A short history of nearly everything - Bill Bryson [124]
By and large, if an element doesn't naturally find its way into our systems—if it isn't soluble in water, say—we tend to be intolerant of it. Lead poisons us because we were never exposed to it until we began to fashion it into food vessels and pipes for plumbing. (Not incidentally, lead's symbol is Pb, for the Latin plumbum, the source word for our modern plumbing.) The Romans also flavored their wine with lead, which may be part of the reason they are not the force they used to be. As we have seen elsewhere, our own performance with lead (not to mention mercury, cadmium, and all the other industrial pollutants with which we routinely dose ourselves) does not leave us a great deal of room for smirking. When elements don't occur naturally on Earth, we have evolved no tolerance for them, and so they tend to be extremely toxic to us, as with plutonium. Our tolerance for plutonium is zero: there is no level at which it is not going to make you want to lie down.
I have brought you a long way to make a small point: a big part of the reason that Earth seems so miraculously accommodating is that we evolved to suit its conditions. What we marvel at is not that it is suitable to life but that it is suitable to our life—and hardly surprising, really. It may be that many of the things that make it so splendid to us—well-proportioned Sun, doting Moon, sociable carbon, more magma than you can shake a stick at, and all the rest—seem splendid simply because they are what we were born to count on. No one can altogether say.
Other worlds may harbor beings thankful for their silvery lakes of mercury and drifting clouds of ammonia. They may be delighted that their planet doesn't shake them silly with its grinding plates or spew messy gobs of lava over the landscape, but rather exists in a permanent nontectonic tranquility. Any visitors to Earth from afar would almost certainly, at the very least, be bemused to find us living in an atmosphere composed of nitrogen, a gas sulkily disinclined to react with anything, and oxygen, which is so partial to combustion that we must place fire stations throughout our cities to protect ourselves from its livelier effects. But even if our visitors were oxygen-breathing bipeds with shopping malls and a fondness for action movies, it is unlikely that they would find Earth ideal. We couldn't even give them lunch because all our foods contain traces of manganese, selenium, zinc, and other elemental particles at least some of which would be poisonous to them. To them Earth might not seem a wondrously congenial place at all.
The physicist Richard Feynman used to make a joke about a posteriori conclusions, as they are called. “You know, the most amazing thing happened to me tonight,” he would say. “I saw a car with the license plate ARW 357. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight? Amazing!” His point, of course, was that it is easy to make any banal situation seem extraordinary if you treat it as fateful.
So it is possible that the events and conditions that led to the rise of life on Earth are not quite as extraordinary as we like to think. Still, they were extraordinary enough, and one thing is certain: they will have to do until we find some better.
17 INTO THE TROPOSPHERE
THANK GOODNESS FOR the atmosphere. It keeps us warm. Without it, Earth would be a lifeless ball of ice with an average temperature of minus 60 degrees Fahrenheit. In addition, the atmosphere absorbs or deflects incoming swarms of cosmic rays, charged particles, ultraviolet rays, and the like. Altogether, the gaseous padding of the atmosphere is equivalent to a fifteen-foot thickness of protective concrete, and without it these invisible visitors from space would slice through us like tiny daggers. Even raindrops would pound us senseless if it weren't for the atmosphere's slowing drag.
The most striking thing about our atmosphere is that there isn't