“Every lifeform we know of evolved in water, and all of them require it for thier biological processes. And although water seems chemically simple - just two hydrogen atoms bound to an oxygen - it is, in fact, an enormously unusual substance. As you know, most compounds contract as they cool and expand as they heat. Water does this, too, until just before it starts to freeze. It then does something remarkable: it begins to expand, even as it grows colder, so that by the time it does freeze, it is actually less dense than it was as a liquid. That is why ice floats instead of sinking, of course. We are so used to seeing that, whether it is ice balls in a beverage or a skin of ice on a pond, that we usually give it no thought. But other substances do not do that: frozen carbon dioxide - what you call dry ice - sinks in liquid carbon dioxide; a lead ingot will sink in a vat of molten lead.”

“But water ice floats - and if it did not, life would be impossible. If lakes and oceans froze from the bottom up, instead of the top down, no sea-floor or lake-bottom ecologies would exist outside equatorial zones. Indeed, once they had started freezing, bodies of water would freeze solid and remain solid forever; it is currents moving unfettered beneath surface ice that promotes melting in the spring - that is why glaciers, which have no such currents beneath them, exist for millennia on dry land adjacent to liquid lakes.”

I returned the eurypterid fossil to its drawer. “I grant that water is an unusual substance, but -” Hollus touched her eyes together. “But this strange expanding-before-freezing is hardly the only remarkable thermal property water has. In fact, it has seven different thermal parameters, all of which are unique or nearly so in the chemical world, and all of which independently are necessary for the existence of life. The chances of any of them having the aberrant value it does must be multiplied by the chances of the other six likewise being aberrant. The likelihood of water having these unique thermal properties by chance is almost nil.”

“Almost,” I said, but my voice was starting to sound hollow, even to me.

Hollus ignored me. “Nor does water's unique nature end with its thermal properties. Of all substances, only liquid selenium has a higher surface tension than does water. And it is water's high surface tension that draws it deeply into cracks in rocks, and, of course, as we have noted, water does the incredible and actually expands as it freezes, breaking those rocks apart. If water had lower surface tension, the process by which soil is formed would not occur. More: if water had higher viscosity, circulatory systems could not evolve - your blood plasma and mine are essentially sea water, but there are no biochemical processes that could fuel a heart that had to pump something substantially more viscous for any appreciable time.”

The alien paused. “I could go on,” she said, “talking about the remarkable, carefully adjusted parameters that make life possible, but the reality is simply this: if any of them - any in this long chain - were different, there would be no life in this universe. We are either the most incredible fluke imaginable - something far, far more unlikely than you winning your provincial lottery every single week for a century - or the universe and its components were designed, purposefully and with great care, to give rise to life.”