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Most stars – and our sun is typical – burn in a stable manner for thousands of millions of years. Very rarely, somewhere in the galaxy a star suddenly flares without obvious warning into a supernova. Within a period of a few weeks, it increases in brightness by a factor of many billions and then dies away to a dark remnant of its former self. During its few high days as a supernova, a star may radiate more energy than in all its previous hundred million years as an ordinary star. If our own sun were to “go supernova,” the entire solar system would be vaporized on the instant. Fortunately this is very unlikely. In our galaxy of a hundred billion stars, only three supernovas have been recorded by astronomers: in 1054, in 1572, and in 1604. The Crab Nebula is the remains of the event of 1054, recorded by Chinese astronomers. (When I say the event “of 1054” I mean, of course, the event of which news reached Earth in 1054. The event itself took place six thousand years earlier. The wave-front of light from it hit us in 1054.) Since 1604, the only supernovas that have been seen have been in other galaxies.

There is another type of explosion a star can sustain. Instead of “going supernova” it “goes information.” The explosion begins more slowly than a supernova and takes incomparably longer to build up. We can call it an information {136} bomb or, for reasons that will become apparent, a replication bomb. For the first few billion years of its build-up, you could detect a replication bomb only if you were in the immediate vicinity. Eventually, subtle manifestations of the explosion begin to leak away into more distant regions of space and it becomes, at least potentially, detectable from a long way away. We do not know how this kind of explosion ends. Presumably it eventually fades away like a supernova, but we do not know how far it typically builds up first. Perhaps to a violent and self-destructive catastrophe. Perhaps to a more gentle and repeated emission of objects, moving, in a guided rather than a simple ballistic trajectory, away from the star into distant reaches of space, where it may infect other star systems with the same tendency to explode.

The reason we know so little about replication bombs in the universe is that we have seen only one example, and one example of any phenomenon is not enough to base generalizations on. Our one case history is still in progress. It has been under way for between three billion and four billion years, and it has only just reached the threshold of spilling away from the immediate vicinity of the star. The star concerned is Sol, a yellow dwarf star lying toward the edge of our galaxy, in one of the spiral arms. We call it the sun. The explosion actually originated on one of the satellites in close orbit around the sun, but the energy to drive the explosion all comes from the sun. The satellite is, of course, Earth, and the four-billion-year-old explosion, or replication bomb, is called life. We humans are an extremely important manifestation of the replication bomb, because it is through us – through our brains, our symbolic culture and our technology – that the explosion may proceed {137} to the next stage and reverberate through deep space.

As I have said, our replication bomb is, to date, the only one we know of in the universe, but this does not necessarily mean that events of this kind are rarer than supernovas. Admittedly, supernovas have been detected three times as frequently in our galaxy, but then supernovas, because of the immense quantities of energy released, are much easier to see from a long distance. Until a few decades ago, when man-made radio waves started to radiate outward from the planet, our own life explosion would have gone undetected by observers even on quite close planets. Probably the only conspicuous manifestation of our life explosion until recent times would have been the Great Barrier Reef.

A supernova is a gigantic and sudden explosion. The triggering event of any explosion is that some quantity is tipped over a critical value, after which things escalate out of control to produce a result far larger than the original triggering event. The triggering event of a replication bomb is the spontaneous arising of self-replicating yet variable entities. The reason self-replication is a potentially explosive phenomenon is the same as for any explosion: exponential growth – the more you have, the more you get. Once you have a self-replicating object, you will soon have two. Then each of the two makes a copy of itself and then you have four. Then eight, then sixteen, thirty-two, sixty-four… After a mere thirty generations of this duplication, you will have more than a billion of the duplicating objects. After fifty generations, there will be a thousand million million of them. After two hundred generations, there will be a million million million million million million million million million million. In theory. In practice it could never {138} come to pass, because this is a larger number than there are atoms in the universe. The explosive process of self-copying has got to be limited long before it reaches two hundred generations of unfettereddoubling.

We have no direct evidence of the replication event that initiated the proceedings on this planet. We can only infer that it must have happened because of the gathering explosion of which we are a part. We do not know exactly what the original critical event, the initiation of self-replication, looked like, but we can infer what kind of an event it must have been. It began as a chemical event.

Chemistry is a drama that goes on in all stars and on all planets. The players in chemistry are atoms and molecules. Even the rarest of atoms are extremely numerous by the standards of counting to which we are accustomed. Isaac Asimov calculated that the number of atoms of the rare element astatine- 215 in the whole of North and South America to a depth often miles is “only a trillion.” The fundamental units of chemistry are forever changing partners to produce a shifting but always very large population of larger units – molecules. However numerous they are, molecules of a given type – unlike, say, animals of a given species or Stradivarius violins – are always identical. The atomic dance routines of chemistry lead to some molecules becoming more populous in the world while others become scarcer. A biologist is naturally tempted to describe the molecules that become more numerous in the population as “successful.” But it is not helpful to succumb to this temptation. Success, in the illuminating sense of the word, is a property that arises only at a later stage in our story. {139}

What, then, was this momentous critical event that began the life explosion? I have said that it was the arising of self-duplicating entities, but equivalently we could call it the origination of the phenomenon of heredity – a process we can label “like begets like.” This is not something molecules ordinarily exhibit. Water molecules, though they swarm in gigantic populations, show nothing approaching true heredity. On the face of it, you might think they do. The population of water molecules (H₂O) increases when hydrogen (H) burns with oxygen (O). The population of water molecules decreases when water is split, by electrolysis, into bubbles of hydrogen and oxygen. But although there is a kind of population dynamics of water molecules, there is no heredity. The minimal condition for true heredity would be the existence of at least two distinct kinds of H₂O molecule, both of which give rise to (“spawn”) copies of their own kind.

Molecules sometimes come in two mirror varieties. There are two kinds of glucose molecule, which contain identical atoms Tinkertoyed together in an identical way except that the molecules are mirror images. The same is true of other sugar molecules, and lots of other molecules besides, including the all-important amino acids. Perhaps here is an opportunity for “like begets like” – for chemical heredity. Could right-handed molecules spawn right-handed daughter molecules and left-handers spawn southpaw daughter molecules? First, some background information on mirror-image molecules. The phenomenon was first discovered by the great nineteenth-century French scientist Louis Pasteur, who was looking at crystals of tartrate, which is a {140} salt of tartaric acid, an important substance in wine. A crystal is a solid edifice, big enough to be seen with the naked eye and, in some cases, worn around the neck. It is formed when atoms or molecules, all of the same type, pile on top of one another to form a solid. They don't pile up hugger-mugger but in an orderly geometric array, like guardsmen of identical size and immaculate drill. The molecules that are already part of the crystal constitute a template for the addition of new molecules, which come out of a watery solution and fit it exactly, so the whole crystal grows as a precise, geometric lattice. This is why salt crystals have square facets and diamond crystals are tetrahedral (diamond-shaped). When any shape acts as a template for building another shape like itself, we have an inkling of the possibility of self-replication.