I’m going to do something a little bit different with this post. I’ve done a few “Species Profiles” in the past (the Introduced Pine Sawfly and the Eastern Band-Winged Hoverfly), and in those I’ve offered a brief overview of the groups those species belong to before focusing in on the species itself. In future, I’d like to zoom in on a species from a distance. Since all living things, from bacteria to Blue Whales, are part of one huge family tree (species have formed out of species) then all of life is related to a greater or lesser extent. So to start my scope as far out as I can, I’m going to begin with the broadest category of all: Life itself.
What are Living Things? You might be surprised to find that it’s actually quite complicated and difficult to draw lines around living and nonliving things. We can intuitively classify large animals (and by this I mean animals that can be seen without aid of a microscope) as living things. They eat, move and reproduce under their own power. Plants and Fungi are similarly easy to class as alive (though some life stages of Plants and Fungi lie on the border, such as seeds or spores). Although Plants and Fungi don’t behave in easily visible ways in our timescale, they still perform the same functions as Animals: reproducing, metabolizing and growing.
So maybe there isn’t a simple definition for life, but Living Things are still easily distinguishable because of a general sense. It’s just common sense that an Elephant is alive and a rock is not. If we bring the scale downward from the living organisms we can see with our own eyes into the rabbithole of microscopy we find that things are (as always) far more complicated. The simplest and perhaps most relevant example to bring confusion to our general sense of what is alive and what is not is… a virus.
I remember the first time I recognized the difference between a bacterial and a viral infection. The major difference from a patient’s point of view is that Bacteria can be treated (ie. killed) by anti-biotics. Anti-biotics literally means “anti-life”. Because Bacteria are alive they can be targeted by anti-biotic medicine. A viral infection is immune to anti-biotics. Viruses are not killed by “anti-life”. This is because they are supposedly not living. This is where the defining border of Life and not Life gets very very fuzzy.
Viruses come in many shapes and sizes and they affect animals and plants in a myriad of ways, but the reason they do any of that is because they exhibit behaviours which enable them to adapt and react to their surroundings in order to reproduce more of their kind. The description I just offered seems to suggest that Viruses are alive. It really depends on where you draw your lines. My Biology textbook from University states that “the characteristics of life that a virus possesses are based on its ability to infect living cells” (Russell et. al. 2010). So, no living cell, no life. A virus contains some portion of DNA or RNA (the information-coding substance that tells cells what proteins to make and how to make them), and sometimes a protein capsule or container. Again, from my Biology textbook: “They essentially highjack the machinery and metabolism of a living cell in order to reproduce. For this reason, most scientists do not consider a virus alive” (Russell et. al. 2010).
For an alternative view of viruses, here is a quote from a book dedicated to Viruses: “they [Viruses] are highly evolved biological entities with an organismal biology that is complex and interwoven with the biology of their hosting species”(Hurst 2000). In this book, Viral Ecology, the editor also recommends placing viruses within a fourth biological Domain of Life (the other three Domains are Bacteria, Archaea, and Eukarya). He proposes that this Domain be called Akamara which means roughly “without chamber”, describing the fact that organisms within this Domain are non-cellular.
I’m not sure where I sit on the issue of whether viruses should be considered Living or non-Living. The more I learn about them, the more complicated the questions and answers are. I think that intuitively I wouldn’t want to define Life by its components such as possessing a cell, but I also see the value in having strict definitions for labels even labels as amorphous as Life. The point I wanted to make with this diversion into Viruses is that Life is actually hard to define or describe and place within limits, even if on a larger scale it’s intuitively simple.
I don’t think I’ll really be delving into Viral Biology on my blog anyway as I personally prefer learning about organisms I can more easily observe, but it is a fascinating aspect and background for my interests in living things in general.
Next up… Animals, Plants, Fungi. They have to be easy to divide and define right?
Hurst, Christon J. ed. Viral Ecology, 2000.
Russell et. al. Biology: Exploring the Diversity of Life, First Canadian Edition, 2010.