5 short stories on the
Origin of Life
What is life, anyways? We can label the items around us as
living or dead fairly easily: Me? Alive. Chair? No. Plant that I haven’t
watered in a month? Might still be kickin’ -- gotta love succulents. Computer?
I wish. Than I could be chatting up Scarlett Johansson like that guy in Her. Below are five short (but true!) stories on the origins of life here on our cozy little planet earth.
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| The Honey Mushroom stretches for miles underground |
But really the division for life and non-life is hard to
pinpoint because it lies at the cellular/acellular divide. From big Blue
whales, Giant Sequoia trees, and the truly massive Honey mushroom in Oregon down to the smallest Paramecium, or the truly tiny Mycoplasma genitalium (one of the smallest bacteria) all of these diverse
organisms get labeled as living because they are all forms of self-replicating, ribosome-utilizing cellular life.
Viruses, Protein, DNA all fall short of this definition of
life. The largest known virus, the Pandoravirus, at 500 nm wide and 1000 nm long is bigger than the tiny
archaeon Nanoarchaeum equitans (300 x 300 nm) and even though both depend on
host cells for survival, the poor Pandoravirus doesn’t get the label of being
alive. M. genitalium at least has the
potential to grow and divide outside of a host cell as it has the ribosomes
necessary to translate RNA to protein.
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| Nanoarchaeum equitans, the smallest cellular life |
The next story on Magma oceans and marauding asteroids soon to come... but this is my first attempt at a blog, so excuse my ignorance as I try to figure it all out.
From reading this article, I do not understand why Nanoarchaeum equitans is classified as a living cell while Pandoravirus is considered non-living and I do not agree with that categorization. Since both organisms meet the requirements to be non-living, I believe they should both be considered as such; in other words, Nanoarchaeum equitans, just like Pandoravirus, relies on a host to survive and reproduce and even lacks the genes to make membrane lipids! Therefore, what gives Nanoarchaeum equitans the nod as a living organism?
ReplyDelete-Elias Khoury
You make a good point, Elias, but which way would you move the bar? Should we lower the bar and consider viruses alive? Or raise the bar and consider anything that isn't free-living--such as Nanoarchaum--as not alive?
ReplyDeleteIf we lower the bar, the issue would be how to characterize prions or free DNA, which are even simpler than viruses, but have the ability to replicate under judicious circumstances.
If we raise the bar, than what qualifies as free-living? One could argue that any heterotroph-even humans--is dependent on other life replicate! Surely, we don't want to end up defining ourselves as "not alive". Where ever we choose to draw this line I think there will be examples that blur the lines. Life defies easy definitions.
From our discussion in class, we spoke of another bacteria that is an obligate intracellular symbiont which, although it is a bacteria, is not considered alive. Carsonella ruddi, for example, is another type of bacteria that lacks the ability to make the lipids required for its cells and relieves on its host to do so for its survival. I guess the question that I am trying to get at is are viruses, which are not defined as being alive, more closely related to the bacteria that are also not considered to be living? Obviously there are major differences between what makes bacteria a bacteria and vice versa for viruses, but does the defining-factor of declaring something as "alive" overlap between these two?
ReplyDelete-Lindsey Kirk
Good question, and first I'll direct you to the animation I made https://www.youtube.com/watch?v=dMtaUSMjsNQ
ReplyDeleteas well as an interesting article from Koonin and Martin, 2005 http://www.ncbi.nlm.nih.gov/pubmed/16223546.
The answer is that the origin of viruses is unresolved. Viruses may have originated several different times in the history of life. They may also pre-date cellular life, but there is definitely no direct connection linking all viruses to obligate intracellular pathogenic bacteria. So in that respect, they have different evolutionary histories.
Blog response:
DeleteAfter reading about how viruses are not considered alive due to the lack of ribosomes, I find myself wondering if such a thing is even possible. How would we know if the virus wouldn't just consume a ribosome similarly to the way our bodies immune systems go haywire if they sense foreign tissues or pathogens? I would be extremely interested in trying to imitate what Griffith did only this time using viruses instead of mice and ribosomes instead of rough/smooth bacteria cells (alive and/or dead).
I feel like there will come a day when a virus evolves and gains the ability to produce its own protein and live without a host. If that day were to come, could we just consider it a new bacteria? I feel like the lines between living and non living are very good and definite, and instead of blurring them is it possible just to consider the new virus something else such as a bacteria? The characteristics of a virus seem definite so wouldn't a new super virus just be something completely different? Also, besides the viruses that help algae blooms, are there any other benefits to viruses? Are there any beneficial virus for humans? I know there are vaccines but I am disregarding those because those are to help protect us from the true viruses.
ReplyDeleteFrom Jenna: "I feel like there will come a day when a virus evolves and gains the ability to produce its own protein and live without a host. If that day were to come, could we just consider it a new bacteria?"
ReplyDeleteIf this happened it would almost certainly constitute a new domain of life distinct from bacteria, archaea and eukarya. But I don't think this is a very likely scenario, viruses lack so many vital components for self-reproduction.
From Jenna: "Are there any beneficial virus for humans?"
See the following article by Carl Zimmer:
http://blogs.discovermagazine.com/loom/2012/02/14/mammals-made-by-viruses/
This describes evidence for a virus that donated a critical gene for embryonic development to primates (Syncytin). Viruses are a mechanism for horizontal gene transfer that have the potential to confer new traits to the host organism.
I have always found the debate over the definition of "alive" slightly silly, similar to the definition of "species." No matter how many different so-called requirements for life we come up with, there are always exceptions. For instance, one might say that in order to be alive, something has to be able to reproduce. If someone is unfertile, does that therefore mean they aren't alive? You could argue that I am taking this too far, that the definition of life in the context we are discussing if only on the species scale. But then, wait, what's a species? Organisms that can reproduce to produce fertile offspring? Well, what about so-called species that occasionally interbreed? If we go back in evolutionary time, it gets even messier... Where do we draw the line between humans and Neanderthals? Are we separate "species" or "subspecies"? In the end, its all just semantics.
ReplyDeleteWhen it comes to viruses, I would lean more towards them being living. In the case of the Pandoravirus and Nanoarchaeum, it seems puzzling that they share similar characteristics, but are classified on opposite spectrums. Like Brianna commented earlier, it comes down to semantics. However, I have always viewed something non-living as stagnant, not having the ability to evolve.Not only can viruses evolve, but they carry information and can pass information to their offspring. Even though they need a host cell to do most of their characteristics, I view it as a part of their survival. In a way, all living things need each other for their survival and viruses are a part of that.
ReplyDeletePoor viruses cannot be included in the living category. This reminds me of the average kid trying to fit in with the cool kids. Even though they may look "cool" on the outside with their Abercrombie jeans, they still do not qualify because of the little details (mom doesn't drive an Escalade). Viruses should be included in the living category even though they do not have ribosomes. The Pandoravirus is a strong example of a virus that could be labeled as living. According to my notes from class, it attacks amoeba and is able to replicate in its environment. Additionally, viruses should be classified as living because they affect our lives. Emerging viruses such as HIV have impacted the human population greatly. I find it difficult to believe a nonliving particle is causing so much complications in our species.
ReplyDeleteHowever, every person has their own criteria of what is considered living and nonliving. In Medical Ethics, we discussed what we considered a living person, and there is a disagreement between being physically alive (breathing and heart beating) versus being mentally alive (interacting with others and responding to others). While Biology includes facts we all agree on, there still remains facts that need to be explored further.
Classifying a virus as a nonliving particle definitely poses some intriguing questions. The criteria on which we choose to classify viruses involves the fact they are unable to produce proteins and self replicate on their own but depend on a host cell. However, this phenomenon reminds me of many common parasites that we consider as "living". In a general sense, what really differentiates them from viruses? After all, they both cannot reproduce without the aid of a host. In addition, without the supplies provided by the host, a parasite cannot "live"!
ReplyDeleteI acknowledge that parasites possess their own cellular network that creates the proteins necessary for life but should we really create disparate groups based on that single factor?
Also, to dive even deeper, could we just consider viruses as living organisms existing in a dormant state until they attach to a host cell? Consider certain bacterium bacilli that remain in an endospore state when deprived of nutrients; many cells will cease most protein syntheses. As soon as some nutrients come around, BOOM! They start actively producing proteins again and dividing.