A Planet of Viruses

Physicist and TV presenter Brian Cox has been gently mocked lately for telling us so often that the Universe is "amazing". The charge is unfair. Amazement remains a staple of successful popular science. Cox's problem is more that his field lacks new amazements.

For those with long memories, not much seems to have happened in fundamental physics and cosmology since Carl Sagan's Cosmos, 30 years ago. There are plenty of ingenious new theoretical speculations, but observations to test them are scarce.

The real action is in biology, where amazing new facts just keep coming. The techniques of genome analysis make it remarkably easy at the moment to make unexpected observations. This little book is packed with them, carefully assembled by another talented populariser, the science writer and Yale University lecturer Carl Zimmer. He has already given us a fine book, Microcosm: E.coli and the New Science of Life, on the workings of life as seen through the molecular anatomy of that ubiquitous bacterium. Here he explores the realm of the (usually) even smaller entities that sit somewhere on the hazy boundary between living and non-living - viruses.

They are not small by physicists' standards. But they are small enough to have passed undetected for almost all of human history. Their effects, of course, were felt long before, in sickrooms and hospitals. But as Zimmer relates, their composition, as tiny packages of genetic material that can subvert the normal business of cells, remained mysterious until the 20th century.

More recently still, cheap, rapid DNA sequencing has revealed a new dimension to biology's microworld. There are far more viruses than we knew, and they are much more diverse. And they have managed to get themselves incorporated into the fabric of complex organisms in hitherto unsuspected ways.

Closest to home, the human genome is full of remnants of old viruses. Their genes - once encoded in RNA rather than DNA - have been stitched into our chromosomes at perhaps 100,000 locations. Virtually all are now non-functional, but they go on being copied every time a cell divides. These zombie viruses in fact make up 8 per cent of our DNA, while our own, functioning genes account for a little over 1 per cent.

If the myriad viruses lurking in our own DNA were a surprise, so was the density of viral populations in Earth's largest, oldest habitat - the oceans. Until the 1980s, biologists believed seafaring viruses were a rarity. Then a more searching investigation established their fantastic profusion - perhaps 100 billion in a litre of seawater. All the viruses in the oceans, writes Zimmer, weigh the same as 75 million blue whales.

Most of these viruses are the kind known as phages, which reproduce in bacteria. They kill roughly half of all ocean-dwelling bacteria every day, and have important influences on ecosystems and even climate. They also harbour enormous unheralded genetic diversity, and are incessantly exchanging genetic information. Thus viruses have been vital agents of evolution, certainly in the ocean, and probably on land as well.

These are the revelations that underwrite Zimmer's title. They pass the crucial non-fiction writer's test - being the kind of thing you want to tell someone else (even, my wife will confirm, if they are reading another book at the time). Although the majority of his brief chapters are devoted to individual viral diseases, Zimmer shows how we really do live on a planet of viruses. And we now know this not because of any advance in theory, but through compiling more careful observations, using more powerful tools. In the current state of biological surveying and experiment, there is no reason to expect that the startling new discoveries are exhausted. The life sciences look like staying newly amazing for the next few decades.