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The rationalist Maimonides sees the ethical and ritual divine commandments as a necessary, but insufficient preparation for philosophical understanding of God, with its love and awe. The Jewish mystical Kabbalah gives complimentary esoteric meanings of life. As well as Judaism providing an immanent relationship with God personal theism , in Kabbalah the spiritual and physical creation is a paradoxical manifestation of the immanent aspects of God's Being panentheism , related to the Shekhinah Divine feminine.

Jewish observance unites the sephirot Divine attributes on high, restoring harmony to creation. In Lurianic Kabbalah , the meaning of life is the messianic rectification of the shattered sparks of God's persona, exiled in physical existence the Kelipot shells , through the actions of Jewish observance. Christianity has its roots in Judaism, and shares much of the latter faith's ontology. Its central beliefs derive from the teachings of Jesus Christ as presented in the New Testament.

Life's purpose in Christianity is to seek divine salvation through the grace of God and intercession of Christ John The New Testament speaks of God wanting to have a relationship with humans both in this life and the life to come, which can happen only if one's sins are forgiven John 3: In the Christian view, humankind was made in the Image of God and perfect, but the Fall of Man caused the progeny of the first Parents to inherit Original Sin and its consequences.

Christ's passion , death and resurrection provide the means for transcending that impure state Romans 6: The good news that this restoration from sin is now possible is called the gospel. The specific process of appropriating salvation through Christ and maintaining a relationship with God varies between different denominations of Christians, but all rely on faith in Christ and the gospel as the fundamental starting point.

Salvation through faith in God is found in Ephesians 2: The gospel maintains that through this belief, the barrier that sin has created between man and God is destroyed, thereby allowing God to regenerate change the believer and instill in them a new heart after God's own will with the ability to live righteously before him. This is what the terms Born again or saved almost always refer to. In the Westminster Shorter Catechism , the first question is: God requires one to obey the revealed moral law, saying: The Apostle Paul also answers this question in his speech on the Areopagus in Athens: Catholicism 's way of thinking is better expressed through the Principle and Foundation of St.

All other things on the face of the earth are created for human beings in order to help them pursue the end for which they are created. It follows from this that one must use other created things, in so far as they help towards one's end, and free oneself from them, in so far as they are obstacles to one's end.

To do this, we need to make ourselves indifferent to all created things, provided the matter is subject to our free choice and there is no other prohibition. Thus, as far as we are concerned, we should not want health more than illness, wealth more than poverty, fame more than disgrace, a long life more than a short one, and similarly for all the rest, but we should desire and choose only what helps us more towards the end for which we are created. Mormonism teaches that the purpose of life on Earth is to gain knowledge and experience and to have joy. Mormons teach that God provided his children the choice to come to Earth, which is considered a crucial stage in their development—wherein a mortal body, coupled with the freedom to choose, makes for an environment to learn and grow.

A recent alternative Christian theological discourse interprets Jesus as revealing that the purpose of life is to elevate our compassionate response to human suffering; [] nonetheless, the conventional Christian position is that people are justified by belief in the propitiatory sacrifice of Jesus' death on the cross.

Hinduism is a religious category including many beliefs and traditions. Since Hinduism was the way of expressing meaningful living for a long time, before there was a need for naming it as a separate religion, Hindu doctrines are supplementary and complementary in nature, generally non-exclusive, suggestive and tolerant in content.

There are four possible aims to human life, known as the purusharthas ordered from least to greatest: In all schools of Hinduism, the meaning of life is tied up in the concepts of karma causal action , sansara the cycle of birth and rebirth , and moksha liberation. Particular goals for life are generally subsumed under broader yogas practices or dharma correct living which are intended to create more favorable reincarnations, though they are generally positive acts in this life as well.

Traditional schools of Hinduism often worship Devas which are manifestations of Ishvara a personal or chosen God ; these Devas are taken as ideal forms to be identified with, as a form of spiritual improvement. In short, the goal is to realize the fundamental truth about oneself. Later schools reinterpreted the vedas to focus on Brahman , "The One Without a Second", [] as a central God-like figure. Dvaita Vedanta and other bhakti schools have a dualist interpretation.

Brahman is seen as a supreme being with a personality and manifest qualities. Vaishnavism is a branch of Hinduism in which the principal belief is the identification of Vishnu or Narayana as the one supreme God. This belief contrasts with the Krishna-centered traditions, such as Vallabha , Nimbaraka and Gaudiya , in which Krishna is considered to be the One and only Supreme God and the source of all avataras.

Vaishnava theology includes the central beliefs of Hinduism such as monotheism , reincarnation , samsara , karma , and the various Yoga systems, but with a particular emphasis on devotion bhakti to Vishnu through the process of Bhakti yoga , often including singing Vishnu's name's bhajan , meditating upon his form dharana and performing deity worship puja. In this, Krishna is worshipped as the single true God, and all living entities are eternal parts and the Supreme Personality of the Godhead Krishna. Thus the constitutional position of a living entity is to serve the Lord with love and devotion.

The purpose of human life especially is to think beyond the animalistic way of eating, sleeping, mating and defending and engage the higher intelligence to revive the lost relationship with Krishna. Jainism is a religion originating in ancient India , its ethical system promotes self-discipline above all else. Through following the ascetic teachings of Jina , a human achieves enlightenment perfect knowledge. Jainism divides the universe into living and non-living beings. Only when the living become attached to the non-living does suffering result.

Therefore, happiness is the result of self-conquest and freedom from external objects. The meaning of life may then be said to be to use the physical body to achieve self-realization and bliss. Jains believe that every human is responsible for his or her actions and all living beings have an eternal soul , jiva. Jains believe all souls are equal because they all possess the potential of being liberated and attaining Moksha. The Jain view of karma is that every action, every word, every thought produces, besides its visible, an invisible, transcendental effect on the soul.

Jains refuse food obtained with unnecessary cruelty. Many practice a lifestyle similar to veganism due to the violence of modern dairy farms, and others exclude root vegetables from their diets in order to preserve the lives of the plants from which they eat. Buddhists practice to embrace with mindfulness the ill-being suffering and well-being that is present in life. Buddhists practice to see the causes of ill-being and well-being in life.

For example, one of the causes of suffering is unhealthy attachment to objects material or non-material. Attaining and perfecting dispassion is a process of many levels that ultimately results in the state of Nirvana. Nirvana means freedom from both suffering and rebirth. Theravada Buddhism is generally considered to be close to the early Buddhist practice. It promotes the concept of Vibhajjavada Pali , literally "Teaching of Analysis", which says that insight must come from the aspirant's experience, critical investigation, and reasoning instead of by blind faith. However, the Theravadin tradition also emphasizes heeding the advice of the wise, considering such advice and evaluation of one's own experiences to be the two tests by which practices should be judged.

The Theravadin goal is liberation or freedom from suffering, according to the Four Noble Truths. This is attained in the achievement of Nirvana , or Unbinding which also ends the repeated cycle of birth, old age, sickness and death. The way to attain Nirvana is by following and practicing the Noble Eightfold Path. Mahayana Buddhist schools de-emphasize the traditional view still practiced in Theravada of the release from individual Suffering Dukkha and attainment of Awakening Nirvana.

In Mahayana, the Buddha is seen as an eternal, immutable, inconceivable, omnipresent being. The fundamental principles of Mahayana doctrine are based on the possibility of universal liberation from suffering for all beings, and the existence of the transcendent Buddha-nature , which is the eternal Buddha essence present, but hidden and unrecognised, in all living beings.

Devotional schools such as Pure Land Buddhism seek the aid of celestial buddhas—individuals who have spent lifetimes [ citation needed ] accumulating positive karma, and use that accumulation to aid all. The monotheistic Sikh religion was founded by Guru Nanak Dev , the term "Sikh" means student, which denotes that followers will lead their lives forever learning.

This system of religious philosophy and expression has been traditionally known as the Gurmat literally "the counsel of the gurus" or the Sikh Dharma. The Sikh Gurus say that salvation can be obtained by following various spiritual paths, so Sikhs do not have a monopoly on salvation: A key distinctive feature of Sikhism is a non- anthropomorphic concept of God, to the extent that one can interpret God as the Universe itself pantheism. Sikhism thus sees life as an opportunity to understand this God as well as to discover the divinity which lies in each individual.

While a full understanding of God is beyond human beings, [] Nanak described God as not wholly unknowable, and stressed that God must be seen from "the inward eye", or the "heart", of a human being: Nanak emphasized the revelation through meditation, as its rigorous application permits the existence of communication between God and human beings.

Taoist cosmogony emphasizes the need for all sentient beings and all man to return to the primordial or to rejoin with the Oneness of the Universe by way of self-cultivation and self-realization. All adherents should understand and be in tune with the ultimate truth. Taoists believe all things were originally from Taiji and Tao , and the meaning in life for the adherents is to realize the temporal nature of the existence. Shinto is the native religion of Japan.

Shinto means "the path of the kami ", but more specifically, it can be taken to mean "the divine crossroad where the kami chooses his way". The "divine" crossroad signifies that all the universe is divine spirit. This foundation of free will , choosing one's way, means that life is a creative process. Shinto wants life to live, not to die.

Shinto sees death as pollution and regards life as the realm where the divine spirit seeks to purify itself by rightful self-development. Shinto wants individual human life to be prolonged forever on earth as a victory of the divine spirit in preserving its objective personality in its highest forms. The presence of evil in the world, as conceived by Shinto, does not stultify the divine nature by imposing on divinity responsibility for being able to relieve human suffering while refusing to do so.

The sufferings of life are the sufferings of the divine spirit in search of progress in the objective world. There are many new religious movements in East Asia, and some with millions of followers: New religions typically have unique explanations for the meaning of life. For example, in Tenrikyo, one is expected to live a Joyous Life by participating in practices that create happiness for oneself and others. The mystery of life and its true meaning is an often recurring subject in popular culture , featured in entertainment media and various forms of art.

At the end of the film, a character played by Michael Palin is handed an envelope containing "the meaning of life", which she opens and reads out to the audience: Uh, try to be nice to people, avoid eating fat, read a good book every now and then, get some walking in, and try to live together in peace and harmony with people of all creeds and nations. Many other Python sketches and songs are also existential in nature, questioning the importance we place on life " Always Look on the Bright Side of Life " and other meaning-of-life related questioning. John Cleese also had his sit-com character Basil Fawlty contemplating the futility of his own existence in Fawlty Towers.

In Douglas Adams ' popular comedy book, movie, television, and radio series The Hitchhiker's Guide to the Galaxy , the Answer to the Ultimate Question of Life, the Universe, and Everything is given the numeric solution " 42 ", after seven and a half million years of calculation by a giant supercomputer called Deep Thought. When this answer is met with confusion and anger from its constructors, Deep Thought explains that "I think the problem such as it was, was too broadly based.

You never actually stated what the question was. Deep Thought then constructs another computer—the Earth—to calculate what the Ultimate Question actually is. Later Ford and Arthur manage to extract the question as the Earth computer would have rendered it. That question turns out to be "what do you get if you multiply six by nine" [] , and it is realised that the program was ruined by the unexpected arrival of the Golgafrinchans on Earth, and so the actual Ultimate Question Of Life, The Universe, And Everything remains unknown.

While 6 x 9 would be written as 42 in the tridecimal numeral system , author Douglas Adams claimed that this was mere coincidence and completely serendipitous. In The Simpsons episode " Homer the Heretic ", a representation of God agrees to tell Homer what the meaning of life is, but the show's credits begin to roll just as he starts to say what it is. Blue season 1 episode 1 the character Simmons asks Grif the question "Why are we here?

In Person of Interest season 5 episode 13, an artificial intelligence referred to as The Machine tells Harold Finch that the secret of life is "Everyone dies alone. But if you mean something to someone, if you help someone, or love someone. If even a single person remembers you then maybe you never really die at all. From Wikipedia, the free encyclopedia. Philosophical and spiritual question concerning the significance of living or existence in general.

This article is about the significance of life in general. For other uses, see Meaning of life disambiguation. For to hope in the possibility of help, not to speak of help by virtue of the absurd, that for God all things are possible—no, that he will not do. And as for seeking help from any other—no, that he will not do for all the world; rather than seek help he would prefer to be himself—with all the tortures of hell, if so it must be.

Chinese philosophy and Japanese philosophy. Abrahamic religion and Iranian philosophy. Indian religions and Indian philosophy. Hinduism , Hindu philosophy , and Dharma. Advaita Vedanta and Dvaita Vedanta. Jainism and Jain philosophy. Philosophy portal Spirituality portal.

Culture of life Bioethics Meaningful life Quality of life Value of life. An Introduction to Philosophy. What's It All About? Philosophy and the Meaning of Life. Thiemann ; William Carl Placher Why Are We Here?: Everyday Questions and the Christian Life. Continuum International Publishing Group. The Scientific Answer to this Age-old Question that you don't need to be a scientist to understand.

Dharma Realm Buddhist Association. The Search for the Origin and Meaning of Life. Archived from the original on 16 June Performance, Participation, and Well-Being. The Physics of Consciousness: The Quantum Mind and the Meaning of Life. Krishnamurti Foundation of America. Archived from the original PDF on 26 September Life, Death, and Subjectivity: Moral Sources in Bioethics. The Psychology of Optimal Experience.

Character strengths and virtues: A handbook and classification. Effect of a purpose in life on risk of incident Alzheimer disease and mild cognitive impairment in community-dwelling older persons. Archives of General Psychiatry. Purpose in life and reduced risk of myocardial infarction among older US adults with coronary heart disease: Journal of Behavioral Medicine. Purpose in life and reduced incidence of stroke in older adults: The Health and Retirement Study. Journal of Psychosomatic Research. Purpose in life is associated with mortality among community-dwelling older persons.

On the Origin of Species. River out of Eden. The Book of Real Answers to Everything! Archived from the original on 13 October University of California Press. Gaia — a New Look at Life on Earth. Information Theory and Evolution. The University of North Alabama. Archived from the original on 1 November Humanity's Fate in the Universe. What Caused the Big Bang? Evolutionary Psychology Versus Ethnography. Creeping Up on the Hard Problem. The Nature of Mind and the Structure of Science.

Little, Brown and Co. Barrow ; Paul C. Davies ; Charles L. Science and Ultimate Reality: Quantum Theory, Cosmology and Complexity. Remote Viewing in Hyperspace. Evidence for an Electromagnetic Field Theory of Consciousness". Journal of Consciousness Studies. Archived from the original on 18 December Studies on the Structure of Time: From Physics to Psycho patho logy.

The Rainbow and the Worm: The Physics of Organisms. Archived from the original on Archived from the original on 27 September The Journal of Parapsychology.

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Proceedings of the 47th PA Convention. Archived from the original PDF on 10 August Aging as an individual process: Towards a theory of personal meaning. The three meanings of meaning in life: Distinguishing coherence, purpose, and significance. The Journal of Positive Psychology, 11 5 , Towards a balanced interactive model of the good life. Canadian Psychology, 52 2 , From logotherapy to meaning-centered counseling and therapy. Theories, research, and applications 2nd ed. A Very Short Introduction. Simon and Schuster; London: Blackwell Publishing, , p.

It is itself the highest political end. Classical Utilitarianism from Hume to Mill. On Liberty , ed. Penguin Classics, , Ed. The Future Of Values: The Affirmation of Life: Nietzsche on Overcoming Nihilism. The Meaning of Truth. The Philosophy of William James. Theistic Perspectives on the Meaning of Life. Kierkegaard and the Treachery of Love.

Schopenhauer, Philosophy, and the Arts. The Sickness Unto Death. Archived from the original on 30 July Archived from the original on 9 August Retrieved 1 August The Meaning of Life". Moral Freedom and the Meaning of Life". An Introduction to Ethics. A guide to the world's greatest thinkers Peter J. Selfhood as Creative Transformation. State University of New York Press, This is relevant because the Will of Allah is not just to worship HIM; to be just and good with humanity is equally important.

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In Hinnells, John Ed. A Popular Dictionary of Hinduism. Gavin Flood; University of Stirling, eds. Caitanya Vaisnava Vedanta of Jiva Gosvami: When knowledge meets devotion. The World of Conquerors. Sussex Academic Press, From Ancient History to the Present. Hamlyn Publishing Group Limited. An Introduction to the Tao in Everyday Life. The Meaning of Shinto. Mean Markets and Lizard Brains: How to Profit from the New Science of Irrationality. John Wiley and Sons. Wood 'N' Barnes Publishing.

I think the problem such as it was, was too broadly based. The Anthology at the End of the Universe: The Hitchhiker's Guide to the Meaning of Everything. The Hitchhiker's Guide to the Galaxy.

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Six by nine… forty-two! I always said there was something fundamentally wrong about the universe! Westminster John Knox Press. The Real Meaning of Life. Let Reality Catch Up: The Book of Positive Quotations. The Rhythm of Life: Living Every Day with Passion and Purpose. Parentalism in the Caring Life. Oxford University Press US. Create Your Own Destiny!: Spiritual Path to Success. The Search for the best way to live. A Jedi craves not these things". Archived from the original on 11 August New Skins for Old Wine. The Meaning of Life. Live Long Enough to Live Forever. On the New Immortality.

The Purpose of Life. Mitochondria and the Meaning of Life. Genetics and the Logic of Evolution. Chance in the House of Fate: A Natural History of Heredity. Exploring the Realm of the Living Cell. Philosophers Explore the Matrix. To reflect the minimum phenomena required, other biological definitions of life have been proposed, [40] with many of these being based upon chemical systems. Biophysicists have commented that living things function on negative entropy. Living systems are open self-organizing living things that interact with their environment. These systems are maintained by flows of information, energy , and matter.

Some scientists have proposed in the last few decades that a general living systems theory is required to explain the nature of life. Instead of examining phenomena by attempting to break things down into components, a general living systems theory explores phenomena in terms of dynamic patterns of the relationships of organisms with their environment.

The idea that the Earth is alive is found in philosophy and religion, but the first scientific discussion of it was by the Scottish scientist James Hutton. In , he stated that the Earth was a superorganism and that its proper study should be physiology. Hutton is considered the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the 19th century. The first attempt at a general living systems theory for explaining the nature of life was in , by American biologist James Grier Miller. Specifically, he identified the "nonfractionability of components in an organism" as the fundamental difference between living systems and "biological machines.

A systems view of life treats environmental fluxes and biological fluxes together as a "reciprocity of influence," [52] and a reciprocal relation with environment is arguably as important for understanding life as it is for understanding ecosystems. Morowitz explains it, life is a property of an ecological system rather than a single organism or species.

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Robert Ulanowicz highlights mutualism as the key to understand the systemic, order-generating behavior of life and ecosystems. Complex systems biology CSB is a field of science that studies the emergence of complexity in functional organisms from the viewpoint of dynamic systems theory. A closely related approach to CSB and systems biology called relational biology is concerned mainly with understanding life processes in terms of the most important relations, and categories of such relations among the essential functional components of organisms; for multicellular organisms, this has been defined as "categorical biology", or a model representation of organisms as a category theory of biological relations, as well as an algebraic topology of the functional organization of living organisms in terms of their dynamic, complex networks of metabolic, genetic, and epigenetic processes and signaling pathways.

It has also been argued that the evolution of order in living systems and certain physical systems obeys a common fundamental principle termed the Darwinian dynamic. The underlying order-generating process was concluded to be basically similar for both types of systems. Another systemic definition called the operator theory proposes that "life is a general term for the presence of the typical closures found in organisms; the typical closures are a membrane and an autocatalytic set in the cell" [61] and that an organism is any system with an organisation that complies with an operator type that is at least as complex as the cell.

Some of the earliest theories of life were materialist, holding that all that exists is matter, and that life is merely a complex form or arrangement of matter. Empedocles BC argued that everything in the universe is made up of a combination of four eternal "elements" or "roots of all": All change is explained by the arrangement and rearrangement of these four elements.

The various forms of life are caused by an appropriate mixture of elements. Democritus BC thought that the essential characteristic of life is having a soul psyche. Like other ancient writers, he was attempting to explain what makes something a living thing. His explanation was that fiery atoms make a soul in exactly the same way atoms and void account for any other thing.

He elaborates on fire because of the apparent connection between life and heat, and because fire moves. Plato's world of eternal and unchanging Forms , imperfectly represented in matter by a divine Artisan , contrasts sharply with the various mechanistic Weltanschauungen , of which atomism was, by the fourth century at least, the most prominent This debate persisted throughout the ancient world.

Atomistic mechanism got a shot in the arm from Epicurus The choice seems simple: In the 19th century, the advances in cell theory in biological science encouraged this view. The evolutionary theory of Charles Darwin is a mechanistic explanation for the origin of species by means of natural selection. Hylomorphism is a theory first expressed by the Greek philosopher Aristotle BC.

The application of hylomorphism to biology was important to Aristotle, and biology is extensively covered in his extant writings. In this view, everything in the material universe has both matter and form, and the form of a living thing is its soul Greek psyche , Latin anima.

There are three kinds of souls: Aristotle believed that while matter can exist without form, form cannot exist without matter, and that therefore the soul cannot exist without the body. This account is consistent with teleological explanations of life, which account for phenomena in terms of purpose or goal-directedness.

Thus, the whiteness of the polar bear's coat is explained by its purpose of camouflage. The direction of causality from the future to the past is in contradiction with the scientific evidence for natural selection, which explains the consequence in terms of a prior cause. Biological features are explained not by looking at future optimal results, but by looking at the past evolutionary history of a species, which led to the natural selection of the features in question.

Spontaneous generation was the belief that living organisms can form without descent from similar organisms. Typically, the idea was that certain forms such as fleas could arise from inanimate matter such as dust or the supposed seasonal generation of mice and insects from mud or garbage. The theory of spontaneous generation was proposed by Aristotle , [75] who compiled and expanded the work of prior natural philosophers and the various ancient explanations of the appearance of organisms; it held sway for two millennia.

It was decisively dispelled by the experiments of Louis Pasteur in , who expanded upon the investigations of predecessors such as Francesco Redi. Vitalism is the belief that the life-principle is non-material. This originated with Georg Ernst Stahl 17th century , and remained popular until the middle of the 19th century.

It is of historical significance because for the first time an organic compound was produced in inorganic reactions. During the s, Hermann von Helmholtz , anticipated by Julius Robert von Mayer , demonstrated that no energy is lost in muscle movement, suggesting that there were no "vital forces" necessary to move a muscle. The age of the Earth is about 4. Although the number of Earth's catalogued species of lifeforms is between 1.

Estimates range from 8 million to million, [] [] with a more narrow range between 10 and 14 million, [] but it may be as high as 1 trillion with only one-thousandth of one percent of the species described according to studies realized in May All known life forms share fundamental molecular mechanisms, reflecting their common descent ; based on these observations, hypotheses on the origin of life attempt to find a mechanism explaining the formation of a universal common ancestor , from simple organic molecules via pre-cellular life to protocells and metabolism.

Models have been divided into "genes-first" and "metabolism-first" categories, but a recent trend is the emergence of hybrid models that combine both categories. There is no current scientific consensus as to how life originated. However, most accepted scientific models build on the Miller—Urey experiment and the work of Sidney Fox , which show that conditions on the primitive Earth favored chemical reactions that synthesize amino acids and other organic compounds from inorganic precursors, [] and phospholipids spontaneously form lipid bilayers , the basic structure of a cell membrane.

Living organisms synthesize proteins , which are polymers of amino acids using instructions encoded by deoxyribonucleic acid DNA. Protein synthesis entails intermediary ribonucleic acid RNA polymers. One possibility for how life began is that genes originated first, followed by proteins; [] the alternative being that proteins came first and then genes.

However, because genes and proteins are both required to produce the other, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because of this, it is unlikely that genes and proteins arose independently. Therefore, a possibility, first suggested by Francis Crick , [] is that the first life was based on RNA , [] which has the DNA-like properties of information storage and the catalytic properties of some proteins.

This is called the RNA world hypothesis , and it is supported by the observation that many of the most critical components of cells those that evolve the slowest are composed mostly or entirely of RNA. The catalytic properties of RNA had not yet been demonstrated when the hypothesis was first proposed, [] but they were confirmed by Thomas Cech in One issue with the RNA world hypothesis is that synthesis of RNA from simple inorganic precursors is more difficult than for other organic molecules.

One reason for this is that RNA precursors are very stable and react with each other very slowly under ambient conditions, and it has also been proposed that living organisms consisted of other molecules before RNA. Geological findings in showed that reactive phosphorus species like phosphite were in abundance in the ocean before 3. In , experiments demonstrated Darwinian evolution of a two-component system of RNA enzymes ribozymes in vitro. Prebiotic compounds may have originated extraterrestrially.

In March , NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, including uracil , cytosine and thymine , have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine , found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons PAHs , the most carbon -rich chemical found in the universe , may have been formed in red giants or in interstellar dust and gas clouds, according to the scientists.

According to the panspermia hypothesis, microscopic life —distributed by meteoroids , asteroids and other small Solar System bodies —may exist throughout the universe. The diversity of life on Earth is a result of the dynamic interplay between genetic opportunity , metabolic capability, environmental challenges, [] and symbiosis. As a consequence of these microbial activities, the physical-chemical environment on Earth has been changing on a geologic time scale , thereby affecting the path of evolution of subsequent life.

Because oxygen was toxic to most life on Earth at the time, this posed novel evolutionary challenges, and ultimately resulted in the formation of Earth's major animal and plant species. This interplay between organisms and their environment is an inherent feature of living systems. The biosphere is the global sum of all ecosystems. It can also be termed as the zone of life on Earth , a closed system apart from solar and cosmic radiation and heat from the interior of the Earth , and largely self-regulating. The biosphere is postulated to have evolved , beginning with a process of biopoesis life created naturally from non-living matter, such as simple organic compounds or biogenesis life created from living matter , at least some 3.

In a general sense, biospheres are any closed, self-regulating systems containing ecosystems. This includes artificial biospheres such as Biosphere 2 and BIOS-3 , and potentially ones on other planets or moons. The inert components of an ecosystem are the physical and chemical factors necessary for life—energy sunlight or chemical energy , water, heat, atmosphere , gravity , nutrients , and ultraviolet solar radiation protection. To live in most ecosystems, then, organisms must be able to survive a range of conditions, called the "range of tolerance.

Beyond these zones are the "zones of intolerance," where survival and reproduction of that organism is unlikely or impossible. Organisms that have a wide range of tolerance are more widely distributed than organisms with a narrow range of tolerance. To survive, selected microorganisms can assume forms that enable them to withstand freezing , complete desiccation , starvation , high levels of radiation exposure , and other physical or chemical challenges.

These microorganisms may survive exposure to such conditions for weeks, months, years, or even centuries. While all organisms are composed of nearly identical molecules , evolution has enabled such microbes to cope with this wide range of physical and chemical conditions. Characterization of the structure and metabolic diversity of microbial communities in such extreme environments is ongoing. Microbial life forms thrive even in the Mariana Trench , the deepest spot in the Earth's oceans.

Investigation of the tenacity and versatility of life on Earth, [] as well as an understanding of the molecular systems that some organisms utilize to survive such extremes, is important for the search for life beyond Earth. All life forms require certain core chemical elements needed for biochemical functioning. These include carbon , hydrogen , nitrogen , oxygen , phosphorus , and sulfur —the elemental macronutrients for all organisms [] —often represented by the acronym CHNOPS. Together these make up nucleic acids , proteins and lipids , the bulk of living matter. Five of these six elements comprise the chemical components of DNA, the exception being sulfur.

The latter is a component of the amino acids cysteine and methionine. The most biologically abundant of these elements is carbon, which has the desirable attribute of forming multiple, stable covalent bonds. This allows carbon-based organic molecules to form an immense variety of chemical arrangements. Deoxyribonucleic acid is a molecule that carries most of the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses.

DNA and RNA are nucleic acid s; alongside proteins and complex carbohydrates , they are one of the three major types of macromolecule that are essential for all known forms of life. Most DNA molecules consist of two biopolymer strands coiled around each other to form a double helix. The two DNA strands are known as polynucleotides since they are composed of simpler units called nucleotides. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone.

According to base pairing rules A with T, and C with G , hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA. The total amount of related DNA base pairs on Earth is estimated at 5. DNA stores biological information. The DNA backbone is resistant to cleavage, and both strands of the double-stranded structure store the same biological information. Biological information is replicated as the two strands are separated. The two strands of DNA run in opposite directions to each other and are therefore anti-parallel.

Attached to each sugar is one of four types of nucleobases informally, bases. It is the sequence of these four nucleobases along the backbone that encodes biological information. Under the genetic code , RNA strands are translated to specify the sequence of amino acids within proteins.

Within cells, DNA is organized into long structures called chromosomes. During cell division these chromosomes are duplicated in the process of DNA replication , providing each cell its own complete set of chromosomes. Eukaryotic organisms animals, plants, fungi , and protists store most of their DNA inside the cell nucleus and some of their DNA in organelles , such as mitochondria or chloroplasts. Within the chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

DNA was first isolated by Friedrich Miescher in Life is usually classified by eight levels of taxa—domains, kingdoms, phyla, class, order, family, genus, and species. In May , scientists reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described. The first known attempt to classify organisms was conducted by the Greek philosopher Aristotle — BC , who classified all living organisms known at that time as either a plant or an animal, based mainly on their ability to move.

He also distinguished animals with blood from animals without blood or at least without red blood , which can be compared with the concepts of vertebrates and invertebrates respectively, and divided the blooded animals into five groups: The bloodless animals were also divided into five groups: Though Aristotle's work in zoology was not without errors, it was the grandest biological synthesis of the time and remained the ultimate authority for many centuries after his death. The exploration of the Americas revealed large numbers of new plants and animals that needed descriptions and classification.

In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced and was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. In the late s, Carl Linnaeus introduced his system of binomial nomenclature for the classification of species. Linnaeus attempted to improve the composition and reduce the length of the previously used many-worded names by abolishing unnecessary rhetoric, introducing new descriptive terms and precisely defining their meaning.

The fungi were originally treated as plants. For a short period Linnaeus had classified them in the taxon Vermes in Animalia, but later placed them back in Plantae. Copeland classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledging their special status.

Evolutionary history shows that the fungi are more closely related to animals than to plants. As new discoveries enabled detailed study of cells and microorganisms, new groups of life were revealed, and the fields of cell biology and microbiology were created. This led to the six-kingdom system and eventually to the current three-domain system , which is based on evolutionary relationships.

As microbiology, molecular biology and virology developed, non-cellular reproducing agents were discovered, such as viruses and viroids. Whether these are considered alive has been a matter of debate; viruses lack characteristics of life such as cell membranes, metabolism and the ability to grow or respond to their environments. Viruses can still be classed into "species" based on their biology and genetics , but many aspects of such a classification remain controversial. In the s a trend called cladistics emerged, arranging taxa based on clades in an evolutionary or phylogenetic tree.

In systems of scientific classification , Biota [] is the superdomain that classifies all life. Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by division. Cell theory was formulated by Henri Dutrochet , Theodor Schwann , Rudolf Virchow and others during the early nineteenth century, and subsequently became widely accepted.

There are two primary types of cells. Prokaryotes lack a nucleus and other membrane-bound organelles , although they have circular DNA and ribosomes.

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Bacteria and Archaea are two domains of prokaryotes. The other primary type of cells are the eukaryotes , which have distinct nuclei bound by a nuclear membrane and membrane-bound organelles, including mitochondria , chloroplasts , lysosomes , rough and smooth endoplasmic reticulum , and vacuoles.

In addition, they possess organized chromosomes that store genetic material. All species of large complex organisms are eukaryotes, including animals, plants and fungi, though most species of eukaryote are protist microorganisms. The molecular mechanisms of cell biology are based on proteins. Most of these are synthesized by the ribosomes through an enzyme-catalyzed process called protein biosynthesis.

A sequence of amino acids is assembled and joined together based upon gene expression of the cell's nucleic acid. Cells reproduce through a process of cell division in which the parent cell divides into two or more daughter cells. For prokaryotes, cell division occurs through a process of fission in which the DNA is replicated, then the two copies are attached to parts of the cell membrane. In eukaryotes , a more complex process of mitosis is followed. However, the end result is the same; the resulting cell copies are identical to each other and to the original cell except for mutations , and both are capable of further division following an interphase period.

Multicellular organisms may have first evolved through the formation of colonies of identical cells. These cells can form group organisms through cell adhesion. The individual members of a colony are capable of surviving on their own, whereas the members of a true multi-cellular organism have developed specializations, making them dependent on the remainder of the organism for survival. Such organisms are formed clonally or from a single germ cell that is capable of forming the various specialized cells that form the adult organism.

This specialization allows multicellular organisms to exploit resources more efficiently than single cells. Cells have evolved methods to perceive and respond to their microenvironment, thereby enhancing their adaptability. Cell signaling coordinates cellular activities, and hence governs the basic functions of multicellular organisms. Signaling between cells can occur through direct cell contact using juxtacrine signalling , or indirectly through the exchange of agents as in the endocrine system.

In more complex organisms, coordination of activities can occur through a dedicated nervous system. Though life is confirmed only on Earth, many think that extraterrestrial life is not only plausible, but probable or inevitable. Other locations within the Solar System that may host microbial life include the subsurface of Mars , the upper atmosphere of Venus , [] and subsurface oceans on some of the moons of the giant planets. The inner and outer radii of this zone vary with the luminosity of the star, as does the time interval during which the zone survives.

Stars more massive than the Sun have a larger habitable zone, but remain on the Sun-like "main sequence" of stellar evolution for a shorter time interval. Small red dwarfs have the opposite problem, with a smaller habitable zone that is subject to higher levels of magnetic activity and the effects of tidal locking from close orbits.

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