Microbiologists Patrick Moreira and Purificación López-García, together with virologists Arturo Ludmir and Lynn Enquist, are at the center of a sharp debate over whether viruses count as living organisms or as fundamentally different genetic entities. Their exchange in the peer‑reviewed literature lays out opposing views on whether viral genomes should appear on the same evolutionary tree as bacteria, archaea and eukaryotes, turning an abstract question about “life” into a concrete dispute over how to classify viruses.
At issue is what belongs on the tree of life and how evolutionary relationships should be drawn. Moreira and López-García defend keeping viruses off that tree, while Ludmir and Enquist argue that viral genomes participate in the same evolutionary processes as cellular genes and therefore deserve a place on it. This clash over classification and ancestry shows how definitions of life can lag behind discoveries in virology and molecular evolution.
Why some researchers say viruses are not alive
One influential camp argues that viruses should be treated as non-living entities, excluded from the same biological family tree that links bacteria, plants and animals. Moreira and López-García set out a structured case for this view in a peer‑reviewed review and opinion article in a microbiology journal. In that 2009 analysis, they list ten distinct criteria for excluding viruses from the cellular tree of life, emphasizing features such as the lack of a cellular structure and the absence of an internal system for copying themselves.
Their argument leans heavily on classic textbook definitions of life that require metabolism, the ability to reproduce independently and internal machinery to keep the system running. Reporting on coronavirus biology describes that outside a host, viral particles remain dormant and can persist in this inert state for extended periods, underscoring that they do not move on their own, burn energy or copy their genetic material in that phase, according to a detailed health explainer. Moreira and López-García build on this kind of reasoning to claim that viruses are better described as mobile genetic elements that depend entirely on cellular hosts, rather than as organisms in their own right.
The “dormant outside, active inside” problem
Everyday experience during outbreaks has made one fact about viruses widely discussed: outside a host, they can sit on surfaces or float in droplets without doing anything that looks like life. That observation is central to the argument that viruses are not alive. The same account of coronavirus biology explains that outside a host, viruses are dormant and can remain in this inert state for quite a long time, based on laboratory measurements of how these particles persist on surfaces and in the air. In this dormant form, often called a virion, the virus does not move on its own, does not carry out metabolism and does not copy its genetic material.
Once a virion enters a suitable cell, however, the picture changes. The viral genome can direct the host’s molecular machinery to make new copies of itself and new viral proteins, turning the cell into a virus factory. Moreira and López-García treat this switch as evidence that the “living” part of the process belongs to the cell, not the virus, because the virus contributes instructions but not the hardware. In their 2009 review, they use that dependence as one of several criteria for excluding viruses from the cellular tree of life, arguing that the active processes belong to the infected cell’s metabolism rather than to an autonomous viral organism.
Keeping viruses off the tree of life
The phrase “tree of life” refers to the branching diagram biologists use to show how cellular organisms share common ancestors. Moreira and López-García argue that viruses should be kept off that tree because they do not share the basic cellular architecture that unites bacteria, archaea and eukaryotes. In their peer‑reviewed article, they present ten defined reasons for exclusion, framing viruses as fundamentally different from cells in origin and structure. They also treat viruses as lacking a direct line of descent comparable to that of cells, because each virus lineage depends on repeated capture of host functions to complete its life cycle.
This position has practical consequences for how evolutionary history is drawn. If viruses sit outside the tree of life, then evolutionary diagrams that compare genes across species should treat viral genes as external additions rather than as branches of the same tree. That framing leads researchers to interpret shared genetic sequences between viruses and cells as transfers or borrowings, not as signs of shared ancestry in the same sense that links two animal species. In this view, viral evolution becomes a history of genetic innovation that rides on top of cellular evolution, rather than a fourth domain of life parallel to bacteria, archaea and eukaryotes.
The counterargument: genomes that belong on the tree
Not all virologists accept that sharp boundary. In a peer‑reviewed correspondence, Ludmir and Enquist argue that viral genomes should be treated as part of the phylogenetic tree of life, even if the virions themselves are inert. Their short article, published as a correspondence in the same journal, directly engages the assertion that viruses are not alive and challenges the idea that this alone justifies excluding them from evolutionary trees. They focus on how viral genetic material behaves and evolves, rather than on whether a virion meets traditional criteria for a living cell.
Ludmir and Enquist make a key distinction between the physical particle and the genetic information it carries. In their view, inert virions, which sit dormant outside cells, are only one stage in a longer process. The viral genome, once inside a host, becomes an active part of the host’s genetic environment. Their correspondence explains that phylogenetic reasoning should track these genomes, not the inert particles, when deciding what belongs on the tree of life, because the genomes participate in the same evolutionary processes as cellular genes. From this angle, the question “are viruses alive?” matters less than whether their genetic sequences share ancestry and exchange information with the rest of life.
Redefining life around processes, not particles
These two positions expose a deeper disagreement about what “life” actually refers to. The Moreira and López-García camp treats life as a property of discrete entities with cells, metabolism and independent reproduction; anything that fails that checklist is excluded from the category of organism. Their review uses defined criteria to police that boundary, and on those terms viruses fall short. The health explainer on coronavirus biology echoes this logic by stressing that viruses have none of the traditional trappings of life, including metabolism, motion and independent reproduction, in its account of why these particles are described as hard to kill and yet not alive. This checklist approach keeps the concept of life tidy but may underplay the role of entities that strongly influence evolution without fitting neatly into the category of autonomous cell.
Ludmir and Enquist, by contrast, push toward a process-based view. Their correspondence treats viral genomes as active players in the same evolutionary processes that shape cellular DNA, even if the virions themselves are inert outside hosts. That shift in focus from particles to genomes suggests that “life” might be better understood as a network of replicating information systems, some of which, like viruses, borrow their machinery from others. From this perspective, the more informative question is how viral genomes help build and reshape the tree of life, even if the particles that carry them spend much of their time dormant.
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*This article was researched with the help of AI, with human editors creating the final content.
