By Philip Ball
Should we be surprised to be here? Some scientists maintain that the origin of life is absurdly improbable--Nobel laureate and 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." Yet others look at the size of the cosmos and conclude that even such extremely low-probability events are inevitable.
One might reasonably argue that the question has little meaning while we still have only a few hundred worlds to compare, and know next to nothing about most of them. 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 luck: 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. Although Darwinian evolution needed billions of years to find a route from microbe to man, it seems that going from mineral to microbe took barely a moment.
Now, Richard Wolfenden and his colleagues at the University of North Carolina, Chapel Hill, say that may be largely a question of chemical kinetics. 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?
In a paper published in the Proceedings of the National Academy of Sciences, the researchers argue that heat 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 degrees Celsius to 100 degrees C, for example, increases the rate of some reactions relevant to the earliest forms of biochemistry 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 explanation for why that is the case.
So this kind of warm beginning could reduce the timescales needed for a working primitive biochemistry from millions to tens of years. What's more, Wolfenden and colleagues say that some of the best non-enzyme catalysts of slow metabolic reactions, which might have served as proto-enzymes, become more effective as the temperature is lowered. If that's what happened on the early Earth, then once catalysis took over from acceleration driven by heat, the biochemical reactions would go on even as the environment cooled or as life spread to cooler regions.
If this scenario is right, it could effectively limit the kind of worlds that can 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: examples include the liquid hydrocarbons of Saturn's moon Titan, ammonia (on Jupiter, say), formamide (HCONH2) or water-ammonia mixtures. One can cite reasons why in some respects these 'cold' liquids might be better solvents for life than water. 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.
What about hotter-than-water liquids? They have their own problems: quite aside from the difficulty of imagining plausible biochemistries in, say, molten silicates, complex molecules would tend more readily to fall apart in extreme heat, both because bonds snap more easily and because entropy favors disintegration over union. All of which could lend credence to the suggestion of biochemist Lawrence Henderson in 1913 that water is peculiarly favorable to the evolution of life. 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 the right amount of heat.