Life is a continuous experiment of single and multi-celled animated forms that exist to consume energy in one form or another so they produce activity within their environment.  These life forms can exist as solids that are mostly liquid within a thin membrane (deep sea creatures) to forms that are predominately solids with little liquid or perceived movement (ancient trees and shrubs).  The definition of life includes viruses (organic and electronic).  If we stretch this definition it might include clouds as a gaseous liquid, however most scientists have not recognized these gaseous forms yet.

Narrow Scientific Definition by Wikipedia

Since there is no unequivocal definition of life, the current understanding is descriptive, where life is a characteristic of organisms that exhibit all or most of the following phenomena:

  1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
  2. Organization: Being structurally composed of one or more cells, which are the basic units of life.
  3. Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
  4. Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
  5. Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism’s heredity as well as the composition of metabolized substances, and external factors present.
  6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multi-cellular organisms. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and by chemotaxis.
  7. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.

Proposed Broader Definition
To reflect the minimum phenomena required, some have proposed other biological definitions of life:

  • Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation.
  • A network of inferior negative feedbacks(regulatory mechanisms) subordinated to a superior positive feedback (potential of expansion, reproduction).
  • A systemic definition of life is that living things are self-organizing and antipoetic (self-producing).
  • Life is a self-sustained chemical system capable of undergoing Darwinian evolution.
  • Things with the capacity for metabolism and motion.

Viruses

Viruses are most often considered replicators rather than forms of life. They have been described as “organisms at the edge of life”, since they possess genes, evolve by natural selection, and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not metabolize and require a host cell to make new products. Virus self-assembly within host cells has implications for the study of the origin of life, as it may support the hypothesis that life could have started as self-assembling organic molecules.

Living Systems Theories

Some scientists have proposed in the last few decades that a general living systems theory is required to explain the nature of life. Such general theory, arising out of the ecological and biological sciences, attempts to map general principles for how all living systems work.

Gaia Hypothesis

The idea that the Earth is alive is probably as old as humankind, but the first public expression of it as a fact of science was by a Scottish scientist, James Hutton. In 1785 he stated that the Earth was a super organism and that its proper study should be physiology. Hutton is rightly remembered as the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the nineteenth century. The Gaia hypothesis, originally proposed in the 1960s by scientist James Lovelock, explores the idea that the life on Earth functions as a single organism which actually defines and maintains environmental conditions necessary for its survival.

Life as a Property of Ecosystems

A systems view of life treats environmental fluxes and biological fluxes together as”reciprocity of influence”, and a reciprocal relation with environment is arguably as important for understanding life as it is for understanding ecosystems. Life is a property of an ecological system rather than a single organism or species. Mutualism is a key to understanding the systemic, order-generating behavior of life and ecosystems.

Origin of Life (Scientific Definition)

Evidence suggests that life on Earth has existed for about 3.7 billion years. All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. There are many different hypotheses regarding the path that might have been taken from simple organic molecules via pre-cellular life to protocells and metabolism. Many models fall into the “genes-first” category or the “metabolism-first” category, but a recent trend is the emergence of hybrid models that combine both categories. There is no scientific consensus as to how life originated and all proposed theories are highly speculative. However, most currently accepted scientific models build in one way or another on the following hypotheses:

  • The Miller-Urey experiment, and the work of Sidney Fox, suggest that that conditions on the primitive Earth may have favored chemical reactions that synthesized some amino acids and other organic compounds from inorganic precursors.
  • Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.

Conditions for Life

The diversity of life on Earth today is a result of the dynamic interplay between genetic opportunity, metabolic capability, environmental challenges, and symbiosis. For most of its existence, Earth’s habitable environment has been dominated by microorganisms and subjected to their metabolism and evolution. As a consequence of such microbial activities on a geologic time scale, the physical-chemical environment on Earth has been changing, thereby determining the path of evolution of subsequent life. For example, the release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced fundamental, global changes in the Earth’s environment. The altered environment, in turn, posed novel evolutionary challenges to the organisms present, which ultimately resulted in the formation of our planet’s major animal and plant species. Therefore this ‘co-evolution’ between organisms and their environment is apparently an inherent feature of living systems.

Types of Life

Life on the Earth can be divided into categories and sub-categories based on the level of complex development within the organism.

  1. Monera Kingdom [10,000 species]: Unicellular and colonial–including the true bacteria (eubacteria) and cyanobacteria (blue-green algae).
  2. Protista Kingdom (Protoctista) [250,000 species]: Unicellular protozoans and unicellular & multi-cellular (macroscopic) algae with 9 + 2 cilia and flagella (called undulipodia).
  3. Fungi Kingdom [100,000 species]: Haploid and dikaryotic (binucleate) cells, multicellular, generally heterotrophic, without cilia and eukaryotic (9 + 2) flagella (undulipodia).
  4. Plant Kingdom [250,000 species]: Haplo-diploid life cycles, mostly autotrophic, retaining embryo within female sex organ on parent plant.
  5. Animal Kingdom [1,000,000 species]: Multi-cellular animals, without cell walls and without photosynthetic pigments, forming diploid blastula.
    1. Invertebrates
      1. Protozoa – Coral, jellyfish, sea anemone
      2. Annelids – Worms
      3. Mollusks – Shell Fish
      4. Echinoderms – Starfish, Sea Urchins
      5. Crustaceans – Lobsters, Crabs
      6. Arachnids – Spiders
      7. Insects – Bugs, Beetles, Flies
    2. Vertebrates
      1. Fish
      2. Birds
      3. Amphibians
      4. Reptiles
      5. Mammals

Range of Tolerance

The inert components of an ecosystem are the physical and chemical factors necessary for life – energy (sunlight or chemical energy), water, temperature, atmosphere, gravity, nutrients, and ultraviolet solar radiation protection. In most ecosystems the conditions vary during the day and often shift from one season to the next. To live in most ecosystems, then, organisms must be able to survive a range of conditions, called ‘range of tolerance’. Outside of that are the ‘zones of physiological stress’, where the survival and reproduction are possible but not optimal. Outside of these zones are the ‘zones of intolerance’, where life for that organism is implausible. It has been determined that organisms that have a wide range of tolerance are more widely distributed than organisms with a narrow range of tolerance.

Death

Death is the permanent termination of all vital functions or life processes in an organism or cell. After death, the remains of an organism become part of the biogeochemical cycle. Organisms may be consumed by a predator or a scavenger and leftover organic material may then be further decomposed by detritivores, organisms which recycle detritus, returning it to the environment for reuse in the food chain. One of the challenges in defining death is in distinguishing it from life. Death would seem to refer to either the moment at which life ends, or when the state that follows life.

Extinction

Extinction is the gradual process by which a group of species dies out, reducing biodiversity. The moment of extinction is generally considered to be the death of the last individual of that species. Because a species’ potential range may be very large, determining this moment is difficult, and is usually done retrospectively after a period of apparent absence. Species become extinct when they are no longer able to survive in changing habitat or against superior competition. Over the history of the Earth, over 99% of all the species that have ever lived have gone extinct; however, mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.