Is there a definition of life? Right from childhood, we are brought up to think that it is easy to differentiate living from non-living. Living beings can see, hear, smell, taste and feel. Humans are alive, but a rock or a table isn’t. Easy human-centric answers are simplistic and don’t tally with facts. One definition defines life as “absorbs compounds from its environment,” “excretes waste,” and “grows”. While fire consumes “food,” excretes “waste,” metabolizes, moves, and grows, it is not life. This shows how fallible definitions are. For a long time, it was not clear if plants were conscious, because they did not have brains and memory. We know better now. Researcher Paco Calvo argues that “Plants can, not only learn and memorize but also make decisions and solve complex problems. Plants can anticipate competition for resources, growing differentially depending upon the future acquisition of minerals and water (Novoplansky, 2015). They exhibit self-recognition and territoriality (Schenk et al., 1999), being able to tell apart their own from others, directing their movements towards their targets of interest (Gruntman & Novoplansky, 2004). Plants can communicate aerially with members of their own kind and with members of other species. They can even communicate bioacoustically, making and perceiving ‘clicking’ noises (Gagliano et al., 2012). Some plants can tell vibrations caused by predators apart from innocuous ones (wind or the chirps of insects), eliciting chemical defenses selectively (Appel and Cocroft, 2014). In a sense, plants can see, smell, hear, and feel (Chamovitz, 2012).”
Philosophers introduced abstract concepts like soul and inner spark, which is a so and so story that can easily be disproved and never be proved. Researchers have identified 123 definitions of life, and condensed the finding as “Life is self-reproduction with variations.” NASA too came up with its own definition “”A self-sustaining chemical system capable of Darwinian evolution.” NASA did accept that its definition wouldn’t apply to viruses. Viruses typically do not have DNA. They are dead the entire time until they meet a host cell. They cannot reproduce by themselves. Even now, many scientists who look at the 3 kingdom framework do not accept viruses as living. But recently discoveries are turning these theories upside down.
Around 2003, a giant virus – Mimivirus was discovered on a French cooling tower which contained 979 genes—as many as some bacteria. Remember humans have 2000-25,000 genes. Megavirus was discovered in 2010, off the coast of Chile. It had 1120 proteins. In 2013, the Pithovirus was discovered. It possesses some replication machinery of its own. Another virus discovered in 2013, Pandoravirus has a staggering 2500 genes, seemingly heralding a new class of viral life. “More than 90 percent of its genes did not resemble anything else found on Earth,” noted a researcher.
Prions are harmless proteins that are present in mammals and birds. But these proteins are in an abnormal form, and once they enter the human brain, they are capable of causing severe brain infections. As soon as prion finds their way into the brain, they cause normal proteins to turn into abnormal ones. Then afterwards, they soon multiply causing severe infection in the brain. Due to this infection, some holes appear inside the brain that can only be treated by incineration. Some of the diseases caused by prion are Mad Cow Disease, Scrapie in sheep and goat, chronic wasting disease in deer and elk, Kuru and Creutz-Jakob disease. If prions are living, then are proteins living too?
Professor Nigel Brown argues that” If a virus is alive, should we not also consider a DNA molecule to be alive? Plasmids–? What about prions?” BBC reporter Josh Gabbatiss argues “Maybe the things we think are essential are really just peculiar to life on Earth. After all, everything from bacteria to lions is derived from a single common ancestor, meaning that on our chart of life in the Universe, we only really have one data point.” And because the definition of life is extremely restricted by our personal experiences and ancestry, it will be very difficult if not impossible, to look for other living forms that could exist. How would Silicon-based life look like if Carbon was not around? We don’t know the answers. And it is possible that our search for extraterrestrial life could be doomed to failure because humans cannot fundamentally reimagine the constraints within which they exist. At the same time, there are contrarian scientists like Carol Cleland who feel that language is vague, and all terms face borderline cases. The fact that there are borderline cases – that we can’t come up with a precise cut-off – doesn’t mean there isn’t a difference. Hence, entities like viruses do not provide interesting challenges to definitions of “life.”
The origin of cellular life: Based on some clues, viruses may have influenced and even created life as we know it. The first clue is that the amount of diversity in viruses far exceeds the diversity found in cellular life. Like the fact that the huge human genomic diversity in Africa is strong evidence of the African origin of Homo Sapiens, viruses may be the fountain of life. In the words of virologist, Valerian Dolja “Where diversity lies, origin lies.” Secondly, viruses are also more diverse in reproduction compared to cellular life. A final argument is that some of the viruses that infect the three oldest domains of life – Archaea, Bacteria and Eukaryote, all share several of the same proteins, suggesting that they may have evolved the earliest life form emerged.
As posited by Patrick Forterre, one of the most famous virologists of our time, “If viruses were already present in the biosphere when the last universal common ancestor (LUCA) was living, one would expect to find some common features between viruses that now infect members of different domains. This is precisely the case. In particular, some archaeoviruses, bacterioviruses and eukaryoviruses share homologues in proteins and ATPases, suggesting that they all evolved from a common virus that existed at the time of LUCA of even before.” Other biologists go so far to argue that “the interaction of hosts with retroviruses, retrotransposons, and retroelements is one of the eternal conflicts that drive the evolution of life.”
It is hypothesized that viruses might have contributed to the formation of the first cellular life form, or that even the eukaryotic cell nucleus originates from an infection by a coated virus. In a research paper published in 2014 by the American society of microbiology, it was argued that “all life was initially a floating stew of genetic information, amino acids, and lipids. The earliest pieces of genetic material were likely short pieces of RNA with relatively few genes that often parasitized other floating bits of genetic material to make copies of themselves. Over time, the parasitic genetic elements remained unable to replicate on their own and evolved into modern-day viruses that mooch off their cellular hosts. The genes they parasitized began to evolve different types of genetic information and other barriers to protect themselves from the genetic freeloaders, which ultimately evolved into cells.” In short, the explanation towards the formation of cellular life (including the universal common ancestor of all kingdoms of life) could probably have begun with the parasitic behavior of virus particles. This interestingly could make selfishness the original sin or virtue, as one would look at it. But there is also a different perspective here. Virologist, Patrick Forterre has suggested that viruses don’t steal genes from cells but rather that cells steal genes from viruses. Viral genomes are full of genes encoding proteins whose function probably is to regulate the interactions between viruses and their hosts. They have no similarities or homologues in cellular genomes. Unlocking the secrets of viral genomes could well unveil functions of genes never seen before and the secrets of life we never knew earlier.