Some like it hot

I have been slack with my postings over the past couple of weeks, so here comes the catching up. First, a Muse for Nature News on a curious paper in PNAS on the origin of life, which seemed to have a corollary not explored by the authors… (I can’t link to the PNAS paper, as it’s not yet been put online, and in the meantime languishes in that peculiar limbo that PNAS commands.)

Heat may have been necessary to ensure that the first prebiotic reactions didn’t take an eternity. If so, this could add weight to the suggestion that water is essential for life in the cosmos.

Should we be surprised to be here? Some scientists maintain that the origin of life is absurdly improbable – Nobel laureate biologist George Wald baldly stated in 1954 that ‘one has only to contemplate the magnitude of [the] task to concede that the spontaneous generation of a living organism is impossible’ [1]. Yet others look at the size of the cosmos and conclude that even such extremely low-probability events are inevitable.

The apparent fine-tuning of physical laws and fundamental constants to enable life’s existence certainly presents a profound puzzle, which the anthropic principle answers only through the profligate hypothesis of multiple universes of which we have the fortune to occupy one that is habitable. But even if we take the laws of nature as we find them, it is hard to know whether or not we should feel fortunate to exist.

One might reasonably argue that the question has little meaning while we still have only a few hundred worlds to compare, about most of which we know next to nothing (not even whether there is, or was, life on our nearest neighbour). But one piece of empirical evidence we do have seems to challenge the notion that the origin of terrestrial life was a piece of extraordinarily good fortune: the geological record implies that life began in a blink, almost the instant the oceans were formed. It is as if it was just waiting to happen – as indeed some have suggested [2]. While Darwinian evolution needed billions of years to find a route from microbe to man, it seems that going from mineral to microbe needs barely a moment.

According to a paper in the Proceedings of the National Academy of Sciences USA by Richard Wolfenden and colleagues at the University of North Carolina, that may be largely a question of chemical kinetics [3]. Just about all the key biochemical processes in living organisms are speeded up by enzyme catalysis; otherwise they would happen too slowly or indiscriminately to make metabolism and life feasible. Some key processes, such as reactions involved in biosynthesis of nucleic acids, happen at a glacial pace without enzymes. If so, how did the earliest living systems bootstrap themselves to the point where they could sustain and reproduce themselves with enzymatic assistance?

The researchers think that temperature was the key. They point out that, not only do reactions speed up with temperature more than is commonly appreciated, but that the slowest reactions speed up the most: a change from 25 C to 100 C, for example, increases the rate of some prebiotically relevant reactions by 10 million-fold.

There’s reason to believe that life may have started in hot water, for example around submarine volcanic vents, where there are abundant supplies of energy, inorganic nutrients and simple molecular building blocks. Some of the earliest branches in the phylogenetic tree of life are occupied by thermophilic organisms, which thrive in hot conditions. A hot, aqueous origin of life is probably now the leading candidate for this mysterious event.

This alone, then, could reduce the timescales needed for a primitive biochemistry to get going from millions to tens of years. What’s more, say Wolfenden and colleagues, some of the best non-enzyme catalysts of slow metabolic reactions, which might have served as prebiotic proto-enzymes, becomes more effective as the temperature is lowered. If that’s what happened on the early Earth, then once catalysis took over from simple temperature-induced acceleration, it would have not suffered as the environment cooled or as life spread to cooler regions.

If this scenario is right, it could constrain on the kinds of worlds that support life. We know that watery worlds can do this; but might other simple liquids act as solvents for different biochemistries? In general, these have lower freezing points than water, such as the liquid hydrocarbons of Saturn’s moon Titan, ammonia (on Jupiter, say), formamide (HCONH2) or water-ammonia mixtures. One can enumerate reasons why in some respects these ‘cold’ liquids might be better solvents for life than water [4]. But if the rates of prebiotic reactions were a limiting factor in life’s origin, it may be that colder seas would never move things along fast enough.

Hotter may not be better either: quite aside from the difficulty of imagining plausible biochemistries in molten silicates, complex molecules would tend more readily to fall apart in extreme heat both because bonds snap more easily and because entropy favours disintegration over union. All of which could lend credence to the suggestion of biochemist Lawrence Henderson in 1913 that water is peculiarly biophilic [5]. In the introduction to a 1958 edition of Henderson’s book, Wald wrote ‘we now believe that life… must arise inevitably wherever it can, given enough time.’ But perhaps what it needs is not so much enough time, but enough heat.

References
1. G. Wald, Sci. Am. 191, 44-53 (1954).
2. H. J. Morowitz & E. Smith, Complexity 13, 51-59 (2007).
3. R. B. Stockbridge, C. A. Lewis Jr, Y. Yuan & R. Woldenden, Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1013647107.
4. S. A. Benner, in Water and Life (eds R. M. Lynden-Bell, S. Conway Morris, J. D. Barrow, J. L. Finney & C. L. Harper, Jr, Chapter 10. CRC Press, Boca Raton, 2010.
5. L. J. Henderson, The Fitness of the Environment. Macmillan, New York, 1913.