A short history of nearly everything

billion years it will have receded so far that it won’t keep us steady and we will have to come up with some other solution, but in the meantime you should think of it as much more than just a pleasant feature in the night sky.

For a long time, astronomers assumed that the Moon and Earth either formed together or that the Earth captured the Moon as it drifted by. We now believe, as you will recall from an earlier chapter, that about 4.5 billion years ago a Mars-sized object slammed into Earth, blowing out enough material to create the Moon from the debris. This was obviously a very good thing for us-but especially so as it happened such a long time ago. If it had happened in 1896 or last Wednesday clearly we wouldn’t be nearly so pleased about it. Which brings us to our fourth and in many ways most crucial consideration:

Timing. The universe is an amazingly fickle and eventful place, and our existence within it is a wonder. If a long and unimaginably complex sequence of events stretching back 4.6 billion years or so hadn’t played out in a particular manner at particular times-if, to take just one obvious instance, the dinosaurs hadn’t been wiped out by a meteor when they were-you might well be six inches long, with whiskers and a tail, and reading this in a burrow.

We don’t really know for sure because we have nothing else to compare our own existence to, but it seems evident that if you wish to end up as a moderately advanced, thinking society, you need to be at the right end of a very long chain of outcomes involving reasonable periods of stability interspersed with just the right amount of stress and challenge (ice ages appear to be especially helpful in this regard) and marked by a total absence of real cataclysm. As we shall see in the pages that remain to us, we are very lucky to find ourselves in that position.

And on that note, let us now turn briefly to the elements that made us.

There are ninety-two naturally occurring elements on Earth, plus a further twenty or so that have been created in labs, but some of these we can immediately put to one side-as, in fact, chemists themselves tend to do. Not a few of our earthly chemicals are surprisingly little known. Astatine, for instance, is practically unstudied. It has a name and a place on the periodic table (next door to Marie Curie’s polonium), but almost nothing else. The problem isn’t scientific indifference, but rarity. There just isn’t much astatine out there. The most elusive element of all, however, appears to be francium, which is so rare that it is thought that our entire planet may contain, at any given moment, fewer than twenty francium atoms. Altogether only about thirty of the naturally occurring elements are widespread on Earth, and barely half a dozen are of central importance to life.

As you might expect, oxygen is our most abundant element, accounting for just under 50 percent of the Earth’s crust, but after that the relative abundances are often surprising. Who would guess, for instance, that silicon is the second most common element on Earth or that titanium is tenth? Abundance has little to do with their familiarity or utility to us. Many of the more obscure elements are actually more common than the better-known ones. There is more cerium on Earth than copper, more neodymium and lanthanum than cobalt or nitrogen. Tin barely makes it into the top fifty, eclipsed by such relative obscurities as praseodymium, samarium, gadolinium, and dysprosium.

Abundance also has little to do with ease of detection. Aluminum is the fourth most common element on Earth, accounting for nearly a tenth of everything that’s underneath your feet, but its existence wasn’t even suspected until it was discovered in the nineteenth century by Humphry Davy, and for a long time after that it was treated as rare and precious. Congress nearly put a shiny lining of aluminum foil atop the Washington Monument to show what a classy and prosperous nation we had become, and the French imperial family in the same period discarded the state silver dinner service and replaced it with an aluminum one. The fashion was cutting edge even if the knives weren’t.

Nor does abundance necessarily relate to importance. Carbon is only the fifteenth most common element, accounting for a very modest 0.048 percent of Earth’s crust, but we would be lost without it. What sets the carbon atom apart is that it is shamelessly promiscuous. It is the party animal of the atomic world, latching on to many other atoms (including itself) and holding tight, forming molecular conga lines of hearty robustness-the very trick of nature necessary to build proteins and DNA. As Paul Davies has written: “If it wasn’t for carbon, life as we know it would be impossible. Probably any sort of life would be impossible.” Yet carbon is not all that plentiful even in humans, who so vitally depend on it. Of every 200 atoms in your body, 126 are hydrogen, 51 are oxygen, and just 19 are carbon.[30]

Other elements are critical not for creating life but for sustaining it. We need iron to manufacture hemoglobin, and without it we would die. Cobalt is necessary for the creation of vitamin B₁₂. Potassium and a very little sodium are literally good for your nerves. Molybdenum, manganese, and vanadium help to keep your enzymes purring. Zinc-bless it-oxidizes alcohol.

We have evolved to utilize or tolerate these things-we could hardly be here otherwise-but even then we live within narrow ranges of acceptance. Selenium is vital to all of us, but take in just a little too much and it will be the last thing you ever do. The degree to which organisms require or tolerate certain elements is a relic of their evolution. Sheep and cattle now graze side by side, but actually have very different mineral requirements. Modern cattle need quite a lot of copper because they evolved in parts of Europe and Africa where copper was abundant. Sheep, on the other hand, evolved in copper-poor areas of Asia Minor. As a rule, and not surprisingly, our tolerance for elements is directly proportionate to their abundance in the Earth’s crust. We have evolved to expect, and in some cases actually need, the tiny amounts of rare elements that accumulate in the flesh or fiber that we eat. But step up the doses, in some cases by only a tiny amount, and we can soon cross a threshold. Much of this is only imperfectly understood. No one knows, for example, whether a tiny amount of arsenic is necessary for our well- being or not. Some authorities say it is; some not. All that is certain is that too much of it will kill you.

The properties of the elements can become more curious still when they are combined. Oxygen and hydrogen, for instance, are two of the most combustion-friendly elements around, but put them together and they make incombustible water.[31] Odder still in combination are sodium, one of the most unstable of all elements, and chlorine, one of the most toxic. Drop a small lump of pure sodium into ordinary water and it will explode with enough force to kill. Chlorine is even more notoriously hazardous. Though useful in small concentrations for killing microorganisms (it’s chlorine you smell in bleach), in larger volumes it is lethal. Chlorine was the element of choice for many of the poison gases of the First World War. And, as many a sore-eyed swimmer will attest, even in exceedingly dilute form the human body doesn’t appreciate it. Yet put these two nasty elements together and what do you get? Sodium chloride-common table salt.

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-