Phillip Ball-
The more we get to know about the molecular machinery of the cell, the more we realise that it could not function in any liquid except water. No other liquid has a structure as subtle as that of water (even though some other compounds can form hydrogen bonds), and this structure seems to be essential for the kind of delicate chemistry that makes life possible. Even if alien organisms use molecules other than proteins and DNA, it's hard to see how they could avoid being comparably complex - and how, therefore, they could relinquish the need for an active, sympathetic solvent and mediator like water.
In the early twentieth century, an American biochemist named Lawrence Henderson argued that water seems so beautifully and uniquely suited for supporting life that it is hard not to perceive it as designed for this purpose. Henderson did not know about the fine details of how water gives a helping hand to the molecules of life (although he would have been quite delighted if he had). But he could see that the many 'anomalous' properties of water already known made it an incomparable 'matrix' for life. The large heat capacity, which helps the oceans maintain a steady temperature, does just the same thing for organisms (which are, remember, mostly water)- it is perfect for temperature regulation. Lawrence pointed out that another method of heat regulation is evaporation: when liquid water changes to water vapour, it imbibes a great deal of energy (more than other liquids). This provides a way to prevent overheating, and it is why we sweat. Lakes can stay at a constant temperature under intense sunlight for the same reason: as water evaporates from the surface, it is rather as if the lake is sweating.
Henderson collected together many other examples of the way water seems fine-tuned to support life. Its unusually large surface tension, for instance, means that water is pulled up through the empty pores and channels in soil by capillary action, making it accessible to plants growing at the surface even if the water table is several feet lower than the roots. Henderson believed that water was uniquely 'fit' in Darwin's sense: it was perfectly adapted to sustain life. Thus he believed that evolution of organisms - survival of the fittest, as the rather crude caricature of those times expressed it- takes place in a 'fit' environment. 'Water, of its very nature', he said, 'as it occurs automatically in the process of cosmic evolution, is fit, with a fitness no less marvelous and varied than that fitness of the organism which has been won by the process of adaptation in the course of organic evolution.' Henderson considered that carbon compounds are also remarkably and uniquely attuned to serve as the building blocks of life- that carbon in some sense makes life inevitable. 'The biologist', he concluded, 'may now rightly regard the universe in its very essence as biocentric.'
