This is a big one! A big question that usually swirls around my head, particularly when I read or consider facts about the microscopic world and its border line with chemistry. This line is not as well defined as you may think. Just to throw us straight into it: how can something with DNA not being certainly consider alive? Yes viruses, I’m looking at you. Probably because they are the most symbolic example of this conundrum. Here for example you may find a few arguments against viruses as alive creatures and a few points about the defining-life dilemma.

The problem

Live beings or very complex molecules that replicate? Even taken the first as a fact, the problem of classifying viruses isn’t precisely new [1]. But as you may know, viruses need a host to replicate and form their genomic nucleic acid, using the host cell machinery in the process. Hence, they are not capable of self-replication and thus some experts will claim that they do not fall inside the alive-beings domain/realm.

There’s even another big problem. The inert characteristics they show when they’re outside a celular host as virions, which can lead to amazingly long periods of inactivity. “That is not death, which can eternal lie. And with strange aeons even death may die” Yes, I hear you Lovecraft. Lack of metabolic activity. We will come to this at the end, but put it in the back burner for now.

Questions that steal my sleep

• If viruses are not alive, how is it that they effectively evolve in a very similar way to how live beings do? Every new year we see how new strains of the common flu adapt and look for new ways of being part of our winter dive in. Like confining summer clothes to the depths of our wardrobes or the arise of a common desire for hot drinks.
•  How did viruses appeared on earth? Did the evolved from bacteria (as some theories point to) or another form of life? Or twas their arise a completely independent process? Being the first case true, does it mean that life can reward to an state of “no life” (that is not death. Cthulhu fans may node as approval here). Being the later, how it is that they share common traits as DNA and RNA with the rest of us?
• If they are not alive, then why is biology studying them since BIO clearly stands for the opposite thing. And which realm of science should they belong to then?

Okay, fear not now, since I spent the last days looking for some answers. Let’s see how good I did.

Origin of viruses

There are three main theories regarding this issue. Nonetheless, some fresh discoveries point to a shared common ancestor for our cells and viruses [2]. DNA in eukaryotic and prokaryotic cells is very hard to compare to that in viruses due to, among other things, the huge diversity and change viruses show in their evolution. Furthermore, some of them only use RNA instead of DNA as genetic source. It’s also true that different virus superfamilies may even have different origins in a quite disfigured evolutionary tree [1]. So that, that you can’t barely call it a tree. This makes phylogenetic studies quite difficult. But comparing the nature and properties (folding) of our proteins has lead to the the conclusion that, at some point in life’s evolution, we may have shared a common ancestor. If that ancestor was or wasn’t “alive” yet is a question I hope to invalidate in the next point.

A change in perspective: replicators

The real breakthrough lays in this argument. Concepts are created so we can understand and describe the world that surrounds us (or may I say, that we perceive). What is the point of keeping an old dusty concept as “life” around if we can’t, not only tell what it is, but distinguish between alive things and not alive chemical molecules either? Dropping something so colossal as that concept leaves a big hole hard to fill. But we may find a new frame of reference for biology picking new clear concepts. Replicator being the first one.

Replicator, as defined by Richard Dawkins in 1982, is an isomorphic concept to genome. Don’t panic here! Very briefly. You are a what we may call a vehicle, and you carry self-replicating units (replicators) that encode the information necessary to their own replication. You guessed it, your genome is a replicator. Just take into account that DNA is not at all the only way of storing and encoding information. So other life forms may have found different ways we don’t now of yet. Different replicators in different vehicles. Also, there are other kinds of replicators that are clearly not part of what we may want to include in the biology field.

The level of autonomy of replicators vary: no replicator can live in complete isolation. Their autonomy is also determined by the replication machinery encoded in its own genome (or equivalent). Thus, we find different levels of what we could call replication-parasitism, opposed to self-sufficient replication and altruism [3]. We have the first corner stone of biology at this point.

Metabolism, as acquiring energy and resources, is the other key stone for our Biology v2.0. Thus, you have the elegant biological expression of entropy and energy (genome and metabolism), as exposed cleverly by E. Schrodinger in his “What is Life” essay. Very recommended lecture, by the way, for people in love with old but still an actual-smelling classic science.

I love this solution to the “what is life?” and “how we define it?” questions. Destroying the questions themselves first, and creating a new field for biology that redefines the question into “how replicators with metabolism behave?” The cathartic experience of destructing and creating from the beginning reevaluating the problem. You may not feel very compel to get rid of the “life” concept at first, but the more I thin about it the more sense it makes. Still, my thoughts will take me back to the topic from time to time I’m sure. Take care folks!

Reviews and articles

[1] The classification of organisms at the edge of life, or problems with virus systematics. E. Rybicki. South African Journal of Science [1990]

[2] A phylogenomic data-driven exploration of viral origins and evolution. A. Nasir, G. Caetano-Anollés. Science Advances [2015]

[3] Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question. E.V. Koonin, P. Starokadomskyy. Stud Hist Philos Biol Biomed Sci. [2016]