Viruses are not classified as being alive because they don’t have their own machinery for reproducing. They can only take over the machinery of cells, turning them into virus factories. But not everyone agrees with this system. Someone might say, “Hey, parasites need to reproduce inside other organisms, but they’re alive.” That would be a reasonable argument for saying that viruses are a form of life. I tell my students that people like to make nice neat categories, but the natural world almost never fits into them. Putting things into categories can be helpful, but we have to remember that the categories are usually artificial and should not get in the way of understanding all of the amazing diversity of the world.

I’m really impressed that you thought about the mitochondria as a possible ancestor for viruses. They best story we have today is that mitochondria were once free-living bacteria. Did viruses evolve from bacteria? Maybe. It is difficult to say because it may have happened 3 billion years ago, and DNA just doesn’t last that long. Viruses don’t seem to fossilize well, either. Since bacteria were around before the cells that we call “eukaryotic” (plant, animal, fungal, and protist cells), that may be what happened. Some scientists think that different groups of viruses evolved independently, maybe some even came from eukaryotic cells, and some from bacterial cells.

If viruses are basically parasites that evolved from living bacteria, would that be another argument for saying that they are a type of life?

We still know very little about viruses and bacteria, compared to what we know about multicellular organisms. You may want to consider a career in microbiology if you want to explore them.

Viruses fascinate me too, so I actually wrote an article all about them. Feel free to check it out here:

I think it will answer your questions (and maybe raise some more!). There's actually a lot of debate over whether they are "alive" or not, and some scientists think that we might have evolved from viruses, or something like them! Viruses have definitely been around for a very long time. See the article for more details.

Viruses are not considered "alive" because they lack many of the properties that scientists associate with living organisms. Primarily, they lack the ability to reproduce without the aid of a host cell, and don't use the typical cell- division approach to replication. Essentially, however, this is just how scientists have defined the word. If viruses were classified as living, other types of self-replicating genes, proteins, and molecules would make the list as well.

There are a few theories on the origin of viruses. Since there is no historical record of the earliest viruses, the only evidence available is from current species. Indeed, one theory suggests that viruses may have arisen from parasitic cells which lost their cellular structure through evolution. However, there isn't a lot of evidence from current cells that shows this type of transition is possible. Another theory is that they evolved along with living cells, from genes or proteins that happened to be self-replicating. It would seem that since the spectrum of viruses around today is so wide, they likely evolved through many different pathways.

Yes, viruses are interesting. Viruses don't fit the definition of life, but they're certainly not dead either! They're an interesting example of how we can't really separate stuff into 2 simple categories - Living and Non-living. Viruses seem to be in between those 2 categories.

And the question gets even more interesting when scientists talk about life on other planets, because this might be very different from life on earth. They call it "Life as we Don't know it."

Some scientists are still debating about What is Life? In fact, I wrote a little article about it last fall, for a collection of journal articles all about that subject.

I think viruses probably evolved from simple cells such as bacteria and archaea that don't have a nucleus. Keep asking questions!

Excellent, viruses ARE fascinating! A virus particle is made up of 2 main parts: genetic material (either RNA or DNA) and a coat that is made up of protein and sometimes lipids (fats) that protect the genetic material. A virus can live outside of a host cell but it can not reproduce without a host!

Scientists argue a lot about whether viruses are in fact living organisms or just organic structures interacting with living organisms. Some properties of viruses make them seem alive like the fact that they have genetic material and that they make copies of themselves to reproduce. On the other hand, people will argue that viruses do not have any structure to their cells (which even simple living organisms have), they do not have the ability to make their own chemical products and need a host cell to do that for them and that is why they can not reproduce without a host cell. This debate is not going to be settled anytime soon, but you can decide for yourself whether you think viruses are living or not!

Where viruses originated is also something that scientists disagree about. There are 3 main ideas for how viruses evolved in the first place. 1) Viruses could have started out as bacteria and just lost all the genes they needed to survive on their own. 2) Viruses could have begun as small pieces of genetic material that escaped from a larger organism and infected another. 3) Viruses simply started out as viruses when proteins and genetic material mixed and that these particles have been living this way (infecting living cells to replicate) for billions of years, ever since life itself began. Viruses do not form fossils, which makes it really hard for scientists to determine where exactly they came from.

For some more information on viruses, check out these two links:

The evolutionary origin of viruses is something that is unknown. It is possible that viruses were actually cellular organisms once, which became adapted to an intracellular life style. This is similar to how mitochondria are believed to have evolved. However, it is unlikely that viruses evolved directly from mitochondria. Viruses were likely around long before mitochondria existed. Another theory is that viruses originated from genetic material that co-evolved with cellular organisms to become separate from the cellular genome and eventually became more complex, resulting in the virus particles we know today.

Viruses lack the cellular machinery to be able to reproduce themselves; without using the genetic code of a cell as their host, the genetic information contained within a virus is meaningless. An analogy I could make is that viruses are basically software, and software requires hardware (in this case, a cell) to run on. For this reason, most definitions of life do not identify viruses as living organisms, because they aren't actually *organisms*.

This said, viruses do possess a lot of life- like qualities, including the ability to carry information, reproduce (with help), and evolve under natural selection. Saying that viruses aren't living in some sense is also missing the point.

Viruses are strands of DNA or RNA contained within protein sheathes and seem to be genetically related to the organisms that they infect, as if they evolved from their hosts' genomes. This means that viruses probably evolved multiple times from different ancestors. I don't know of any viruses thought to have evolved from mitochondria, or that can even infect mitochondria, but I see no reason why it isn't possible that some could have.