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ScienceWeek
ASTROBIOLOGY: WATER AND EXTRATERRESTRIAL LIFE
The following points are made by Philip Ball (Nature 2004 427:19):
1) Is life possible without water? NASA has stated explicitly that its strategy in searching for extraterrestrial life is to "follow the water". But is the space agency thereby overlooking other potentially fertile environments? A recent meeting of physicists, chemists, biochemists, and microbiologists grappled with the question of whether water-free life is feasible. No one can give a definitive negative answer, and neither can we expect the issue to be resolved by a show of hands. Rather, the task has to be that of reducing the basic question to smaller, tractable ones, in the hope that a framework might emerge for moving the discussion beyond mere speculation.
2) The naive response might be to suppose that the question is absurdly terracentric. If one allows -- and it seems a reasonable, though not invulnerable, starting point -- that a liquid of some kind is required simply for efficient mass transport in living systems, the cosmos could provide plenty of alternatives: ammonia, sulphuric acid, liquid carbon dioxide, even the putative hydrocarbon lakes of Saturn's moon Titan.
3) But there is much more to water than that. It has long been recognized as a profoundly anomalous liquid, with properties that set it apart from all others. High heat capacity, expansion on freezing, maximum density at 4 C, high dielectric constant --all of these so-called anomalies, and others, seem critical to its biological role. They are in fact relatively easy to rationalize on the grounds of water's hydrogen-bonded structure, which joins the H2O molecules into a fluctuating, three-dimensional network. Unlike "simple" liquids, water's molecular structure is dominated not by the hard core repulsions between molecules but by the directional, attractive interactions of hydrogen bonds.
4) Is this unusual character an essential, or just an incidental, factor in water's life-giving agency? The apparent "specialness" of water was pointed out in 1913 by Lawrence Henderson (1879-1942), who argued that the Universe seems remarkably "fit" to foster life -- a precursor to the anthropic principle. But there is an inherent danger of circularity here: because life is adaptive, who is to say that it has not simply found ways to exploit what water has to offer? For example, some proteins make use of the fast proton conduction that takes place in water, a consequence of bond-flipping along chains of hydrogen-bonded molecules. (The details are, however, more complicated than implied by the classical Grotthuss mechanism; see N. Agmon Chem. Phys. Lett. 244, 456-462; 1995.) Some proteins use one-dimensional chains of water molecules to carry protons rapidly to active sites in their interior. The hydrogen-bonded network makes water particularly well suited to providing such "proton wires". But if this trick were not available, is there any reason to suppose that life would be stymied?
5) One can postulate that life of any sort will require enzyme-like selectivity of molecular interactions for transmitting chemical information. Water does seem to play many subtle parts in enzyme function, but is it really irreplaceable? Solvation shells can be seen to be active components in protein function. But it is not obvious that other small-molecule solvents could not substitute, in principle.
Nature http://www.nature.com/nature
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ASTROBIOLOGY: ON THE UNIVERSAL NATURE OF BIOCHEMISTRY
In contrast to the view that extraterrestrial life may involve a chemistry and physics not yet imagined on Earth, there is the view that the fundamentals of extraterrestrial life will be very similar to the fundamentals of life on Earth. Although this latter view is considered by many to be too conservative [see related background material below], it has the marked practical advantage, in the context of an actual search for extraterrestrial life, of familiarity: at least we know what we are looking for.
The following points are made by Norman R. Pace (Proc. Natl. Acad. Sci. US 2001 98:805):
1) The author suggests that considering the properties of molecules likely to be needed to replicate and evolve (two of the characteristics that define life), it is predictable that life that we encounter anywhere in the Universe will be composed of organic chemicals that follow the same general principles as our own organic-based terrestrial life. The operational definition of life then becomes: Life is a self-replicating, evolving system expected to be based on organic chemistry.
2) The author suggests that the basic drive of life is to make more of itself. The chemical reactions required for the faithful propagation of a free-living organism necessarily require high degrees of specificity in the interactions of the molecules that carry out the propagation. The author suggests that such specificity requires information in the form of complex molecular structure -- large molecules. The molecules that serve terrestrial organisms typically are very large -- proteins and RNAs with molecular weights of thousands to millions of daltons, or even larger, as in the case of genetic DNA. The author suggests that it is predictable that life, wherever we encounter it, will be composed of macromolecules.
3) The author suggests that only two of the natural atoms, carbon and silicon, are known to serve as the backbone of molecules sufficiently large to carry biological information. Thought on the chemistry of life has generally focused on carbon as unique. As the structural basis for life, one of the important features of carbon is that unlike silicon it can readily engage in the formation of chemical bonds with many other atoms, thereby allowing for the chemical versatility required to conduct the reactions of biological metabolism and propagation. The various organic functional groups, composed of hydrogen, oxygen, nitrogen, phosphorus, sulfur, and a host of metals, such as iron, magnesium, and zinc, provide the enormous diversity of chemical reactions necessarily catalyzed by a living organism. Silicon, in contrast, interacts with only a few other atoms, and the large silicon molecules are monotonous compared with the combinatorial universe or organic macromolecules.
Proc. Nat. Acad. Sci. http://www.pnas.org
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ASTROBIOLOGY: ON INTELLIGENT LIFE IN THE UNIVERSE
The following points are made by J. Cohen and I. Stewart ((Nature 22 Feb 01 409:1119):
1) The authors point out that it is possible to imagine the existence of forms of life very different from those found on Earth, occupying habitats that are unsuitable for our kind of life. Some of those aliens might be technological, because technology is an autocatalytic process, and it follows that some aliens might possess technology well in advance of our own, including interstellar transportation. So much is clear, but this train of logic begs the obvious question of where these intelligent non-humanoid aliens might be.
2) The authors point out that the subject area of this discussion is often called "astrobiology", although in science fiction circles (where the topic has arguably been thought through more carefully than it has been in academic circles) the term "xenobiology" is favored. The authors suggest the difference is significant: Astrobiology is a mixture of astronomy and biology, and the tendency is to assume that the field must be assembled from contemporary astronomy and biology; in contrast, xenobiology is the biology of the strange, and the name inevitably involves the idea of extending contemporary biology into new and alien realms.
3) The authors ask: Upon what science should xenobiology be based? The authors suggest that the history of science indicates that any discussion of alien life will be misleading if it is based on the presumption that contemporary science is the ultimate in human understanding. Consider the position of science a century ago. We believed then that we inhabited a newtonian clockwork Universe with absolute space and absolute time; that time was independent of space; that both were of infinite extent; and that the Universe had always existed, always would exist, and was essentially static. We knew about the biological cell, but we had a strong feeling that life possessed properties that could not be reduced to conventional physics; we had barely begun to appreciate the role of natural selection in evolution; and we had no idea about genetics beyond mendelian numerical patterns. Our technology was equally primitive: cars were inferior to the horse, and there was no radio, television, computers, biotechnology or mobile phones. Space travel was the stuff of fantasy. If the past is any guide, then almost everything we now think we know will be substantially qualified or proven wrong within the next 25 years, let alone another century. Biology, in particular, will not persist in its current primitive form. At present, biology is at a stage roughly analogous to physics when Newton (1642-1727) discovered his law of gravity. "There is an awfully long way to go."
4) The authors point out that evolution on Earth has been in progress for at least 3.8 billion years. "This is deep time --too deep for scenarios expressed in human terms to make much sense. A hundred years is the blink of an eye compared with the time that humans have existed on Earth. The lifespan of the human race is similarly short when compared with the time that life has existed on Earth. It is ridiculous to imagine that somehow, in a single century of human development, we have suddenly worked out the truth about life. After all, we do not really understand how a light switch works at a fundamental level, let alone a mitochondrion."
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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