Viruses and Life on Earth

Abrupt changes in the number of living animals in past millennia? Major dips indicate mass extinctions following cataclysmic events (last one: picture above uc.sc). Minor dips represent declines in dominant species likely to be caused by viral pandemics

By prof. Kirthi Tennakone

National Institute of Fundamental Studies

Viruses exist everywhere in association with living things, decisively influencing their evolution as well as behaviour. A virus secures a message encoded in its genome by a cover of proteins and lipids – carrying a warning, as if written and kept in a sealed envelope to be opened.

When a species attempts to be too smart by exploiting common resources and procreate endlessly; the virus intervenes to control the expansion.

Pandemics and epidemics caused by viruses had adversely affected humans, colonies of animals and cultivations. Nonetheless, the mission of viruses to alert species to not monopolise and expand; appears to be a crucial factor that diversified and preserved life on earth. What viruses do today to teach us is painful, but they were our progenitors and saviors.

If not for viruses and pandemics they created in the past; we ourselves and lively animals and plants around us would not have existed. Viruses may also have acted as the precursor agent which created life.

The origin of life on earth, its evolution into species has been shaped by viruses. Until humans evolved, viruses did not permit one species to dominate and rule the earth. They might dictate terms to us in future and sway our destiny.

Origin of viruses and origin of life

How viruses came to being remains a puzzle intimately connected with the mystery of the origin of life. Although life occurs everywhere on earth in different forms and perhaps in planets circling distant stars – what life means, evades rigorous and consistent definition. Generally, entities capable of storing information and replicate by digestion of substances in the environment are considered as living. Another characteristic of life is the aptness to undergo Darwinian evolution – the inherent capacity to mutate into variants so that ones fitting the environment survive and reproduce. For this reason, United States National Aeronautics and Space Agency (NASA) defined life as self-sustaining chemical system capable of Darwinian evolution. In terms of above interpretations; viruses fall outside the domain of living systems, because they can replicate only by entering a living cell and hijacking its machinery to reproduce. Unlike bacteria, viruses cannot grow in dead animal or plant tissue. Possibly they stand in between living and non-living, based exclusively on ribonucleic acid (RNA) when they first originated. Quite a number of present day pathogenic viruses are also RNA based.

RNA is a chain like motif decorated with four different types of molecular beads referred to as nucleotides bases, denoted by symbols A, C, G and U. The sequential arrangement of the beads encode information; just like a message written in a four letter alphabet language.

According to one school of thought, viruses came first and evolved into advanced forms of life. Later they learned to invade living organisms as parasites. This idea supports RNA hypothesis of the origin of life, where most primitive replicating biomolecules were considered to be forms of RNA; naturally synthesised out of nucleotides floating in prebiotic oceans.

If viruses came first; why did they resort to parasitism later? Possibly as life in prebiotic oceans advanced evolving into cellular organisms; chemical substances essential for primitive viruses to breed were exhausted, forcing those viral variants with a taste for cellular life to proliferate.

Viruses are the smallest self-replicating entities; size ranging from 200 – 400 nanometers (1 nanometer = one millionth of a millimeter), but their reproduction happens inside a living cell. Recently several types of giant viruses, larger than 1000 nanometers and comparable to size of an average bacterium has been discovered. Amazingly these giant viruses self-replicate, just like unicellular microbes without a host cell – suggesting virus-like entities finally evolved into advanced forms of life. The very thing from which we may have been created is now threatening us!

How viruses infect cells?

Viruses attack animals, plants, bacteria and all the other cellular organisms. They are host specific; a virus sickening an animal, rarely infect humans directly. The host specificity generally depends on ability of the pathogen to bind or attach to the host cell membrane. To impregnate genetic information, the virus must affix itself to the tissues of the host. Viruses have acquired intricate strategies to invade the cells by this mechanism. The outer shell of a virus known as the capsid serves to protect its genome. The structure of the capsid and proteins there have evolved to anchor the virus into specific sites in host tissue known as receptors. In the case of the corona virus, a protein in the spikes bind to a receptor on the host cell membrane named ACE2 – kind of protein found in a wide range of human cell membranes – notably those in mouth, nose, throat and lungs.

After binding to the receptor, virus injects the genetic substance to the interior of the cell. Viral RNA or DNA (some viruses are DNA based) intermingle with DNA of the cell. Thereafter, the virus commandeer the cell to obey instructions written in its genome and make copies of itself, using energy and resources of the cell. The sickened cell burst open releasing virus particles, which infect other cells. The above process, known as the lytic pathway of viral reproduction, kills the host cell. Sometimes virus inside the cell, replicate when the cell divides, via so-called lysogenic mode of reproduction. Lysogenic reproduction helps virus to evade host immunity and prolong the infection. Coronavirus is lytic, whereas the HIV virus switches from one mode to the other.

Virus variations: the ability viruses to undergo genetic changes

The invasive potential of RNA viruses rests largely on their inherent flair to undergo genetic changes frequently to exploit Darwinism. When a RNA virus replicate errors would occur in the sequence of the nucleotide bases A, C.G and U. For example the sequence AACU may be wrongly copied as AACG. Such accidental changes or mutations alter the character of the virus progeny. Often the mutations turn out be ineffective or deleterious to survival of the virus; but occasionally, the variant (one produced by mutation) may acquire qualities more favorable for its proliferation, such as resistance to host immunity or faster transmission. The probability of a mutation in a RNA virus per replication exceed that of a host organism million fold. Furthermore, viruses replicates at rates orders of magnitude faster than the host and their numbers are astronomically larger. Thus in the case of RNA viruses the likelihood of emergence of variants spreading faster would be significantly high. That is why in a period less than two years we have seen several potentially dangerous variants of the coronavirus. More people getting infected and longer the pandemic lasts; the chances of virus mutating to variants is higher.

Viruses also have disposition to undergo major genetic changes described as antigenic shifts. When different viruses infect the same cell, a segment of RNA from one virus could get inserted into the genome of the other as a recombination. Some viruses have more than one strands of RNA, in this situation, strands could be exchanged by a process known as re-assortment. Spanish flu virus is believed to have originated by re-assortments involving viruses from avian, swine and human sources.

When we allow opportunities for the virus to breed; we are at the risk of being confronted by new mutant variants or antigenically shifted strains that spread even faster. The strategy a virus adopts to achieve this objective may inadvertently turn out be a more virulent attack that escalates the death toll. Virus gains no benefit by being virulent.

Host congregation and overpopulation

Despite the advantage of fast mutability (Darwinian superiority), viruses predisposes a frailty. They being delicate and minute; cannot survive outside host for very long; without getting denatured by heat, sunlight and other environmental conditions. As such, viruses find hard to move from one host to another, unless the hosts position close proximity to each other.

If a kind of animals or plants dominate segregating into densely populated colonies, at the expense of common resources to be shared by other species; viruses invariably gain access to the system, sometime or other, creating an epidemic or pandemic! The result would be limitation of the population, but not up to the point of extinction; because when the susceptible host population thins out as a result of immunity and deaths, the virus stops spreading. In the vacant niches opened – up, other varieties of plants and animal flourish. Even the original affected species, may regain strength and reappear. Thus far, because of ingenuity, humans have succeeded in evading the eventuality of this phenomenon, but for how long?

From prehistoric to modern times, pandemics have abetted the diversity of life and paved way for social reforms. Similarly, past extinction events had eventually made the world of flora and fauna more diverse and sustainable.

History tells when destructive calamities end or made to retract; new opportunities surface – communities of organisms have progressed that way.

Extinctions and aftermath

Fossil records reveal life on earth suffered several mass extinctions wiping out a large percentage of plants and animals in a short time. Planetological evidence points to the conclusion; volcanic eruptions, climate change and an asteroid impact as the causes of the major destructions. In between mass extinctions there had also been more frequent minor ones indicative of disappearance or marked decline of the population of some species. Arguments have presented to explain these endangerments to as viral pandemics.

Mass extinctions events initially interrupted life drastically curtailing the diversity. Amazingly habitats recovered – diversity regained or increased beyond the original index.

The Permian- Triassic extinction originating from catastrophic volcanic eruptions in Siberia 252 million years ago, killed over 90 and 70 percent of marine and land animals. In about six million years; biodiversity regained, new species adopting more advanced life styles appeared.

The extinction that changed the world for ever was caused by impact of an asteroid 66 million years ago. Gigantic dinosaurs who dominated the world for millennia vanished leading the way for mammals to takeover. Perhaps viruses helped our ancestor mammalians to establish by killing medium size reptiles survived the asteroid catastrophe, but immunocompromised by food shortage – opening the way for humans to evolve. Brain became more important than the body size and one species dominated the biosphere. Human innovation succeeded in resisting natural forces limiting the undue expansion of the species. They controlled illnesses, cultivated crops on large scale, using science based techniques. However, maintaining a population continuing to increase, present new challenges; because we are getting vulnerable to the same natural predicaments which limited the growth of animal populations.

In between major extinctions arising from environmental calamities, there were more frequent disappearances of many species. When animals and plants overpopulate, viruses intervene to bring forth pandemics, limiting the population. Are we approaching a similar scenario?

Emerging zoonotic diseases and future pandemics

Everyone knows COVID-19 is caused by an agent termed a virus. Viruses are all over latently hiding in the bodies of animals and humans. Infrequently, dependent on environmental conditions, a virus from an animal could move to a human, get adapted to the new host and cause an epidemic or a pandemic. Almost all previous viral epidemics and pandemics including measles, rubella, smallpox, Spanish flu initially surfaced in this manner are said to zoonotic in origin.

Zoonotic diseases or zoonoses reached epidemic proportions when humans segregated into settlements, as viruses spread when hosts live close together. Settlers domesticated animals; close contact transferred pathogens from animals to humans. Measles is believed be the dog distemper virus adapted to humans and Spanish flu a zoonosis associated with pigs and birds.

Following industrialisation and multiplication of urban localities of increasing population density, more zoonotic diseases turned into epidemics. Polio has been an ancient disease but epidemics did not occur until early 20^th century. Dengue spread all over tropics; beginning late 1960s, when urbanisation congregated people into cities.

Dengue, chikungunya, zika and other existing or emerging insect vector mediated viral infections poses a severe threat for of the following reason. Insects are a very successful species. They have stubbornly survived all mass extinctions, since they evolved 500 million years ago. No other widespread species has been exposed to viruses for so long and as such they have acquired strong innate immunity to viruses. Therefore, they can harbor viruses asymptomatically, without getting manifestly sick and transfer the virus to vulnerable humans and animals.

Recently, within a short span of time, a number of zoonotic diseases have emerged; Ebola, Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS), Avian flu (H5N1), Zika, Nipah and lately COVID- 19 . When greedy humans encroach habitats occupied by animals or spoil the environment by their activities; wild life has no alternative than to move closer to humans. The viruses causing the diseases; SARS, MERS, Nipah and COVID -19 are believed to have disseminated from bats, when their habitats got disturbed. Rearing farm animals in torturous congested conditions, create situations conducive for them to catch infections of wild species and transfer them to humans after genetic modifications. Loss of biodiversity create conditions favorable for zoonotic disease to emerge and turn into epidemics for pandemics.

So many species of animals and have gone extinct because of anthropogenic activities and millions are threatened. On this scale, viruses have not exterminated a single species – they only limit unwarranted expansion of species. We are accustomed to think that the non-living senseless virus is the culprit; when the real cause has been our behavior.

In 1901 the French Chemist Le Chatelier enunciated a principle, which goes after his name. Le Chatelier’s principle states “If a system is stressed, the system reacts in such a way to relieve the stress “. If the system is taken as the collection of flora and fauna of the biosphere and stress as human activities endangering the biosphere; it follows from the principle that natural forces in the biosphere will react to human activities. The present pandemic and emerging zoonotic diseases are example of such reactions.

World needs to be prepared to counter pandemics. Swift action once they emerge would not solve the problem as the virus may propagate and mutate much faster than our response. Vaccines are proven to be effective. Understanding required to design vaccines and installing manufacturing plants and rolling inoculations to the global population takes years. The real offender that bring forth pandemics is our behavior. Environmental destruction, occupation of the habitats of wild species and unwarranted congregation at all levels of association, prompts pandemics to emerge and propagate. The other factor is ignorance and irrationality of thought, continuing to prevail – many advocate myth and pseudoscience. The pandemic is a signal that humanity needs to adjust and change collectively for betterment.