Viruses are routinely left off, including in popular versions such as the Interactive Tree of Life. Without viruses, one cannot fully understand the mechanisms of evolution, says Hill. Viruses are wildly abundant. They infect all cellular life, from single-celled bacteria to elephants, and they are especially dense in the ocean, where they work as a gigantic recycling network, ripping apart 20 percent of the bacteria and other microbes there each day to release tons of carbon, which is then used by other microorganisms to grow.
Viral DNA is transmitted not only from one viral particle to its progeny, but also to other viruses and other species. Because of this, viral genetic sequences have permanently taken up residence in the genomes of all organisms, including ours, and we rely on them. Viral DNA is required for the formation of the mammalian placenta; it is crucial in the growth of early embryos; and the human innate immune system is made up, in part, of ancient viral proteins.
When a person is fighting COVID, they are doing it with the help of viruses that colonized our cells long ago. Viruses are not a missing branch of the tree of life; they are woven into every limb and leaf.
Scientists may always dispute whether viruses are alive or not, but they can hopefully agree on the importance of viruses to life as we know it. Scientists and journalists share a core belief in questioning, observing and verifying to reach the truth. Science News reports on crucial research and discovery across science disciplines. We need your financial support to make it happen — every contribution makes a difference. Subscribers, enter your e-mail address for full access to the Science News archives and digital editions.
Not a subscriber? Become one now. Skip to content. Science News Needs You Support nonprofit journalism. By Megan Scudellari November 1, at am. Read more. Coming to Terms It is easy to see why viruses have been diffi cult to pigeonhole.
They seem to vary with each lens applied to examine them. Because they were clearly biological themselves and could be spread from one victim to another with obvious biological effects, viruses were then thought to be the simplest of all living, gene-bearing life-forms. Their demotion to inert chemicals came after , when Wendell M. Stanley and his colleagues, at what is now the Rockefeller University in New York City, crystallized a virus— tobacco mosaic virus—for the fi rst time.
They saw that it consisted of a package of complex biochemicals. But it lacked essential systems necessary for metabolic functions, the biochemical activity of life. Stanley shared the Nobel Prize— in chemistry, not in physiology or medicine—for this work. Further research by Stanley and others established that a virus consists of nucleic acids DNA or RNA enclosed in a protein coat that may also shelter viral proteins involved in infection.
By that description, a virus seems more like a chemistry set than an organism. But when a virus enters a cell called a host after infection , it is far from inactive. These behaviors are what led many to think of viruses as existing at the border between chemistry and life. More poetically, virologists Marc H.
Molecular biologists went on to crystallize most of the essential components of cells and are today accustomed to thinking about cellular constituents—for example, ribosomes, mitochondria, membranes, DNA and proteins—as either chemical machinery or the stuff that the machinery uses or produces. This exposure to multiple complex chemical structures that carry out the processes of life is probably a reason that most molecular biologists do not spend a lot of time puzzling over whether viruses are alive.
For them, that exercise might seem equivalent to pondering whether those individual subcellular constituents are alive on their own. This myopic view allows them to see only how viruses co-opt cells or cause disease. The more sweeping question of viral contributions to the history of life on earth, which I will address shortly, remains for the most part unanswered and even unasked. For example, a living entity is in a state bounded by birth and death.
Living organisms also are thought to require a degree of biochemical autonomy, carrying on the metabolic activities that produce the molecules and energy needed to sustain the organism. This level of autonomy is essential to most definitions.
Viruses, however, parasitize essentially all biomolecular aspects of life. That is, they depend on the host cell for the raw materials and energy necessary for nucleic acid synthesis, protein synthesis, processing and transport, and all other biochemical activities that allow the virus to multiply and spread.
One might then conclude that even though these processes come under viral direction, viruses are simply nonliving parasites of living metabolic systems. But a spectrum may exist between what is certainly alive and what is not. A rock is not alive. A metabolically active sack, devoid of genetic material and the potential for propagation, is also not alive. A bacterium, though, is alive. Although it is a single cell, it can generate energy and the molecules needed to sustain itself, and it can reproduce.
But what about a seed? A seed might not be considered alive. Yet it has a potential for life, and it may be destroyed. In this regard, viruses resemble seeds more than they do live cells. They have a certain potential, which can be snuffed out, but they do not attain the more autonomous state of life. On the other end of the spectrum, a different criterion for defining life would be the ability to move a genetic blueprint into future generations, thereby regenerating your likeness.
In the second, more simplistic definition, viruses are definitely alive. They are undeniably the most efficient entities on this planet at propagating their genetic information. Although there is no definitive resolution to the question of whether viruses can be considered living entities, their ability to pass on genetic information to future generations makes them major players in an evolutionary sense.
It is a surprise to most who think of viruses simply as parasites that they make up the largest component of biomass on this planet Bamford , Research in Microbiology ; So far every living organism that has been studied to date has had at least one virus associated with it, and viruses out number all other life forms by at least an order of magnitude Ackerman , Research in Microbiology ; When considering that not only is viral presence on this planet all encompassing, but every sequenced organism to date has a major component of its genome that is viral in origin, it becomes apparent that viruses are integral players in the evolution of what we presently consider life.
Organization and complexity have slowly increased from the time that macromolecules began to assemble in the primordial soup. One has to ponder the existence of an inexplicable principal that is in direct opposition to the 2nd law of thermodynamics that drives evolution toward higher organization. Not only have viruses been extremely efficient at propagating their own genetic material, they have also been responsible for untold movement and mixing of genetic code between other organisms.
Variability of genetic code is arguably the driver of evolution. Through the expression of variable phenotypes, organisms are able to adapt and become more efficient in changing environments.
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