At any rate, it very much looks as if all that `junk' DNA in your genome is not junk at all. On the contrary, it may be a crucial part of what makes you human.

This lesson is driven home by those business associates of parasitic wasps, the symbiotic polydnaviruses, sneakily buried inside the wasps' own DNA. There is a message there about human evolution, and it's a very strange one.

Genome-sequencing may have been oversold as the answer to human diseases, but it's very good basic science. The activities of the sequencers have revealed that wasps are not the only organisms to have bits of viral DNA hanging around in their genomes. In fact, most creatures do, humans included. The human genome even contains one complete viral genome, and only one, called ERV-3 (Endogenous RetroVirus). This may seem an evolutionary oddity, a bit of `junk DNA' that really is junk ... but, actually, without it none of us would be here. It plays the absolutely crucial role of preventing rejection of the foetus by the mother. Mother's immune system `ought to' recognise the tissue of a developing baby as `foreign', and trigger actions that will get rid of it. By `ought to' here we mean that this is what the immune system normally does for tissue that is not the mother's own.

Apparently, the ERV-3 protein closely resembles another one called p15E, which is part of a widespread defence system used by viruses to stop their hosts killing them off. The p15E protein stops lymphocytes, a key type of cell in the immune system, from responding to antigens, molecules that reveal the virus's foreign nature. At some stage during mammalian evolution, this defence system was stolen from the viruses and used to stop the female placenta responding to antigens that reveal the foreign nature of the foetus's father. Perhaps on the principle of being hung for a sheep as well as for a lamb, the human genome decided to go the whole hog [1] and steal the entire retroviral genome.

When evolution carried out the theft, however, it did not just dump its booty into the human DNA sequence unchanged. It threw in a couple of introns, too, splitting ERV-3 into several separate pieces. It's complete, but not connected. No matter: enzymes can easily snip out the introns when that bit of DNA is turned into protein. But no one knows why the introns are there. They might be an accidental intrusion. Or - pursuing the RNA interference idea - they might be much more significant. Those introns might be an important part of the genetic regulatory system, `text messages' that let the placenta use ERV-3 without running the risk of setting the corresponding virus loose.

At any rate, whatever the introns are for, warm-bloodedness is not the only trick that mammalian evolution managed to find and exploit.

[1] Sorry about the proliferation of barnyard metaphors.

278

279

I'HE SCIENCE OF DISCWORLD

It also indulged in wholesale theft of a virus's genome, to stop mother's immune system booting out baby because it `smelled' of father. And we also get another lesson that DNA isn't selfish. ERV-3 is present in the human genome, but not because it's a bit of junk that gets copied along with everything else and has remained because it does no harm. It's there because, in a very real sense, humans could not survive - could not even reproduce - without it.

NOUGAT SURPRISE

THE ACTIVITY IN THE GREAT HALL was slowing down now. All along the rainbow lines of time, the nodes were closed. Or shut, or denoded, Rincewind thought. Whatever you did with nodes, anyway.

There was a little cheer as the last glowing wizard symbol faded away, and a roar from outside as three wizards and a lot of tentacles landed in the fountain. Rincewind had been surprised about that, and then dismayed. Since it did not have his name on it, this meant that Ridcully had something worse in mind.

`Looks like I'm not needed now,' he said hopefully, just in case.

'Haha, professor, what a card you are and no mistake,' said the lobster next to him. `The Archchancellor was very definite that we was to keep you here no matter what you said.'

`But I wasn't running away!'

`No, you was only inspecting the wall with the loose bricks in it,' said the lobster.[1] `We quite understand. Lucky for you we caught you before you dropped over into the alley, eh? Could've done yerself a mischief.'

Rincewind sighed. The lobsters were always hard to outrun. They hunted in packs, appeared to share a common brain, and many years

[1] The University's proctors were known as lobsters because they went very red when hot and had a grip that was extremely hard to shake off. They were generally ex-army sergeants, had depths of cynicism unplumbable by any line, and were fuelled by beer.

of harrying erring students had given them a malignant street cunning that verged on the supernatural.

Some of the senior wizards swept in ... There was an argument going on between Ridcully and the Dean.

`I don't see why I shouldn't.'

`Because you get too excited in the presence of combat, Dean. You run around making silly "hut, hut" noises,' said Ridcully. `Remember why we had to stop those paintball afternoons? You didn't seem to be able to get the hang of the term "people on my side".'

`Yes, but this is-'

`It took us a week to get the Senior Wrangler looking halfway suitable for polite company - Ah, Rincewind. Still with us, I see. Good man. Very well, Mr Stibbons: report!'

Ponder coughed.

`Hex confirms that, er, that our recent activities may have left camiloops between our word and Roundworld, Sir. That is to say, residual connections that may be used deliberately or inadvertently from either side. Magic doors, in fact, drifting without anchor. These will evaporate within a matter of days. Um ...'

`I don't want to hear "um", Mr Stibbons. "Um" is not a word we entertain here.'

`Well, the fact is that since the camiloops are spread over centuries, the Auditors may very well have been in Roundworld for some time. We have no way of knowing for how long. Hex, um, sorry, does report some circumstantial evidence that humans are dimly aware of their malign meddling, albeit at a very mundane level as evidenced by the findings of a researcher called Murphy. Roundworld would be difficult for them. They would be bewildered. Things would not work the way they expect. They are not flexible thinkers.'

`They were able to mess around with Mr Darwin's voyage!'

`By doing lots of small and rather stupid things at great effort, sir. They don't react well to adversity. They get petulant. From what Professor Rincewind tells us, many hundreds of them have to combine to perform even a simple physical action.'

He stood back and indicated some items laid out on a dining table.

`There is some evidence that Auditors, being embodiments of physical laws, find it hard to deal with nonsensical or contradictory instructions. Therefore, I have prepared these.'

He flourished something that looked like a table-tennis bat. On it were printed the words: `Do Not Read This Sign.'

`That works, does it?' said the Dean, doubtfully.

`It's said to put their minds into a fugue state, Dean. They feel confused and alone, and evaporate instantly. Being alone means having a sense of self, and any Auditor that develops an individuality is said to die instantly.'

`And the catapult bows?' What are they for?' said the Archchancellor, slapping the Dean's hand off one of them.

`In addition, it is possible that a collective of Auditors with sufficient presence in the material world may develop crude physical senses, and so I have adapted some catapult bows to fire a mixture of intense, er, stimuli. Old references suggest chilli, essence of Wahoonie or Blissberry blossoms, but modern thinking inclines to Higgs & Meakins Luxury Assortment.'