Jessica asked if I think viruses are alive. John has given his opinion. I will waffle, but I hope in an interesting way. The hard thing about answering that question is that we’d have to agree on what it means to be alive.
We all have a sense that we know what’s alive and what’s not, but I think that sense is really just an intuition. We use different circuits in our brains for recognizing biological motion, for example, as opposed to the motion of rocks or cars or other dead things. But the trouble comes when we try to turn that intuition into definition. We can see that things that look alive to us–tigers, roses, lobsters–share some things in common. And when we get tools to let us see new things, such as bacteria, we wonder, are they like us–in other words, are they alive? I find it interesting that in the nineteenth century, bacteria seemed to be at the hazy border of life and non-life. They seemed to be featureless bags of protoplasm. That was why the research on E. coli I write about in Microcosm was so astonishing. Down to many fine details, E. coli is a lot like us. Their genes are made of DNA. So are ours. They use a genetic code to read those genes and build corresponding proteins out of amino acids. So do we. There are actually dozens and dozens of different amino acids in nature, but E. coli only uses 20 of them to build proteins. We use a nearly identical set. Nobody would claim that E. coli is not alive anymore, because it is so much like us.
But just making a list of traits shared by us and E. coli is not a good definition of life. All known living things use the same set of amino acids. Or at least they did till some scientists engineered E. coli to use “unnatural” amino acids a few years ago. Are they no longer alive? Perhaps there are just a few basic things that qualify somehting as alive. A lot of people like to put metabolism on that short list–the ability to take in food and turn it into living matter. Some would say viruses are not alive, because they don’t have their own metabolism. The classic picture of a virus is a package of genes that uses a host’s cells to make more packages of genes. Yet some viruses appear to grow and undergo other changes outside their hosts, making this a dubious standard.
I also think it’s a mistake to try to cordon off viruses in some non-living quarantine because they evolve, and their evolution is intertwined with the evolution of their hosts. A sizeable chunk of E. coli’s genome is made up of genes delivered by viruses–many of which are essential to the microbe’s survival. The same goes for all the microbes in the ocean, the soil, and in our bodies. I think now of life as a global matrix of genes, shuttling from node to node and changing over time.
So viruses may or may not be alive, but they are definitely a part of life.
I think it’s better to think about life not in terms of hard definitions, but in terms of rules–ways in which species tend to work, no matter how different they seem superficially. The fact that all living things use 20 amino acids is not part of the definition of life, but it certainly is a rule that applies to all life on Earth outside of laboratories. Some scientists think this rule probably the result of some sort of frozen accident early in the evolution of life, or perhaps natural selection zeroing in on the most efficient or reliable system for building proteins.
In the book I also point out other surprisingly widespread rules of life. Life, for example, is robust. In other words, the ways in which genes interact allows living things to stay stable in a world full of change. E. coli copes with rising and falling temperatures, times of feast and famine–all sorts of change–while maintaining an even keel. Its robustness, like our own, is the result of how its genes are organized, like the parts of an airplane. (That’s why engineers are now helping make sense of E. coli’s genes, using the same tools they might use to build autopilot systems.) But that doesn’t necessarily mean that life started out robust to begin with. In each lineage, robustness was a good long-term strategy.
When I imagine the day when we discover alien life, I wonder about whether aliens will be robust too. I also wonder if they will also obey the rules of Earthly life. E. coli and other microbes are surprisingly social, for example, communicating, cooperating, and sometimes even killing themselves for their fellow microbe. Perhaps to be alive is to be social? And the fact that E. coli ages like we do–as an evolutionary strategy to cope with the inescapable decay of biological molecules–makes me wonder if aliens get old too.