section_path: Life
section_name: Life
paragraph_text: Life
  
  Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life is characterized by organization, metabolism, growth, adaptation, response to stimuli, and reproduction. Life may also be said to be simply the characteristic state of organisms.
  
  Present day organisms from viruses to humans possess a self-replicating informational molecule (genome), either DNA or RNA (as in some viruses), and such an informational molecule is probably intrinsic to life. It is likely that the earliest forms of life were based on a self-replicating informational molecule (genome), perhaps RNA or a molecule more primitive than RNA or DNA. The specific deoxyribonucleotide / ribonucleotide sequence in each currently extant individual organism contains sequence information that functions to promotes survival, reproduction, and the capacity to acquire resources necessary for reproduction, and such sequences probably emerged early in the evolution of life. Survival functions present early in the evolution of life likely also included genomic sequences that promote the avoidance of damage to the self-replicating molecule and also the capability to repair such damages that do occur. Repair of some genome damages may have involved using information from another similar molecule by a process of recombination (a primitive form of sexual interaction).
  
  Properties common to terrestrial organisms (plants, animals, fungi, protists, archaea, and bacteria) are that they are cellular, carbon-and-water-based with complex organization, having a metabolism, a capacity to grow, respond to stimuli, and reproduce. An entity with these properties is generally considered life. However, not every definition of life considers all of these properties to be essential. Human-made analogs of life may also be considered to be life.
  
  The biosphere is the part of Earth's outer shell—including land, surface rocks, water, air and the atmosphere—within which life occurs, and which biotic processes in turn alter or transform. From the broadest geophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere (rocks), hydrosphere (water), and atmosphere (air). The entire Earth contains over 75 billion tons (150 trillion pounds or about 6.8×10 kilograms) of biomass (life), which lives within various environments within the biosphere.
  
  Over nine-tenths of the total biomass on Earth is plant life, on which animal life depends very heavily for its existence. More than 2 million species of plant and animal life have been identified to date, and estimates of the actual number of existing species range from several million to well over 50 million. The number of individual species of life is constantly in some degree of flux, with new species appearing and others ceasing to exist on a continual basis. The total number of species is in rapid decline.