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Life

Life (Biota / Vitae / Eobionti)
Plants in the Rwenzori Mountains, Uganda
Scientific classification
Domains and kingdoms

Life on Earth:

Life is a characteristic distinguishing physical entities having biological processes (such as signaling and self-sustaining processes) from those that do not,[1][2] either because such functions have ceased (death), or because they lack such functions and are classified as inanimate.[3][4][5] Various forms of life exist such as plants, animals, fungi, protists, archaea, and bacteria. The criteria can at times be ambiguous and may or may not define viruses, viroids or potential artificial life as living. Biology is the primary science concerned with the study of life, although many other sciences are involved.

The smallest contiguous unit of life is called an genetic information.

age of the Earth is about 4.54 billion years.[6][7][8] The earliest life on Earth arose at least 3.5 billion years ago,[9][10][11] during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. The earliest physical evidence of life on Earth is biogenic graphite from 3.7 billion-year-old metasedimentary rocks found in Western Greenland[12] and microbial mat fossils in 3.48 billion-year-old sandstone found in Western Australia.[13][14] Some theories, such as the Late Heavy Bombardment theory, suggest that life on Earth may have started even earlier,[15] and may have begun as early as 4.1 billion years ago according to a 2015 study,[16][17] or even, 4.25 billion years ago,[18] or even earlier yet, 4.4 billion years ago, according to another study.[19] According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."[16]

The mechanism by which life began on Earth is unknown, although many hypotheses have been formulated. Since emerging, life has evolved into a variety of forms, which have been wide range of conditions. Nonetheless, more than 99 percent of all species, amounting to over five billion species,[20] that ever lived on Earth are estimated to be extinct.[21][22] Estimates on the number of Earth's current species range from 10 million to 14 million,[23] of which about 1.2 million have been documented and over 86 percent have not yet been described.[24]

The chemistry leading to life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the Universe was only 10–17 million years old.[25][26][27] Though life is confirmed only on the Earth, many think that extraterrestrial life is not only plausible, but probable or inevitable.[28][29] Other planets and moons[30] in the Solar System and other planetary systems are being examined for evidence of having once supported simple life, and projects such as SETI are trying to detect radio transmissions from possible alien civilizations.

The meaning of life—its significance, origin, purpose, and ultimate fate—is a central concept and question in philosophy and religion. Both philosophy and religion have offered interpretations as to how life relates to existence and consciousness, and on related issues such as life stance, purpose, conception of a god or gods, a soul or an afterlife. Different cultures throughout history have had widely varying approaches to these issues.

Contents

  • Early theories 1
    • Materialism 1.1
    • Hylomorphism 1.2
    • Vitalism 1.3
  • Definitions 2
    • Biology 2.1
      • Alternatives 2.1.1
      • Viruses 2.1.2
    • Living systems theories 2.2
  • Origin 3
  • Environmental conditions 4
    • Range of tolerance 4.1
  • Form and function 5
  • Classification 6
  • Extraterrestrial life 7
  • Death 8
  • Artificial life 9
  • Phanerozoic Eon 10
    • Paleozoic Era 10.1
      • Cambrian 10.1.1
      • Ordovician 10.1.2
      • Silurian 10.1.3
      • Devonian 10.1.4
      • Carboniferous 10.1.5
      • Permian 10.1.6
    • Mesozoic Era 10.2
      • Triassic 10.2.1
      • Jurassic 10.2.2
      • Cretaceous 10.2.3
    • Cenozoic Era 10.3
      • Paleogene 10.3.1
      • Neogene 10.3.2
      • Quaternary 10.3.3
  • See also 11
  • Notes 12
  • References 13
  • Further reading 14
  • External links 15

Early theories

Materialism

Plant growth in the Hoh Rainforest
Herds of zebra and impala gathering on the Maasai Mara plain
An aerial photo of microbial mats around the Grand Prismatic Spring of Yellowstone National Park

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 (430 BC) argued that every thing in the universe is made up of a combination of four eternal "elements" or "roots of all": earth, water, air, and fire. 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.[31]

Democritus (460 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.[32]

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... while the Stoics adopted a divine teleology... The choice seems simple: either show how a structured, regular world could arise out of undirected processes, or inject intelligence into the system.[33]
— R. J. Hankinson, Cause and Explanation in Ancient Greek Thought

The mechanistic materialism that originated in ancient Greece was revived and revised by the French philosopher René Descartes, who held that animals and humans were assemblages of parts that together functioned as a machine. In the 19th century, the advances in cell theory in biological science encouraged this view. The evolutionary theory of Charles Darwin (1859) is a mechanistic explanation for the origin of species by means of natural selection.[34]

Hylomorphism

Hylomorphism is a theory, originating with Aristotle (322 BC), that all things are a combination of matter and form. Biology was one of his main interests, and there is extensive biological material in his extant writings. In this view, all things in the material universe have both matter and form, and the form of a living thing is its soul (Greek psyche, Latin anima). There are three kinds of souls: the vegetative soul of plants, which causes them to grow and decay and nourish themselves, but does not cause motion and sensation; the animal soul, which causes animals to move and feel; and the rational soul, which is the source of consciousness and reasoning, which (Aristotle believed) is found only in man.[35] Each higher soul has all the attributes of the lower one. Aristotle believed that while matter can exist without form, form cannot exist without matter, and therefore the soul cannot exist without the body.[36]

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.[37]

Vitalism

  • Wikispecies – a free directory of life
  • Resources for life in the Solar System and in galaxy, and the potential scope of life in the cosmological future
  • "The Adjacent Possible: A Talk with Stuart Kauffman"
  • Stanford Encyclopedia of Philosophy entry
  • The Kingdoms of Life

External links

  • Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman
  • Seeding the Universe With Life Legacy Books, Washington D. C., 2000, ISBN 0-476-00330-X
  • Walker, Martin G. LIFE! Why We Exist...And What We Must Do to Survive Dog Ear Publishing, 2006, ISBN 1-59858-243-7

Further reading

  1. ^ a b
  2. ^ The American Heritage Dictionary of the English Language, 4th edition, published by Houghton Mifflin Company, via Answers.com:
    • "The property or quality that distinguishes living organisms from dead organisms and inanimate matter, manifested in functions such as
    • "The characteristic state or condition of a living organism."
  3. ^ .inanimateDefinition of WordNet Search by Princeton University.
  4. ^
  5. ^
  6. ^
  7. ^
  8. ^
  9. ^ a b Schopf, JW, Kudryavtsev, AB, Czaja, AD, and Tripathi, AB. (2007). Evidence of Archean life: Stromatolites and microfossils. Precambrian Research 158:141–155.
  10. ^ a b Schopf, JW (2006). Fossil evidence of Archaean life. Philos Trans R Soc Lond B Biol Sci 29;361(1470) 869-85.
  11. ^ a b
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  17. ^ Early edition, published online before print.
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  35. ^ Aristotle, De Anima, Book II
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  80. ^ Cleland and Chyba wrote a chapter in Planets and Life: "In the absence of such a theory, we are in a position analogous to that of a 16th-century investigator trying to define 'water' in the absence of molecular theory." [...] "Without access to living things having a different historical origin, it is difficult and perhaps ultimately impossible to formulate an adequately general theory of the nature of living systems".
  81. ^
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  87. ^ *
  88. ^ *
  89. ^ a b
  90. ^ Michod RE. (1999) Darwinian Dynamics: Evolutionary Transitions in Fitness and Individuality. Princeton University Press, Princeton, New Jersey ISBN 0691050112, 9780691050119
  91. ^ The pursuit of complexity KNNV Publishing, Zeist, The Netherlands, (2012) pages: 27–29, 87–88 and 94–96.
  92. ^ Towards a hierarchical definition of life, the organism, and death. Jagers op Akkerhuis G.A.J.M. (2010). Foundations of Science 15: 245–262.
  93. ^ Explaining the origin of life is not enough for a definition of life. Jagers op Akkerhuis G.A.J.M. (2010). Foundations of Science 16: 327–329.
  94. ^ The Role of Logic and Insight in the Search for a Definition of Life. Jagers op Akkerhuis G.A.J.M. (2012). J. Biomol Struct Dyn 29(4), 619–620 (2012).
  95. ^ Contributions of the Operator Hierarchy to the field of biologically driven mathematics and computation. Jagers op Akkerhuis G.A.J.M. (2012). In: Integral Biomathics: Tracing the Road to Reality.
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References

  1. ^ The 'evolution' of viruses and other similar forms is still uncertain. Therefore, this classification may be paraphyletic because cellular life might have evolved from non-cellular life, or polyphyletic because the most recent common ancestor might not be included.
  2. ^ Infectious protein molecules prions are not considered living organisms, but can be described as ‘organism-comparable organic structures’.
  3. ^ As specific virus-dependent organic structures can be considered satellites and defective interfering particles, both of which require another virus help for their replication.

Notes

See also

The Holocene ranges from 12,000 years ago to present day. Also known as "the Age of Man", the Holocene features the rise of man on his path to sentience. All recorded history and "the history of the world" lies within the boundaries of the Holocene epoch.[201] Human activity, however, is being blamed for a die-out that has been going on since 10,000 B.C.E. commonly referred to as "the Sixth Extinction" with an estimated extinction rate of 140,000 species per year.[202]

The Pleistocene lasted from 3 million to 12,000 years ago. This epoch features the ice ages which is a result from the cooling effect that started in the Mid-Eocene. As the ice progressively migrated towards the equator, the areas north and south of the tropic line featured intense winters yet mild summers. Meanwhile, Africa experienced terrible droughts which resulted in the creation of the Sahara, Namib, and Kalahari deserts. To cope, many animals evolved including Mammoths, Giant ground sloths, Dire wolves and most famously Homo sapiens. 100,000 years ago marked the end of one of the worst droughts of Africa, and the expansion of primitive man. As the Pleistocene draws to a close, one of the largest die-outs causes many mega-fauna to die off, including the last hominid species (excluding Homo sapiens). All continents are effected, but Africa isn't hit quite as hard.[200]

Mega-fauna of the Pleistocene (Mammoths, Cave lions, Woolly Rhino, Megaloceros, American Horses
The Quaternary ranges from 3 million to present day, and features modern animals, and dramatic climate changes and features two epochs: the Pleistocene and the Holocene.

Quaternary

The Pliocene ranges from 5 to 2 million years ago. The Pliocene features dramatic climactic changes, which ultimately leads to modern species and plants. The most dramatic are the formation of Panama, and the accumulation of ice at the poles, leading to a massive die-off, India and Asia collide forming the Himalayas, the Rockies and Appalachian mountain ranges were formed, and the Mediterranean Sea dried up for the next several million years. Along with these major geological events, the Australopithecus evolves in Africa, beginning the human branch. Also, with the isthmus of Panama, animals migrate across North and South America, wreaking havoc on the local ecology. Climactic changes bring along savannas that are still continuing to spread across the world, Indian monsoons, deserts in East Asia, and the beginnings of the Sahara desert. The earth's continents and seas move into their present shapes, and the world map hasn't changed much since.[198][199]

The Miocene spans from 23 to 5 million years ago and is a period in which grass spreads further across, effectively dominating a large portion of the world, diminishing forests in the process. Kelp forests evolved, leading to new species such as sea otters to evolve. During this time, perissodactyls thrived, and evolved into many different varieties. Alongside them were the apes, which evolved into a staggering 30 species. Overall, arid and mountainous land dominated most of the world, as did grazers. The Tethys Sea finally closed with the creation of the Arabian Peninsula and in its wake left the Black, Red, Mediterranean and Caspian Seas. This only increased aridity. Many new plants evolved, and 95% of modern seed plants evolved in the mid-Miocene.[197]

[196]. It features 2 epochs: the Miocene, and the Pliocene.Phanerozoic EonThe Neogene spans from 23 million to 3 million years ago, and is the shortest geological period in the
Animals of the Miocene (Chalicotherium, Hyenadon, Entelodont...)

Neogene

The Oligocene Epoch spans from 33 million to 23 million years ago. The Oligocene feature the expansion of grass which had led to many new species to evolve, including the first elephants, cats, dogs, marsupials and many other species still prevalent today. Many other species of plants evolved in this period too, such as the evergreen trees. A cooling period was still in effect and seasonal rains were as well. Mammals still continued to grow larger and larger. Paraceratherium, the largest land mammal to ever live evolved during this period, along with many other perissodactyls in an event known as the Grand coupre.[195]

The Eocene Epoch ranged from 55 million years to 33 million years ago. In the Early-Eocene, life was small and living in cramped jungles, much like the Paleocene. There was nothing over the weight of 10 kilograms.[192] Among them were early primates, whales and horses along with many other early forms of mammals. At the top of the food chains were huge birds, such as Gastornis. It is the only time in recorded history that birds ruled the world (excluding their ancestors, the dinosaurs). The temperature was 30 degrees Celsius with little temperature gradient from pole to pole. In the Mid-Eocene, the circum-Antarctic current between Australia and Antarctica formed which disrupted ocean currents worldwide and as a result caused a global cooling effect, shrinking the jungles. This allowed mammals to grow to mammoth proportions, such as whales which are, by now, almost fully aquatic. Mammals like Andrewsarchus were now at the top of the food-chain and sharks were replaced by whales such as Basilosaurus as rulers of the seas. The Late-Eocene saw the rebirth of seasons, which caused the expansion of savanna-like areas, along with the evolution of grass.[193][194]

The Paleocene ranged from 65 million to 55 million years ago. The Paleocene is a transitional point between the devastation that is the K-T extinction, to the rich jungles environment that is the Early Eocene. The Early Paleocene saw the recovery of the earth. The continents began to take their modern shape, but all continents (and India) were separated from each other. Afro-Eurasia is separated by the Tethys Sea, and the Americas are separated by the strait of Panama, as the isthmus has not yet formed. This epoch features a general warming trend, with jungles eventually reaching the poles. The oceans were dominated by sharks as the large reptiles that had once ruled went extinct. Archaic mammals filled the world such as creodonts and early primates that evolved during the Mesozoic, and as a result, there was nothing over 10 kilograms. Mammals are still quite small.[191]

. Oligocene and Eocene, Paleocene: the epochsThe Paleogene spans from the extinction of the dinosaurs, some 65 million years ago, to the dawn of the Neogene twenty three million years ago. It features three

Paleogene

The Cenozoic features the rise of mammals on their conquest to rule the land, as the dinosaurs have now left a huge opening as top dog. There are three division of the Cenozoic: the Paleogene, the Neogene and Quaternary.

Cenozoic Era

The Late Cretaceous spans from 100 million to 65 million years ago.[188] The Late Cretaceous featured a cooling trend that would continue on in the Cenozoic period. Eventually, tropics were restricted to the equator and areas beyond the tropic lines featured extreme seasonal changes in weather. Dinosaurs still thrived as new species such as Tyrannosaurus, Ankylosaurus, Triceratops and Hadrosaurs dominated the food web. Pterosaurs, however, were going into a decline as birds took to the skies. The last pterosaur to die off was Quetzalcoatlus. Marsupials evolved within the large conifer forests as scavengers. In the oceans, Mosasaurs ruled the seas to fill the role of the Ichthyosaurs, and huge plesiosaurs, such as Elasmosaurus, evolved. Also, the first flowering plants evolved. At the end of the Cretaceous, the Deccan traps and other volcanic eruptions were poisoning the atmosphere. As this was continuing, it is thought that a large meteor smashed into earth, creating the Chicxulub Crater in an event known as the K-T Extinction, the fifth and most recent mass extinction event, in which 75% of life on earth went extinct, including all non-avian dinosaurs. Everything over 10 kilograms went extinct. The age of the dinosaurs was officially over.[189][190]

The Early Cretaceous spans from 145 million to 100 million years ago.[188] The Early Cretaceous saw the expansion of seaways, and as a result, the decline and extinction of sauropods (except in South America). Many coastal shallows were created, and that caused Ichthyosaurs to die out. Mosasaurs evolved to replace them as head of the seas. Some island-hopping dinosaurs, like Eustreptospondylus, evolved to cope with the coastal shallows and small islands of ancient Europe. Other dinosaurs rose up to fill the empty space that the Jurassic-Cretaceous extinction left behind, such as Carcharodontosaurus and Spinosaurus. Of the most successful would be the Iguanodon which spread to every continent. Seasons came back into effect an the poles got seasonally colder, but dinosaurs still inhabited this area like the Leaellynasaura which inhabited the polar forests year-round, and many dinosaurs migrated there during summer like Muttaburrasaurus. Since it was too cold for crocodiles, it was the last stronghold for large amphibians, like Koolasuchus. Pterosaurs got larger as species like Tapejara and Ornithocheirus evolved. More importantly, the first true birds evolved which sparked competition between them and the pterosaurs.

[188]The Cretaceous is the longest era in the Mesozoic, but has only two periods: the Early Cretaceous, and the Late Cretaceous.

Cretaceous

The Late Jurassic spans from 163 million to 145 million years ago.[184] The Late Jurassic featured a massive extinction of sauropods and Ichthyosaurs due to the separation of Pangaea into Laurasia and Gondwana in an extinction known as the Jurassic-Cretaceous extinction. Sea levels rose, destroying fern prairies and creating shallows in its wake. Ichthyosaurs went extinct whereas sauropods, as a whole, did not die out in the Jurassic; in fact, some species, like the Titanosaurus, lived up to the K-T extinction.[187] The increase in sea-levels opened up the Atlantic sea way which would continue to get larger over time. The divided world would give opportunity for the diversification of new dinosaurs.

(Inaccurately portrayed) Stegosaurus
[186] were flourishing. This epoch was the peak of the reptiles.Ichthyosaurs were quite common, and Plesiosaurs. Conifer forests made up a large portion of the forests. In the oceans, Allosaurus, filled the fern prairies of the Middle Jurassic. Many other predators rose as well, such as Diplodocus and Brachiosaurus During this epoch, reptiles flourished as huge herds of sauropods, such as [184]The Middle Jurassic spans from 175 million to 163 million years ago.

The Early Jurassic spans from 200 million years to 175 million years ago.[184] The climate was much more humid than the Triassic, and as a result, the world was very tropical. In the oceans, Plesiosaurs, Ichthyosaurs and Ammonites fill waters as the dominant races of the seas. On land, dinosaurs and other reptiles stake their claim as the dominant race of the land, with species such as Dilophosaurus at the top. The first true crocodiles evolved, pushing out the large amphibians to near extinction. All-in-all, reptiles rise to rule the world. Meanwhile, the first true mammals evolve, but never exceed the height of a shrew.[185]

[184]The Jurassic ranges from 200 million years to 145 million years ago and features 3 major epochs: The Early Jurassic, the Middle Jurassic, and the Late Jurassic.

Jurassic

The Late Triassic spans from 237 million to 200 million years ago. Following the bloom of the Middle Triassic, the Late Triassic featured frequent heat spells, as well as moderate precipitation (10-20 inches per year). The recent warming led to a boom of reptilian evolution on land as the first true dinosaurs evolve, as well as pterosaurs. All this climactic change, however, resulted in a large die-out known as the Triassic-Jurassic extinction event, in which all archosaurs (excluding ancient crocodiles), synapsids, and almost all large amphibians went extinct, as well as 34% of marine life in the fourth mass extinction event of the world. The cause is debatable.[182][183]

The Middle Triassic spans from 247 million to 237 million years ago. The Middle Triassic featured the beginnings of the breakup of Pangaea, and the beginning of the Tethys Sea. The ecosystem had recovered from the devastation that was the Great Dying. Phytoplankton, coral, and crustaceans all had recovered, and the reptiles began to get bigger and bigger. New aquatic reptiles evolved such as Ichthyosaurs and Nothosaurs. Meanwhile, on land, Pine forests flourished, bringing along mosquitoes and fruit flies. The first ancient crocodilians evolved, which sparked competition with the large amphibians that had since rule the freshwater world.[181]

[180] evolved during this time and would be the dominant predator for much of the Triassic.Temnospondyli. the Great Dying along with many other creatures that managed to survive Euparkeria, and Labyrinthodont, Lystrosaurus had not yet broken up, thus the interior was nothing but arid. The Earth had just witnessed a massive die-off in which 95% of all life went extinct. The most common life on earth were PangaeaThe Early Triassic lived between 250 million to 247 million years ago and was dominated by deserts as

The Triassic ranges from 250 million to 200 million years ago. The Triassic is a desolate transitional state in Earth's history between the Permian Extinction and the lush Jurassic Period. It has three major epochs: the Early Triassic, the Middle Triassic and the Late Triassic.[179]

Triassic

Also known as "the Age of the dinosaurs", the Mesozoic features the rise of reptiles on their 150 million year conquest to rule the earth from the seas, the land, and even in the air. There are 3 periods in the Mesozoic: the Triassic, the Jurassic, and the Cretaceous.

Mesozoic Era

The Permian spans from 300 million to 250 million years ago and was the last period of the Paleozoic. At the beginning, all continents formed together to form the super-continent the Great Dying", and is the third mass extinction event of the world.[177][178]

Permian

[176].Carboniferous Rainforest Collapse as much of it was situated around the south pole in an event known as the Permo-Carboniferous glaciation or the Gondwana evolved in the swamps. Throughout the Carboniferous, there was a cooling pattern, which eventually led to the glaciation of synapsids Tropical swamps dominated the earth, and the large amounts of trees created much of the carbon for the coal that is used today (hence the name "Carbon-iferous"). Perhaps the most important evolutionary development of the time was the evolution of amniotic eggs, which allowed amphibians to head farther inland and remained the dominant vertebrae throughout the duration of this period. Also, the first reptiles and [175]The Carboniferous spans from 360 million to 300 million years ago. During this time, average global temperatures were exceedingly high; the early Carboniferous averaged at about 20 degrees Celsius (but cooled down to 10 degrees during the Middle Carboniferous).

Carboniferous

[174] and is the second mass extinction event the world has seen.Late Devonian extinction. On land, plant groups diversified incredibly in an event known as the Devonian Explosion where the first trees evolved, as well as seeds. This event also diversified arthropod life. The first amphibians also evolved, and the fish were now at the top of the food chain. Near the end of the Devonian, 70% of all species went extinct in an event known as the tetrapods and lobe-finned fish which eventually evolved into the first DunkleosteusThe Devonian spans from 415 million years to 360 million years ago. Also known as "The Age of the Fish", the Devonian features a huge diversification of fish, including armored fish like
Eogyrinus (an amphibian) of the Carboniferous

Devonian

The Silurian spans from 440 million years to 415 million years ago. The Silurian saw the healing of the earth that recovered from the snowball earth. This period saw the mass evolution of fish, as jaw-less fish became more numerous, jawed fish evolved, and the first freshwater fish evolved, though arthropods, such as sea scorpions, were still apex predators. Fully terrestrial life evolved, which included early arachnids, fungi, and centipedes. Also, the evolution of vascular plants (Cooksonia) allowed plants to gain a foothold on land. These early plants are the forerunners of all plant life on land. During this time, there are four continents: Gondwana (Africa, South America, Australia, Antarctica, Siberia), Laurentia (North America), Baltica (Northern Europe), and Avalonia (Western Europe). The recent rise in sea levels provided many new species to thrive in water.[173]

Silurian

[172] in which 60% of marine invertebrates and 25% of families went extinct, and is considered the first mass extinction and the second deadliest extinction.Ordovician-Silurian extinction, and the snowball earth. By the end of the period, Gondwana was at the south pole, early North America had collided with Europe, closing the Atlantic Ocean. Glaciation of Africa resulted in a major drop in sea level, killing off all life that staked a claim along coastal Gondwana. Glaciation caused a GondwanaThe Ordovician spans from 485 million years to 440 million years ago. The Ordovician is a time in earths history in which many species still prevalent today evolved, such as primitive fish, cephalopods, and coral. The most common forms of life, however, were trilobites, snails and shellfish. More importantly, the first arthropods went ashore to colonize the empty continent of
Cephalaspis (a jaw-less fish)

Ordovician

[171].Gondwana begins to break up, most of which becomes the super-continent Rodinia. Almost all marine phyla evolved in this period. During this time, the super-continent trilobites in which the largest number of creatures evolve in the history of Earth during one period. Creatures like algae evolve, but most of the water is populated by armored arthropods, like Cambrian Explosion. The Cambrian sparks a boom in evolution in an event known as the Phanerozoic EonThe Cambrian spans from 540 million years to 485 million years ago and is the first period of the Paleozoic and of the

Cambrian

The Paleozoic is a time in earth's history when complex life forms evolve, take their first breath of oxygen on dry land, and when the forerunner of all life on earth begin to diversify. There are seven periods in the Paleozoic eras: the Cambrian, the Ordovician, the Silurian, the Devonian, the Carboniferous and the Permian.[170]

Paleozoic Era

The Phanerozoic Eon is the current eon in Earth's history. The Phanerozoic began 540 million years ago and continues to the present. The Phanerozoic comprises the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. This eon is when the diversity of life increases dramatically, starting with the Cambrian Explosion.

Phanerozoic Eon

Synthetic biology is a new area of biotechnology that combines science and biological engineering. The common goal is the design and construction of new biological functions and systems not found in nature. Synthetic biology includes the broad redefinition and expansion of biotechnology, with the ultimate goals of being able to design and build engineered biological systems that process information, manipulate chemicals, fabricate materials and structures, produce energy, provide food, and maintain and enhance human health and the environment.[169]

Artificial life is a field of study that examines systems related to life, its processes, and its evolution through simulations using computer models, robotics, and biochemistry.[168] The study of artificial life imitates traditional biology by recreating some aspects of biological phenomena. Scientists study the logic of living systems by creating artificial environments—seeking to understand the complex information processing that defines such systems. While life is, by definition, alive, artificial life is generally referred to as data confined to a digital environment and existence.

Artificial life

rock formations 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.[165] 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 billion years old.[166][167]

[164]

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.[161] 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 or reincarnation for the soul, or resurrection of the body at a later date.

Death is the permanent termination of all vital functions or life processes in an organism or cell.[160][161] It can occur as a result of an accident, detritus, returning it to the environment for reuse in the food chain.

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

Death

Beyond the solar system, the region around another main sequence star that could support Earth-like life on an Earth-like planet is known as the habitable zone. 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 main sequence for a shorter time interval. Small red dwarf stars 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. Hence, stars in the intermediate mass range such as the Sun may have a greater likelihood for Earth-like life to develop.[158] The location of the star within a galaxy may also have an impact on the likelihood of life forming. Stars in regions with a greater abundance of heavier elements that can form planets, in combination with a low rate of potentially habitat-damaging supernova events, are predicted to have a higher probability of hosting planets with complex life.[159]

Earth is the only planet known to harbor life. Other locations within the microbial life include subsurface Mars, the atmosphere of Venus,[155] and subsurface oceans on some of the moons of the gas giant planets.[156] The variables of the Drake equation are used to discuss the conditions in solar systems where civilization is most likely to exist.[157]

Extraterrestrial life

Linnaeus
1735[150]
Haeckel
1866[151]
Chatton
1925[152]
Copeland
1938[144]
Whittaker
1969[153]
Woese et al.
1990[146]
Cavalier-Smith
1998[154]
2 kingdoms 3 kingdoms 2 empires 4 kingdoms 5 kingdoms 3 domains 6 kingdoms
(not treated) Protista Prokaryota Monera Monera Bacteria Bacteria
Archaea
Eukaryota Protoctista Protista Eucarya Protozoa
Chromista
Vegetabilia Plantae Plantae Plantae Plantae
Fungi Fungi
Animalia Animalia Animalia Animalia Animalia
[149].evolutionary or phylogenetic tree in an clades emerged, arranging taxa based on cladisticsIn the 1960s a trend called

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.[148]

As new discoveries enabled detailed study of protozoa as animals and protophyta/thallophyta as plants, but were united by Haeckel in the kingdom Protista; later, the prokaryotes were split off in the kingdom Monera, which would eventually be divided into two separate groups, the Bacteria and the Archaea. This led to the six-kingdom system and eventually to the current three-domain system, which is based on evolutionary relationships.[146] However, the classification of eukaryotes, especially of protists, is still controversial.[147]

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.[144] The problem was eventually solved by Whittaker, when he gave them their own kingdom in his five-kingdom system. Evolutionary history shows that the fungi are more closely related to animals than to plants.[145]

The exploration of the American continent 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 1740s, Carl Linnaeus introduced his system of binomial nomenclature for the classification of species.[143] 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. By consistently using this system, Linnaeus separated nomenclature from taxonomy.

The first known attempt to classify organisms was conducted by the Greek philosopher Aristotle (384–322 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: viviparous quadrupeds (mammals), oviparous quadrupeds (reptiles and amphibians), birds, fishes and whales. The bloodless animals were also divided into five groups: cephalopods, crustaceans, insects (which included the spiders, scorpions, and centipedes, in addition to what we define as insects today), shelled animals (such as most molluscs and echinoderms) and "zoophytes." 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.[142]

Classification

Cells have evolved methods to perceive and respond to their microenvironment, thereby enhancing their adaptability. nervous system.[141]

[140]

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.[139]

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.[138] In eukaryotic cells, these proteins may then be transported and processed through the Golgi apparatus in preparation for dispatch to their destination.

There are two primary types of cells. endosymbiosis between bacteria and the progenitor eukaryotic cell.[137]

Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by energy flow occurring within and between them. Cells contain hereditary information that is carried forward as a genetic code during cell division.[135]

Form and function

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.[118] For example, lichen could survive for a month in a simulated Martian environment.[132][133]

Microbial life forms thrive even in the Mariana Trench, the deepest spot on the Earth.[129][130] Microbes also thrive inside rocks up to 1900 feet below the sea floor under 8500 feet of ocean.[129][131]

To survive, selected microorganisms can assume forms that enable them to withstand 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.[128]

Deinococcus radiodurans is an extremophile that can resist extremes of cold, dehydration, vacuum, acid, and radiation exposure.

[127] The inert components of an ecosystem are the physical and chemical factors necessary for life — energy (sunlight or

Range of tolerance

[125][124] or other chemical properties.chiralities have been proposed that eliminate one or more of these elements, swap out an element for one not on the list, or change required hypothetical types of biochemistry Alternative [123] All life forms require certain core

[118] The diversity of life on Earth is a result of the dynamic interplay between

oxygen-intolerant organisms.

Environmental conditions

According to the microscopic life—distributed by meteoroids, asteroids and other small Solar System bodies—may exist throughout the universe.[117]

In March 2015, NASA scientists reported that, for the first time, complex 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.[116]

Prebiotic compounds may have extraterrestrial origin. outer space.[112][113][114][115]

In 2009, experiments demonstrated Darwinian evolution of a two-component system of RNA enzymes (ribozymes) in vitro.[110] The work was performed in the laboratory of Gerald Joyce, who stated, "This is the first example, outside of biology, of evolutionary adaptation in a molecular genetic system."[111]

Geological findings in 2013 showed that reactive phosphorylated biomolecules, like RNA.

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.[106] However, the successful synthesis of certain RNA molecules under the conditions that existed prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor phosphate present throughout the reaction.[107] This study makes the RNA world hypothesis more plausible.[108]

Therefore, a possibility, first suggested by Francis Crick,[103] is that the first life was based on RNA,[102] 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. Also, many critical cofactors (ATP, Acetyl-CoA, NADH, etc.) are either nucleotides or substances clearly related to them. The catalytic properties of RNA had not yet been demonstrated when the hypothesis was first proposed,[104] but they were confirmed by Thomas Cech in 1986.[105]

However, since 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.[102]

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;[100] the alternative being that proteins came first and then genes.[101]

There is no current scientific consensus as to how life originated. However, most accepted scientific models build on the following observations:

Evidence suggests that life on Earth has existed for at least 3.5 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.[98]

Origin

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',[91] and also proposes that an organism is 'any system with an organisation that complies with an operator type that is at least as complex as the cell.[92][93][94][95] Life can also be modeled as a network of inferior negative feedbacks of regulatory mechanisms subordinated to a superior positive feedback formed by the potential of expansion and reproduction.[96]

It has also been argued that the evolution of order in living systems and certain physical systems obey a common fundamental principle termed the Darwinian dynamic.[89][90] The Darwinian dynamic was formulated by first considering how macroscopic order is generated in a simple non-biological system far from thermodynamic equilibrium, and then extending consideration to short, replicating RNA molecules. The underlying order generating process for both types of system was concluded to be basically similar.[89]

networks of metabolic, genetic, epigenetic processes and signaling pathways.

[85] (2009) highlights mutualism as the key to understand the systemic, order-generating behavior of life and ecosystems.Robert Ulanowicz He argues that an ecosystemic definition of life is preferable to a strictly biochemical or physical one. [84] A systems view of life treats environmental

The first attempt at a general [82]

[79] 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

Living systems theories

[75][74][73]

Electron micrograph of adenovirus with a cartoon to demonstrate its icosahedral structure

Viruses

Others take a autopoietic (self-producing). Variations of this definition include Stuart Kauffman's definition as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle.[69]

[68][67].Darwinian evolution Hence, life is a self-sustained chemical system capable of undergoing [66][65] That is, life is matter that can reproduce itself and evolve as survival dictates.[61] many of these are based upon chemical systems. [58] To reflect the minimum phenomena required, other biological definitions of life have been proposed,

Alternatives

These complex processes, called physiological functions, have underlying physical and chemical bases, as well as signaling and control mechanisms that are essential to maintaining life.

  1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.
  2. cells — the basic units of life.
  3. [51]
  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. heredity, diet, and external factors.
  6. Response to phototropism), and chemotaxis.
  7. [57] "with an error rate below the sustainability threshold."[57][56]

Since there is no unequivocal definition of life, the current understanding is descriptive. Life is considered a characteristic of something that exhibits all or most of the following traits:[51][54][55]

Biology

It is a challenge for scientists and philosophers to define life.[43][44][45][46] This is partially because life is a process, not a substance.[47][48][49] Any definition must be general enough to both encompass all known life and any unknown life that may be different from life on Earth.[50][51][52] Some may even consider that life is not real at all, but a concept instead.[53]

Definitions

During the 1850s, Helmholtz, anticipated by Mayer, demonstrated that no energy is lost in muscle movement, suggesting that there were no "vital forces" necessary to move a muscle.[41] These results led to the abandonment of scientific interest in vitalistic theories, although the belief lingered on in pseudoscientific theories such as homeopathy, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.[42]

[39]

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