|A magnified virus|
First seen as poisons, then as life-forms, then biological chemicals, viruses today are thought of as being in a gray area between living and nonliving: they cannot replicate on their own but can do so in truly living cells and can also affect the behavior of their hosts profoundly. The categorization of viruses as nonliving during much of the modern era of biological science has had an unintended consequence: it has led most researchers to ignore viruses in the study of evolution.We have to also take into account other causal possibilities, for example, you would think that water is a necessary condition for life. According to astrobiologist Benton Clark not so:
Life on Earth evolved with water, and so today life on Earth is dependent on that resource. But we cannot say that without water, life is impossible. On Earth, life has been able to adapt to the harshest environments, so it is possible that life may have found a way to survive on worlds that have no liquid water.According to Clark, living organisms exhibit at least 102 observable qualities. Adding all these qualities together into a single - if exceedingly long - definition still does not capture the essence of life.
For the sake of problematizing, take a look at proposed conditions:
1. A network of inferior negative feedbacks (regulatory mechanisms) subordinated to a superior positive feedback (potential of expansion, reproduction).
2. A systemic definition of life is that living things are self-organizing and autopoietic (self-producing). Variations of this definition include Stuart Kauffman's definition as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle.
3. Living beings are thermodynamic systems that have an organized molecular structure.
3(a). Things with the capacity for metabolism and motion.
4. Life is a delay of the spontaneous diffusion or dispersion of the internal energy of the biomolecules towards more potential microstates.
5. Life is a way to "hydrogenate carbon dioxide," at least at its very beginnings, according to physicist Sean Carroll.
6. Life is a self-sustained chemical system capable of undergoing Darwinian evolution.
7. Life is matter that can reproduce itself and evolve as survival dictates.
8. Life is self-reproduction "with variations."
9. Life is self-reproduction "with an error rate below the sustainability threshold."