I don’t feel we will have a definition of life until we more fully understand the origins of life at least 3.7 billion years ago. The current theory is that the transition from pre-biotic chemistry to life occurred via RNA. RNA works as a molecule of data-storage, replication and it can be folded to catalyze reactions without proteins. Ribosomes which translate RNA codons to amino-acid sequences are composed of RNA and the active site is entirely compared of RNA. Many of the important energy carrying molecules and co-factors common to all life such as ATP, Acetyl-CoA and NADH are related to RNA in their synthesis. Harold White in 1972 called them "fossils of nucleic acid enzymes." I love that :) DNA evolved later as a more stable form of storage and proteins largely replaced RNA as catalysts due to the greater variety of their structural components. But the basis would have been RNA.
I have several criteria for defining life but to me personally the single most fundamental criteria is that life uses a template based digital mechanism that can potentially encode, decode and replicate genetic data that defines, organizes and sustains the phenotype of itself and its replication units under specific environmental conditions. And as an information storage system and self-organizing unit that can reproduce it by definition decreases system entropy. But I hesitate to define life at the earliest stage as being able to control enthalpy.
Crystals are fascinating because they can replicate across a substrate based on the nucleation of a single crystal. They exhibit some of the features of template based reproduction but they lack any means of digitally encoding, decoding and replicating their own genetic data.
Computers can encode digital data about their own design and manufacturing process but have no template based mechanism to construct their own phenotype or replicate.
I differ from most biologists because I believe that viruses are a relatively advanced form of life and that they meet the most demanding aspect of the definition. I feel that all living organisms must exist within strict environment al conditions or they will cease to be alive. We can’t exist in outer space for example. We require certain temperatures, gases, food sources, etc. The existence of a cell wall to me is just another environmental condition for some forms of life. To a virus the cell wall is provided by its environment. It has no need to manufacture one. We require certain conditions which we don’t need to manufacture ourselves so I find the distinction artificial at best. Some would argue that the definition of life should include at least the ability to complete a thermodynamic work cycle and that in turn requires a cell wall, etc. That is certainly true of life as we know it today. I’m not sure if that was the case during the transition to life when the system could have included a non-living energy source.
Interestingly while we tend to think of a virus as an RNA or DNA molecule surrounded by a protein coat there is a gray area: viroids. Viroids infect plants and may be comprised of just a couple of hundred nucleotides and no protein coat at all. They are the simplest naturally reproducing entities we know of. And their RNA is very simple and non-coding possibly the vestige of our earliest ancestors. Notably they lack any protein coding ability at all reflecting possibly an age where proteins were no a part of the biotic process. Diener published about their significance in 1989 but like all things related to botany it was completely ignored by the rest of academia. Remember Gregor Mendel and his pea experiment which quantified the rules of genetic heredity? Or the discovery of “jumping genes” (transposons) in corn by Barbara McClintock? Academia has a tendency to ignore any discovery made no matter how important if it relates to the field of botany :(
If we look at current theories regarding the origins of life RNA is supposed to have catalyzed reactions before the existence of a cell membrane possible under “flow cell” conditions. The flow cell theory has not been proven yet but certainly other methods of providing components necessary for life couldn’t have existing in the tar like goo created in Miller–Urey experiment.
We have to be careful to define “potentially reproduce” because a spayed cat or the last Dodo bird cannot reproduce but they could have reproduced in the correct environment but they are / were very much alive and otherwise fit the definition of a living organism.
And the phenotype requires a “specific environment.” My genes encode my fair skin but if you leave me in the sun for too long I become as red as a Maine lobster.
But when we look at RNA we see that it encompasses all the requirements for the transition from pre-biotic chemistry to life 1) It has the ability to self-replicate 2) I can catalyze reactions 3) It can catalyze polypeptide bonds between amino acids to form structural and enzymatic proteins. It can also conjugate with amino-acids and other compounds itself making very complex molecules that could have played a role we know nothing about in early biotic systems. And it has been demonstrated that purine RNA bases can be produced without a biological system. Adenine is merely a pentamer of hydrogen cyanide. Sutherland has shown that abiogenic production of Pyrimidines is possible with a chirality of 60% although this is much more difficult than production of purines. In 2011 NASA showed that some meteorites on Earth contained building blocks for RNA (adenine and guanine) and last year NASA showed that complex strands of RNA and DNA could be formed in the conditions of outer space near carbon rich red giants. This biggest obstacle in understanding the origins of life is that assembly of RNA nucleotides requires a high energy phosphate bond on earth which is usually provided by ATP. Activation of AMP to ADP or ATP is not likely to occur abiogenically. But once you pass this hurdle you only need an autocatalytic ribosome to form life and this can be demonstrated with as few as five nucleotides. Several theories including flow-cells, polycyclic aromatic hydrocarbons and catalytic transition metal centers formed in volcanoes have all been put forth as plausible explanations for the provision and concentration of energetic compounds.
There are many other defining features of life as it exists now. But these include many advanced states that may not have been present in the first transition to a living form.
