05 April 2015

Lenton & Watson, Revolutions that Made the Earth, and the prevalence of intelligent life in the universe

I'm reading Lenton & Watson, Revolutions that Made the Earth (Oxford, 2010). It's a whole evolutionary history, from an Earth-System ("Gaia") point of view. Very interesting. Although hardly central to their thesis, they agree in general with Brownlee & Ward (Rare Earth) that complex life may be very rare in the universe. The "Archean" revolution, (Genetic Code, origin of life, replicating organisms, some kind of sustainable autotrophy; the emergence of the enzyme Rubisco, or something very like it); and probably the second "revolution" that resulted in photosynthesis (not necessarily oxygen-producing, there are at least two other systems still extant on Earth).... may be relatively "easy." Thus living worlds that have accomplished these developments may be common elsewhere in the universe. Other "revolutions," however, including the endosymbiotic adaptation that resulted in eukaryotes, the remarkable combination of Photosystem I and Photosystem II to create a really powerful system of oxygenating photosynthesis (resulting in the evolution of cyanobacteria, which were subsequently endosymbiotically combined with eukaryotes to produce plants), may have relied on chance circumstances sufficiently unlikely that comparable events may not frequently occur in the history of life elsewhere, such that complex life may be quite rare in the universe. The evolution of macroscopic organization, i.e., the Cambrian Explosion, they seem to treat as more or less inevitable, but it couldn't have happened without these other, less likely, earlier revolutions. Then there's the Great Fourth Biological Revolution: the emergence of human culture. We are already processing 1/10 of the 100,000 gW/sec. of energy that the entire rest of the biosphere produces, and, as Lovelock discusses in his most recent book (A Rough Ride to the Future), our "rate of evolution" (transmitted as information outside our bodies, not just our genes), is about 1 million times faster than previous biological evolution. So our existence is a very big deal in the history of life on earth, objectively. (Many people are resistant to this idea, but if you really think about it, it's actually undeniable). These guys seem to think this development is also probably rather unlikely. In 600 million years, since the emergence of macroscopic animals, no other animal, including our close relatives the chimps and gorillas, even came close. Hard to say, but you could imagine, as Stephen Gould used to analogize, "replaying the tape," a number of times, even starting with, say, the Mesozoic, and not getting the equivalent of humans most of the time. 

Incidentally, I am not at all sure that the first of the "revolutions," which Lenton and Watson seem to treat as pretty likely, namely the origin of life at all (what they refer to as "Inception") isn't just possibly the most unlikely of all. We just don't know. Other than the fact that it seems to have occurred on Earth at just about the earliest physically possible date, I've not seen an explanation for why this should be considered an "easy" transition. From non-life to life? Seems to me quite conceivable, as old fashioned thinkers used to argue, that this one could turn out to have been spectacularly unlikely. We modern folks (including me) prefer to think that life is common in the universe, but there is no real hard evidence for that presumption. 
All of this has implication for our favorite topic, the prevalence, or non-prevalence, of human-equivalent civilized life elsewhere in the universe. Of course no one knows, for sure. But there is a pretty robust intellectual case for the idea that even planets as favorably situated at the outset for the emergence of life as Earth was at the outset, may only quite rarely result in the emergence of intelligent beings and technological civilizations. 

Lurking behind all of this ratiocination is one or other level of the anthropic principle. We cannot, of course, really say whether the combined probability of all of these "unlikely" revolutions adds up to the Earth being a nearly impossible miracle, or something much more likely to occur, in broad outlines. Because, it almost goes without saying at this point, but for all of this having occurred, just as it did, we would not be here to think about it. So we cannot assess, without more information about other instances of life, how likely rough alternatives may or may not have been, which might have led to our rough equivalents. Or not. 
The authors are relying on two things. The complexity of the adaptations involved, which they plausibly translate into a measure of the "difficulty" for evolution to come up with a specific major adaptive change. And the other is that certain kinds of change, like the evolution of complex body plans from single celled organisms, apparently happened over and over again, which is more than a hint that it's an "easy" development. But the "revolutions" they consider to be "unlikely" occurred only once, and usually after long periods of time at any point during which they could have happened but did not.

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