Metabolic and replication abilities are generally required for the conditions of life. Because genes and translation systems in cells drive metabolism and replication, “translation system (including gene)” is more fundamental element of life. Translation system itself is not life; however, it could be regarded as a marker that characterizes life. Life, matter, and their intermediates can be classified by virtue of the marker. This conception, which implies that translation mechanism is the essence of life, leads to another viewpoint on the origin of life: “translation-first” rather than “metabolism or replication-first”.
According to my previous works, the definition of life is reconsidered. A point of my view regarding the definition of life is based on taking into account the relationship between a life-like system and the environments. The analysis of life-like systems regarding the metabolism, replication and mutation of information, assignment method between information and function, individuality, and subjectivity towards the environments clearly demonstrated the analogies among life-like systems at different hierarchical levels. Based on these analysis, civilization is regarded as a similar type of biosystem to cell-type organisms. The analysis of characteristics and emergence of civilization as comparing to cell-type organisms would imply both the emergence of life and the definition of life.
Definition of life has been investigated by many researchers for many years. The researchers belonging to different fields have stressed their definitions from their view points. Therefore, they draw the line between life and non-life at different positions. Now there are many proposals to allow the meaningless of the definition of life to lead the goal of origin of life. However, I pretend to address a brief history of the definition of life and to seek some criterion of the definition of life, which may not be useful for researchers but useful for teachers. The criterion of definition of life must be a good foothold to teach students sciences including biology.
The answer to “what is life?” may be given through the complementarity principle of life between the becoming viewpoint of non-linear non-equilibrium thermodynamics and the mechanistic viewpoint of molecular biology. Thus, we propose the evolvability viewpoint to life based on theory and experimental results of evolutionary molecular engineering. Life at its origin must have the most concentrated properties for the definition of life. Life should be defined at the ecosphere level and in evolutionary time scale in order to avoid confusion from taking "parts" of life as the object. At the origin, the ecosphere level of life was the same as the biopolymer system level of life. Life is a molecular information system with much higher evolvability than non-living. Several examples of molecular mechanisms of evolvability were discussed. Fitness information as a measure of value of information was presented. Correlation between definition/origin of life and definition/origin of information was discussed.