Life might be defined as the capacity to conduct respiration and to use the energy from that process to construct and repair the structural and functional chemicals of an organism (guided by the nucleic acids, DNA and RNA); once life fails (due to disease, injury or aging) and death ensues, the organism degrades and ceases to exist. While the term "respiration" is commonly associated with breathing and lung function, biologic respiration refers to the process of energy production from glucose (which, itself, may be derived from complex carbohydrates, fats or proteins). This intracellular process may be aerobic (relying on the presence of oxygen and generating carbon dioxide and water) or anaerobic, in which case lactate is the waste product. (See also Carbon & Life)
Life first evolved in the primordial seas of Earth about 3.6 billion years ago; for the first half of that history, all life was unicellular. Once more complex, multicellular life forms evolved, cellular specialization, governed by the regulation of gene expression, resulted in the development of organs and tissues that provided unique physical characteristics and functional capabilities; through the process of natural selection, those traits that favored survival were retained.
Though reproductive capability is essential to the survival of a species, it is not vital to the life of an individual except with regard to tissue repair and growth. All life forms must be able to ingest food, generate energy, repair defects and dispose of waste but the ability or inability to reproduce has no significant impact on an individual's lifespan; indeed, risks associated with pregnancy and birthing have long shortened the life of many female animals. Finally, while many humans believe that there is a spiritual element to life, science has yet to confirm its presence.
