions."
This has a thermodynamic energy conservation law written all over it and similar molecules form in layers, so...
Caleb, your questions are right on. I struggle with them, too.
Consider this:
Q: Why are there many people, not just me?
A: The easiest way to make me is for there to be evolution, sexual reproduction and a population. This produces other people as a side effect.
Q: Why are there many planets, not just Earth?
A: The beginning of evolution, the transition from chemistry to life is so hard, it cannot be made very likely in one spot by tuning a few fundamental parameters. The existence of many planets is a way to make it likely overall.
Q: Why is there just one occurrence of life, not many?
A: See the previous answer.
As to "phenomenocentrism", I think it is valid. You could postulate any sort of molecule, or "thing", and there would be a "most likely", or "least tuned" hypothetical universe that contains it. However, this universe would not matter to us, because we would not be part of it. From the viewpoint of the molecule, obviously, that universe is "real", but for us it is hypothetical, one amongst uncountable others. The universe we are in is just as hypothetical, except it looks real to us because we are in it.
Taken to the extreme, this position leads to a solipsistic view, where EVERYTHING is just there to give rise to me, directly or as a side effect. Not life, not consciousness, just ME.
We can, even more preposterously, carry on and assure ourselves of our own immortality: Because any universe in which we do not exist is not real, there cannot be a reality in which we die. Maybe this is the reason we were not born earlier, when physical immortality was much less likely than it is now, with medicine advancing the way it is.
Far out stuff, but fascinating....
Eniac, thanks for the comments, absolutely, it is fascinating to sometimes have a go at these more 'far out' ideas.
Island - thanks for your comments. I think one thing that makes me take issue with the idea of homeostasis is that the Earth has gone through a pretty wide variety of 'equlibria' during the past 3.5 billion years - each of which (from snowball Earth to oxygen free to hot wet etc etc) has nonetheless been conducive to life since we have no evidence of the utter extinction of life at those times. Homeostatis certainly is at play, but its absence at certain periods has not eliminated life here. I'm not sure those periods are any less 'valid' than those in which environment is fully linked with the biosphere in a feedback loop.
Caleb,
Your mention of Bayesian statistics is well placed here, too.
In many ways, cosmology is a forensic science. We find ourselves in the situation where we know the outcome, and want to "postdict" the chain of events that led us there. Physics, though, usually works the other way around: We have initial conditions and use our Hamiltonian to predict what happens after.
Bayes' theorem is exactly concerned with this inversion of causality, although I do not claim to understand the whole Bayesian philosophy very well. The theorem itself is quite trivial in the context of probability theory, and the fuss is all about interpretation, just like in quantum mechanics.
I'm not sure those periods are any less 'valid' than those in which environment is fully linked with the biosphere in a feedback loop.
Unless maybe this to is a part of the dynamical process that eventually leads to the convergence of equilibrium and optimal conditions for a specific form of life.
I'm just throwing that out there as the first thing that comes to mind while maintaining my confidence because of the strength of the point that lies the fact that apparent commonality that produces our balanced universe is also a critical feature of our own local ecobalances. This commonality indicates that there is a direct connection between the mechanism that defines the structure of the universe, and our existence, in other words, and that commonality most naturally indicates that there may be a bio-oriented cosmological principle in effect that requires carbon based life to appear over a specifically defined region and time in the history of the observed universe.
What does a flat balanced barely expanding universe do that a wide open expanding universe does not?
It maximizes entropy production and work over time because less and perhaps no energy is wasted to heat death.
It gets the most bang for the big bang... a maximum action principle... or maybe a laziness principle... ;)
And thank you for your reply, Caleb. Your knowledge is greatly appreciated and I have been reading all along, fascinated.
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