One encouraging thing the Guide does have to say on the subject of parallel universes is that you don't stand the remotest chance of understanding it. You can therefore say `What?' and `Eh?' and even go cross-eyed and start to blither if you like without any fear of making a fool of yourself.
The first thing to realise about parallel universes, the Guide says, is that they are not parallel.
It is also important to realise that they are not, strictly speaking, universes either, but it is easiest if you try and realise that a little later, after you've realised that everything you've realised up to that moment is not true.
The reason they are not universes is that any given universe is not actually a thing as such, but is just a way of looking at what is technically known as the WSOGMM, or Whole Sort of General Mish Mash. The Whole Sort of General Mish Mash doesn't actually exist either, but is just the sum total of all the different ways there would be of looking at it if it did.
The reason they are not parallel is the same reason that the sea is not parallel. It doesn't mean anything. You can slice the Whole Sort of General Mish Mash any way you like and you will generally come up with something that someone will call home.
Please feel free to blither now.
The Earth with which we are here concerned, because of its particular orientation in the Whole Sort of General Mish Mash, was hit by a neutrino that other Earths were not.
A neutrino is not a big thing to be hit by.
In fact it's hard to think of anything much smaller by which one could reasonably hope to be hit. And it's not as if being hit by neutrinos was in itself a particularly unusual event for something the size of the Earth. Far from it. It would be an unusual nanosecond in which the Earth was not hit by several billion passing neutrinos.
It all depends on what you mean by `hit', of course, seeing as matter consists almost entirely of nothing at all. The chances of a neutrino actually hitting something as it travels through all this howling emptiness are roughly comparable to that of dropping a ball bearing at random from a cruising 747 and hitting, say, an egg sandwich.
Anyway, this neutrino hit something. Nothing terribly important in the scale of things, you might say. But the problem with saying something like that is that you would be talking cross- eyed badger spit. Once something actually happens somewhere in something as wildly complicated as the Universe, Kevin knows where it will all end up --- where `Kevin' is any random entity that doesn't know nothin' about nothin'.
This neutrino struck an atom.
The atom was part of a molecule. The molecule was part of a nucleic acid. The nucleic acid was part of a gene. The gene was part of a genetic recipe for growing... and so on. The upshot was that a plant ended up growing an extra leaf. In Essex. Or what would, after a lot of palaver and local difficulties of a geological nature, become Essex.
The plant was a clover. It threw its weight, or rather its seed, around extremely effectively and rapidly became the world's dominant type of clover. The precise causal connection between this tiny biological happenstance, and a few other minor variations that exist in that slice of the Whole Sort of General Mish Mash --- such as Tricia McMillan failing to leave with Zaphod Beeblebrox, abnormally low sales of pecan-flavoured ice-cream and the fact that the Earth On which all this occurred did not get demolished by the Vogons to make way for a new hyperspace bypass --- is currently sitting at number 4,763,984,132 on the research project priority list at what was once the History Department of the University of MaxiMegalon, and no one currently at the prayer meeting by the poolside appears to feel any sense of urgency about the problem.