One of the things we often debate here is the definition of “consciousness,” but consciousness is far from the only concept that is difficult to define. Others include religion, democracy, free will, and biological life.
Life has a number of definitions, many of which are suitable for particular purposes. If I recall correctly, NASA, for purposes of detecting it in extraterrestrial environments, defines life as anything that reproduces and undergoes evolution. But many biologists insist that homeostasis is crucial aspect.
Cells are often referred to as the basic unit of life. That certainly seems true for the homeostasis definition.
(Interestingly, cells may be the basic unit of life, but they’re not the basic unit of capability. For that, we have to drop down to proteins, molecular nanomachines that do the actual work in biology. A simple cell has tens of millions of proteins. More complex cells, such as neurons, reportedly have tens of billions. It pays to remember that life began as, and remains, a molecular enterprise, with even cells being vast ecosystems.)
But here’s a question. Are viruses alive? It depends on which definition we use. They reproduce and undergo evolution, producing new species and strains all the time, as most of us are only too aware of these days. But they don’t have homeostasis.
Outside of a cell, in their “virion” phase, they consist of some genetic material (RNA or DNA) surrounded by a protein coat, both for protection and containing penetration machinery to get through cellular membranes. Once they’ve invaded the cell, they hijack that cell’s reproductive mechanisms for their own use, reproducing themselves.
Viruses appear to be evolutionarily ancient, perhaps co-evolving along with the earliest cells. Both cells and viruses may have evolved from pre-cellular biology. It’s tempting to think of the cells as farmers putting up a fence to protect their mechanisms and resources, and viruses as the raiders coming in to hijack and ransack those mechanisms and resources.
Thinking back to Aristotle’s three layers of soul, he would almost certainly have attributed a nutritive soul to cells. But I’m not sure he necessarily would have for viruses. He might have seen them as soulless parasites.
But is that entirely fair? It seems like viruses simply use the resources available to propagate, just as all heterotrophic life consumes other life to survive and propagate. In evolution, they also provide an important means of horizontal gene transfer, increasing genetic diversity in non-sexually reproducing species. And they’re often used today as vehicles for delivery gene editing therapies.
It’s worth noting that viruses are not the smallest infectious agent. Viroids are even smaller. Composed of a short circle of RNA with no protein coat, they only infect plant cells. Their existence seems to have inspired the idea that life began in an RNA world. Viroids may be leftovers from the earliest primeval life, or proto-life forms. If viruses are alive, then it seems hard to exclude viroids.
There are even smaller replicating agents: prions. These are proteins that have become misfolded, with the disturbing ability to transmit their misfolding, leading to a number of neurodegenerative conditions, such as “mad cow” disease. Are prions alive? Most biologists say no. They do reproduce, but I haven’t seen any assertions they undergo evolution. They may be more like crystals or clay, albeit of a more organic variety.
So what is life? Does it require a cell? Or do viruses and viroids count? Thinking in terms of extraterrestrial life, it seems possible to conceive of mechanisms we might discover that would challenge any definitions we come up with. Suppose we encountered a sort of organic clay somewhere.
Reality often seems to delight in ruining our little categories.