Ranting about planets got me thinking about another old favourite – are viruses alive?
Once again, I’ve always felt that this is something of a non-question, easily resolved with the answer “no”. Think back in time to your first ever high school biology lesson. You probably had to make a list entitled something like “characteristics of living things”. The list goes something like this: they move, they grow, they respire, they excrete, they reproduce, they sense their environment. It’s sometimes pointed out that a flame has some of these characteristics, but we instinctively recognise that combustion by a flame is not the same as cellular respiration.
Evolution is often described as the one theory that forms the framework for all of biology but for me there’s another – chemiosmotic theory. All cells use some variant of chemiosmosis – even bizarre extremophiles that require only hydrogen and carbon dioxide and pump sodium, not hydrogen ions, across membranes. I think cellular respiration is the defining characteristic of “life as we know it”. So what are viruses? Molecular machines. Intricate, complex and amazing machines, but unable to generate energy or reproduce unaided – and so not alive.
The presence of viruses in all 3 domains of cellular life and the unrelatedness of viral and cellular proteins suggests an important role for viruses in evolution. Try these references for starters:
The origin of viruses and their possible roles in major evolutionary transitions
Did DNA come from viruses?