Forget Genesis, forget the Big Bang. The earth was not created by some higher being (though they themselves might argue otherwise), but by a bunch of roudy wizards from another part of the multiverse.
In the original Science of the Discworld, Earth, or rather 'Roundworld' (but that's not possible, things would fall off the bottom), was created as an annomilous by-product of splitting the 'thaum' - the magical equivalent of the atom, understood by few, abused by many
(especially if it involves fireworks directed at other wizards).
The Science of the Discworld told the story of Roundworld, from creation to destruction, and the scientific princpals behind it. The narrative of Terry Prachett provided a witty medium through which Messieurs Stewart and Cohen could explain to the masses why things are as they are.
But wizards are wizards. Once the initial excitement wore off, the mini-universe got shunted out of the way. After all "It mostly had ice ages, and was less engrossing than an ant farm." But they missed something. Us. More to the point, the elves didn't.
The Discworldian story can be read pretty much as a self-contained novella, but a)that is not the point and b)you miss half the fun. While all the ingredients of Pratchett's humour and 'narrativium' remain, we are gently lead into the seemingly daunting world of science.
It is a world which many would have hoped to have left behind at school, but while we are not spoon-fed the 'children-lies' told at schools, we are not inundated with overly complicated description and jargon. The balance is very difficult to maintain, but Messieurs Stewart and Cohen manage admirably.
It is a study of humanity, from prehistoric origins to modern existence; from birth and through life; exploring why we are here (because a meteorite killed the crabs - see Science of the Discworld I) and why London still isn't a Neandathal encampment. It is the exploration of mind, intelligence, art and science. With gags.
Some previous knowledge of the Discworld setting is needed to give full weight to the book; the development of Hex, for example is covered in numerous novels. However, for the general purposes of the book, one can read it having only read the first Science of the Discworld.
To those who love the Discworld, I'd say that even just the story is worth it, but the sciency bits are not incomprehensible and makes the series better. If this is the first book on Discworld that you've read, I'd say wait until reading #1, or even a couple of the novels themselves (Lords and Ladies is particularly relevant to this one).
Over all, a brilliant read, definitely something to fill some of the gap before the next novel.