ife is a characteristic distinguishing physical entities having biological processes, such assignaling and self-sustaining processes, from those that do not, either because such functions have ceased, or because they never had such functions and are classified asinanimate. Various forms of life exist, such asplantsanimalsfungiprotistsarchaea, andbacteria. The criteria can at times be ambiguous and may or may not definevirusesviroids, or potential artificial life as “living”. Biology is the primary scienceconcerned with the study of life, although many other sciences are involved.

The definition of life is controversial. The current definition is that organisms maintainhomeostasis, are composed of cells, undergometabolism, can growadapt to their environment, respond to stimuli, andreproduce. However, many other biological definitions have been proposed, and there are some borderline cases, such as viruses. Throughout history, there have been many attempts to define what is meant by “life” and many theories on the properties and emergence of living things, such asmaterialism, the belief that everything is made out of matter and that life is merely a complex form of it; hylomorphism, the belief that all things are a combination of matter and form, and the form of a living thing is its soul;spontaneous generation, the belief that life repeatedly emerges from non-life; andvitalism, a now largely discredited hypothesis that living organisms possess a “life force” or “vital spark”. Modern definitions are more complex, with input from a diversity of scientific disciplines. Biophysicists have proposed many definitions based on chemicalsystems; there are also some living systems theories, such as the Gaia hypothesis, the idea that the Earth itself is alive. Another theory is that life is the property of ecological systems, and yet another is elaborated in complex systems biology, a branch or subfield ofmathematical biologyAbiogenesis describes the natural process of life arising from non-living matter, such as simple organic compounds. Properties common to all organisms include the need for certain corechemical elements to sustain biochemicalfunctions.

Life on Earth probably arose between 3.8 and 4.1 billion years ago. It is widely accepted that all of Earth’s current life descended from anRNA world, but RNA-based life may not have been the first. The mechanism by which life began on Earth is unknown, though many hypotheses have been formulated and are often based on the Miller–Urey experiment. In July 2016, scientists reported identifying a set of 355 genes believed to be present in the last universal common ancestor (LUCA) of all living organisms.[1]

Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale. To survive in most ecosystems, life must often adapt to a wide range of conditions. Some microorganisms, calledextremophiles, thrive in physically or geochemically extreme environments that are detrimental to most other life on Earth.Aristotle was the first person to classifyorganisms. Later, Carl Linnaeus introducedhis system of binomial nomenclature for the classification of species. Eventually new groups and categories of life were discovered, such as cells and microorganisms, forcing dramatic revisions of the structure of relationships between living organisms. Cells are sometimes considered the smallest units and “building blocks” of life. There are two kinds of cells, prokaryotic and eukaryotic, both of which consist of cytoplasm enclosed within a membrane and contain manybiomolecules such as proteins and nucleic acids. Cells reproduce through a process ofcell division, in which the parent cell divides into two or more daughter cells.

Though currently only known on Earth, life need not be restricted to it, and many scientists believe in the existence ofextraterrestrial lifeArtificial life is a computer simulation or man-made reconstruction of any aspect of life, which is often used to examine systems related to natural life. Deathis the permanent termination of all biological functions which sustain an organism, and as such, is the end of its life. Extinction is the process by which an entire group or taxon, normally a species, dies out. Fossils are the preserved remains or traces of organisms.

DEATH

Animal corpses, like this African buffalo, are recycled by theecosystem, providing energy and nutrients for living creatures

Death is the permanent termination of all vital functions or life processes in an organism or cell.[183][184] It can occur as a result of an accident, medical conditionsbiological interactionmalnutritionpoisoning,senescence, or suicide. After death, the remains of an organism re-enter thebiogeochemical cycle. Organisms may beconsumed by a predator or a scavenger and leftover organic material may then be further decomposed by detritivores, organisms that 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 life ends, or when the state that follows life begins.[184]However, determining when death has occurred requires drawing precise conceptual boundaries between life and death. This is problematic, however, because there is little consensus over how to define life. The nature of death has for millennia been a central concern of the world’s religious traditions and of philosophical inquiry. Many religions maintain faith in either a kind of afterlife orreincarnation for the soul, or resurrection of the body at a later date.

Extinction

Main article: Extinction

Extinction is the process by which a group oftaxa or species dies out, reducing biodiversity.[185] The moment of extinction is generally considered 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 changinghabitat or against superior competition. InEarth’s history, over 99% of all the species that have ever lived are extinct;[186][187][188][189]however, mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.[190]

Fossils

Main article: Fossils

Fossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossil-containing rockformations and sedimentary layers (strata) is known as the fossil record. A preserved specimen is called a fossil if it is older than the arbitrary date of 10,000 years ago.[191]Hence, fossils range in age from the youngest at the start of the Holocene Epoch to the oldest from the Archaean Eon, up to 3.4 billionyears old.[192][193]

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