A virus is a seriously weird thing, a refrigerator which is able to morph into a heatpump, and then into a toaster. I give you…Ebola. And Marburg.
These strange crystalline creatures can exist in outer space which is a unique capability not available to biological organisms. Indeed it is theorised that Earth’s viruses arrive from outside the planet.
For the sake of balance it is possible for a biological cell to hibernate and float in space but without water and energy it will die in time. Viruses appear to transcend that limitation.
Viruses need DNA-based cells in order to replicate, therefore, if viruses originate outside the planet, this implies one or other of the following:
[ul]
[li]DNA-based living cells also originate off-planet (panspermia)[/li][li]DNA-based living cells exist elsewhere because of some kind of parallel evolution[/li][/ul]
Pet peeve. The issue here is not that there’s missing knowledge. It’s that that we don’t know if viruses are dead or alive. We understand viruses very well. We know their molecular biology, we know their reproductive strategies, we know very well how they get along in this world and do what they do. It’s not an issue of knowledge; it’s a question of semantics.
The issue, if you want to call it that, is that our definition of life isn’t very good. It leaves a big ol’ grey area, and viruses happen to exist right in the middle of the grey. The question of if viruses are alive or not is not a question to be settled by biologists; it’s to be settled by English majors.
It seems to me they would like to classify viruses as alive, but the problem is such definitions would also classify other things as alive, such as computer viruses, which they cannot accept.
Are robots alive? Will they be in the future? One could argue that as long as they’re not completely self-sufficient (“You need to perform my annual overhaul, Master!”) they’re not alive. If necessary overhauls can be performed by other types of robot, then the collection of robots is a living system.
Similarly, while viruses can survive indefinitely by remaining dormant, they need higher-life hosts to reproduce.
But does this definition exclude some parasites? Or even the non-reproducing individuals in an ant colony?
Best may be to rephrase the issue: Does a planet (or other well-defined realm) host life or not?
Yeah, that’s the typical list, except that it’s a description of life as we know it rather than a definition. Still, it illuminates the subject.
IMHO, the first point (life is cellular) is an interesting one. It’s definitely true of LAWKI, but is it really necessary for life of any kind? My guess is that if we were to find 100 completely unrelated types of life (not necessarily chemical, and yes, including self-sustaining storms of Jupiter), the vast majority would be cellular, simply because it helps to define an “inside” vs “outside” and as a unit for construction. But if I find something that meets the other criteria, I’d still call it “life”.
The second point is similar. My guess is that any lifeforms would involve emergent layers of organization for the obvious reasons. The example of tissues and organs is just an example, not a requirement: bacteria have substructures, but not tissues. Again, though, if I found something that met all the other criteria but wasn’t particularly structured, I’d still probably call it life.
Good point. Viruses are excellent examples to discuss when trying to define life, because they meet many of the criteria, but not quite all. Yet without falling back on “cells”, it’s hard to pinpoint what about them makes them not organisms (“living things”). The simplest is that, except when they’re in a host cell, they’re really not doing much in the way of living.
My favorite bottle-cap definition of life is my own, but I bet someone came up with the same one before me:
Life is that which evolves itself.
I admit that definition is a bit circular, but I think it hits the nub of what life really is.
I also admit that it has a bit of a grammatical/semantic blunder, as “evolve” isn’t really the kind of verb that admits that kind of object. Grammarians can probably improve the discription of what’s wrong here. But I’m sticking with it anyway, since it’s necessary. Things that evolve include man-made designs that are successively refined – not living! But anything that successively evolves its own design is either alive or will become alive. It might not be what we normally call “biological”, though.
Oops, that 7-point list left out something important: life has to consume “unlike” ingredients. That is, it has to eat, and it has to eat stuff that’s not identical to itself. Otherwise, we get some crystal processes that fit too many of the other criteral (except the cellular stuff). Consumes energy, grows, reproduces, responds to the environment (sorta). But it doesn’t evolve itself. It just produces similar crystals, the blueprints for which never change.