Tuesday, November 14, 2006

Was life inevitable?

Here’s the unexpurgated version of my latest story for news@nature. There’s a lot of really interesting back story here, which I hope to return to at some point. This is far and away some of the most interesting “origin of life” work I’ve seen for some time.

Life may be the ultimate in planetary stress relief, a new theory claims

The appearance of life on Earth seems to face so many obstacles that scientists often feel forced to regard it almost as miraculous. Now two scientists working at the Santa Fe Institute in New Mexico suggest that, on the contrary, it may have been inevitable.

They argue that life was the necessary consequence of the build-up of available energy on the early Earth, thanks to purely geological processes. They regard it as directly analogous to the way lightning relieves the build-up of electrical charge in thunderclouds.

In other words, say Harold Morowitz and Eric Smith in a preprint posted on the Santa Fe Institute archive [1], the geological environment "forced life into existence".

This view, the researchers say, implies not only that life had to emerge on the Earth, but that the same would happen on any similar planet. And they hope that ultimately it will be possible to predict the first steps in the origin of life based on the laws of physics and chemistry alone.

Their proposal is "instructive and inspiring", says Michael Russell, a specialist in the origin of life at the California Institute of Technology in Pasadena.

Morowitz and Smith admit that they don't yet have the theoretical tools to clinch their arguments, or to show what form this "inevitable life" must take. But they argue that it is likely to have used the same chemical processes that now drive our own metabolism – but in reverse.

They say that the young Earth would have been accumulating energy from geological processes much as a dam accumulates gravitational potential energy by piling up water. Sooner or later, something had to give.

One source of such energy would have been energy-rich compounds called polyphosphates, generated in volcanic processes. These are 'battery molecules', analogous to the compound ATP, the ubiquitous source of metabolic energy in living cells.

Another source would have been hydrogen molecules, which are likely to have been abundant in the early atmosphere even though they are almost absent today. Hydrogen would have been generated, for example, by reactions between seawater and dissolved iron.

Energy-releasing reactions between hydrogen and carbon dioxide (a volcanic gas) in the atmosphere can produce complex organic molecules, the precursors of living systems.

In our own metabolism, a series of biochemical reactions called the citric-acid cycle breaks down organic compounds from food into carbon dioxide. Horowitz and Smith say that the energy reservoirs of the young Earth could have driven a citric-acid cycle in reverse, spawning the building blocks of life while relaxing the 'energy pressure' of the environment. Eventually these processes will have become encapsulated in cells, which makes the 'energy-conducting' flows more efficient.

Life, agrees Russell is "a chemical system that drains and dissipates chemical energy." He has used similar ideas to argue that "life would emerge using the same pathways on any sunny, wet rocky planet" [2,3]. But he believes that the most likely place for it to occur was at miniature subsea volcanoes called hydrothermal vents, where the ingredients and conditions are just right for energy-harnessing chemical machinery to develop [4].

The biochemical processes of living organisms are highly organized. Scientists have long puzzled over how these 'ordered' systems can come spontaneously into being, when the Second Law of Thermodynamics suggests that the universe as a whole tends to generate increasing disorder.

The answer, broadly speaking, is that local clumps of order come at the expense of increasing the disorder in their environment. But Horowitz and Smith suggest a rationale for why such concentrations of order should happen in the first place. They draw on the idea, proposed in the 1980s by Rod Swenson of the University of Connecticut, that ordered states are much better 'lightning' conductors' for discharging excess energy.

Thus, they say, despite several major extinctions throughout geological time, when most of life on Earth was obliterated, life itself was never in danger of disappearing – because an Earth with life is always more stable than one without. They call this 'condensation' of life from the energy-rich environment a "collapse to life", which in their view is as inevitable as the appearance of snowflakes in cold, moist air.

References
1. Morowitz, H. & Smith, E. Santa Fe Institute Working Paper (2006).
2. Russell, M. J. & Hall, A. J. in Hiscox, J. A. (ed.) The Search for Life on Mars, 26-36 (British Interplanetary Society, 1999).
3. Russell, M. J. et al. in Ikan, R. (ed.) Natural and Laboratory-Simulated Thermal Geochemical Processes, 325-388 (Kluwer, Dordrecht, 2003).
4. Martin, W. & Russell, M. J. Phil. Trans. Roy. Soc. B online publication doi:10.1098/rstb.2006.1881 (2006).

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