I have always thought of viruses as being non-cellular lifeforms. And no, it isn't just because of Wikipedia. I have heard that there are viruses that are so complex, that they are very close to being considered cellular. And some viruses infect other viruses. Wow, I never thought parasitism would scale all the way down to viruses as the host until I found out about viruses that infect other viruses. So, here are the criteria scientists have used for defining living vs non-living for centuries:
- Made of at least 1 cell -> Some viruses have membranes and some cells have cell walls. Sure, cell walls are normally made up of complex sugars, but what is to say that there couldn't be lifeforms with proteins as the most prevalent component of a cell wall? And some very large viruses get close to being actual cells.
- Metabolism -> This is one thing that even the most complex of viruses fail.
- Responding to stimulus -> I think viruses do respond to a stimulus, just not in the way we normally think of responding to a stimulus. I mean, they clearly respond to temperature(cold temperature, won't reproduce or at least will reproduce very slowly, moderate temperature, will definitely reproduce, but not at peak reproduction rate, hot temperature, peak reproduction rate reached) and temperature is a stimulus.
- Grow, Develop, Die -> The virus does develop, sort of, when viruses get built in the host cell(which is kind of analogous to an egg cell becoming a baby, only on a much much smaller scale). And viruses do die as well. The only thing they fail at here is growth. The virus doesn't get bigger or more complex than the parent virus unless it mutates and the mutated virus has size and/or complexity promoting genes that the original virus didn't have.
- Highly organized -> Small viruses like the flu virus fail at this. Some bigger viruses though don't fail this criterion
- Maintain Homeostasis -> All viruses fail at this, even the complex viruses
- The ability to reproduce -> Viruses don't reproduce independently from a host but they do reproduce. If viruses were to be considered non-living because of the inability to reproduce independently of a host, than wouldn't your average parasite also be considered non-living for the same reason? Clearly parasites aren't considered non-living just because they can't independently reproduce. Viruses are just parasites of the cell, so I would say that they do meet this criterion, because viruses and their relationship to cells is parasitic. Viruses infecting other viruses also makes me think that viruses meet this criterion
- DNA changes and thus the organism evolves -> This is definitely true for viruses. Viruses evolve super fast, even just 1 cell getting infected by 2 viruses simultaneously is enough for completely different viruses to form as the genetic material and capsid proteins are exchanged between viruses as they are built(I have seen this illustrated before with 2 different flu viruses infecting one cell and 4 different viruses coming out of the cell, both the original flu viruses and 2 completely different flu viruses). So the evolution rate of viruses can be as fast as a single generation compared to the thousands of years minimum it takes for the same amount of evolution relatively speaking to occur in a multicellular organism.
Clearly, viruses do meet a lot of the criteria for a living organism. So with this and all the new discoveries of things like viruses getting ever closer to the cell in complexity and viruses that infect other viruses, why aren't viruses considered to be non-cellular lifeforms?