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Our earth is very old. Based on the estimation of the oldest rock, its around 4. 5 billion years of age. Scientists from all over the world use astronomy, geology, chemistry, biology, archaeology and other sciences to investigate the Earths formation as well as the emergence and extinction of life on Earth. …Then theres life! Around 13. 8 billion years ago, an enormous explosion that scientists call the Big Bang spurred the formation of our planet. The explosion produced increasingly dense, cloud-like masses of hydrogen dust; the biggest turned into our sun, while smaller ones became planets. One of those planets is our Earth. Some scientists believe around 600 to 700 million years later, meteor showers bombarded the earth, carrying with it large volumes of water and amino acids. Life, in the form of single-cell bacteria, began. Since then, bacteria has evolved into more complex forms, though different beings have also become extinct. Geological eras Geologists divide the periods from the Earths formation up until now into a number of eras based on the changes that happened in each of them. We are currently in the Holocene era, which started around 11, 700 years ago when the Ice Age ended. More recently though, a number of scientists have argued that because of the nuclear bomb testings of the 1950s and population explosion, humans have entered a new era, called the Anthropocene. They argue that with more than seven billion people, human activity has drastically influenced nature and the extinction of a number of wildlife. The Earth is no stranger to disappearing life forms. There are have been many periods of extinction, from when the first organism emerged on Earth until today. However, according to fossil records, only five eras have drastically reduced the population of living beings on earth to warrant the label of mass extinction. First period of extinction Entering early to mid period of the Ordovician Era, the Earth was still warm with an ideal humidity level for living. However, towards the end of the period - around 443 million years ago - everything changed suddenly, when the old continent Gondwana reached the South Pole. The temperature dropped drastically and ice formed everywhere, lowering the water level. Subsequently, the level of carbon dioxide in the atmosphere and in the sea dropped, causing the number of plants to decrease dramatically and an ecosystem chaos ensued because certain plants, used as sources of food, became scarce. Some 86% of the population of living beings disappeared within three million years. Some of the organisms affected by the first extinction were Brachiopods, Conodonts, Acritarchs, Bryozons, and also Trilobites that lived in the ocean. Second period of extinction The second period of extinction, during the Devon Age, happened around 359 million years ago. A relentless meteor shower is believed to be one of the causes of mass extinction. Other causes include a drastic decrease in oxygen levels globally, the increased activity of tectonic plates, and climate change. These changes caused around 75% of living creatures to die. Extinction in this period impacted life in the sea which, at the time, was dominated by corals and stromatoporoids. Third period of extinction The third period of extinction, around 251 million years ago, during the Permian Age, was the biggest and worst that ever happened on Earth. The formation of the giant continent Pangea caused immense changes in geology, climate and the environment. Volcanic eruptions that continued for 1 million years released around 300 million square kilometres of lava while more than 1750 metres of sediment was formed in the Siberian Traps. The eruptions burned forests four times the size of Korea. It produced large volumes of carbon dioxide that caused global warming. As a result, frozen methane below the sea melted, producing a global warming effect 20 times more powerful than carbon dioxide. The global warming lasted for approximately 10 million years. A terrible mass extinction was inevitable. Only 5% of the population of life on Earth survived and 95% perished from massive drought, lack of oxygen and acid rain that made plants unable to survive. Fourth period of extinction The fourth period of extinction happened around 210 million years ago, during the Late Triassic Age. The slow splitting of Pangea caused volcanoes to form in the Central Atlantic Magmatic Province. After a spike in atmospheric carbon dioxide, global warming started again, with scientists speculating it lasted as long as eight million years. This caused coral and [conodonts. an eel-like ancient sea creature to face serious crisis. Coral-based creatures did not survive. A meteor rain also hastened the destruction in this period: Around 80% of living creatures, including reptiles died, with some 20% of the creatures that became extinct sea-based lifeforms. Additionally, a number of creatures that lived on land that died in this period were pseudosuchia, crocodylomorphs, theropods and several large amphibians. Fifth period of extinction The fifth period of extinction happened around 65 million years ago and is more popularly known as Cretaceous-Tertiary extinction. It was the fastest period of mass extinction, occurring over one to 2. 5 million years. Its possibly the most known period of mass extinction because this was when dinosaurs were wiped out from the face of the earth. Scientists believe a meteor fall in todays Gulf of Mexico compounded with high volcanic activity which produced a significant amount of carbon dioxide, killed half of the earths living population. How does the future look? Some scientists believe that we have entered the sixth period of extinction since 2010. The massive emission of carbon dioxide from fossil fuels has affected the lives of many plants and animals. Scientists predict that this will affect many life forms on Earth in the next three to four decades. Who knows.

Extinction is an important process of learning that is typically defined as the removal of reinforcers and/or biologically relevant stimuli from a previously established stimulus relationship that results in a reduction in conditioned responding (Pavlov, 1927. From: International Review of Neurobiology, 2016 Animal Models of Drug Addiction George F. Koob, Michel Le Moal, in Neurobiology of Addiction, 2006 Resistance to Extinction Associated with Drug Self-administration Extinction procedures can provide measures of the motivational properties of drugs by assessing the persistence of drug-seeking behavior in the absence of response-contingent drug availability. In an extinction paradigm, subjects are trained to self-administer a drug until stable self-administration patterns are achieved, and then the drug is removed ( Schuster and Woods, 1968. Fig. 2. 16. Extinction testing sessions are identical to training sessions except that no drug is delivered after completion of the response requirement. Measures provided by an extinction paradigm reflect the degree of resistance to extinction and include the duration of extinction responding, the total number of responses emitted during the entire extinction session, and the probability of reinitiating responding under extinction conditions at a later time after successful extinction of the self-administration behavior (i. e., propensity to relapse. FIGURE 2. 16. Average response rate for two monkeys M#6 and M#7. The far left point is the final 5 day average response rate for morphine reinforcement. The monkeys were placed on a 2. 5 min variable internal schedule of reinforcement for both food and morphine. Each 24 h day was broken into four cycles of 6 h each. Brackets indicate the range of daily response rate over the 5 days. Points to the right of the dashed line show extinction data following 15 drug-free days of rest. The closed circles indicate the absence of response consequences for drug responding, and the open circles indicate the consequences of infusion of saline and the presentation of a red light. [Reproduced with permission from Schuster and Woods, 1968. Invertebrate Learning and Memory Dorothea Eisenhardt, in Handbook of Behavioral Neuroscience, 2013 Conclusion Extinction learning and the molecular mechanisms underlying extinction memory formation are surprisingly similar between honeybees and vertebrates. In honeybees and in vertebrates, a context dependency of extinction has been shown. Furthermore, a reactivation and reconsolidation of the initially formed acquisition memory and the existence of two contrasting memories after extinction have been demonstrated. Recent results on extinction in honeybees extend these findings by showing that extinction memory formation (i. e., its underlying molecular mechanisms) is dependent on the learning parameters of reward learning. This might ensure that only meaningful changes of the reinforcing stimulus are memorized. Extinction resembles a basic learning phenomenon that enables animals to adequately react to a fluctuating environment by providing them with the ability to learn about environmental changes and memorize this information. The similarity of extinction learning and its underlying molecular mechanisms between honeybees and vertebrates suggests that extinction is a conserved, phylogenetically old mechanism. Given such conserved functions, findings on the mechanisms of extinction in honeybees will be most important to further elucidate the basic mechanisms of extinction learning in both invertebrates and vertebrates. Extinction: Anatomy K. A. Corcoran, G. J. Quirk, in Encyclopedia of Neuroscience, 2009 Introduction Extinction occurs when a conditioned stimulus (CS) is repeatedly presented in the absence of reinforcement; it is measured as a decline in the frequency and amplitude of conditioned responses. Current theories of extinction suggest that this decline reflects the development of an inhibitory memory that competes with the conditioning memory to drive behavior. Like other forms of memory, extinction consists of three phases: acquisition, consolidation, and retrieval ( Figure 1. Acquisition of extinction is evident as decreased responding to the CS across an extinction session. Consolidation begins some time during the extinction session and continues for several hours afterward to form a more stable, long-term memory trace. Retrieval of extinction is triggered when the CS is once again presented after the extinction session. Research on the mechanisms of extinction has pinpointed many of the molecular mechanisms involved in these processes and has identified a critical neural circuit, which includes the amygdala, medial prefrontal cortex (mPFC) and hippocampus. Figure 1. Extinction is a learning process that occurs in three phases. During extinction training, when conditioned stimuli (CSs) are presented without any reinforcement, the frequency and magnitude of conditioned responses decline. Consolidation of the extinction memory begins some time during extinction training and continues for several hours after the end of the extinction session. When the CS is again presented sometime later, extinction is retrieved, as evidenced by low levels of conditioned responding. A failure to retrieve extinction (high levels of responding) occurs when CS is presented outside the extinction context. Learning Theory and Behaviour M. E. Bouton, A. M. Woods, in Learning and Memory: A Comprehensive Reference, 2008 Extinction is the decrease in strength of a learned behavior when the conditioned stimulus is presented without the unconditioned stimulus (in Pavlovian learning) or when the behavior is no longer reinforced (in operant or instrumental learning. This chapter reviews the behavioral literature on extinction. The first section reviews several relapse phenomena, such as the renewal effect, which suggest that extinction does not destroy original learning, but instead involves new context-dependent learning. The second section reviews theoretical explanations of the cause of extinction. The final section reviews behavioral and pharmacological ways of optimizing extinction learning and decreasing the likelihood of relapse. THE NEGLECT SYNDROME Elsdon Storey, in Neurology and Clinical Neuroscience, 2007 Extinction Extinction (or sensory extinction to double simultaneous stimulation) is said to be present when the patient does respond to sensory stimulation on the contralesional side but then fails to do so when another stimulus is applied simultaneously. The extinguishing stimulus is typically similar to that being extinguished and is usually applied to the corresponding contralateral area, but transmodal extinction (e. g., of a left-sided tactile stimulus by a right-sided visual stimulus) can occur, as can extinction of one stimulus by a second ipsilateral stimulus. When this occurs, the rightward stimulus typically extinguishes that further to the left: this allocentric effect can be seen in the ipsilesional as well as the contralesional receptive field. Extinction, too, may be unimodal or multimodal. Cetacean Evolution R. Ewan Fordyce, in Encyclopedia of Marine Mammals (Second Edition) 2009 XI Extinction Extinction, the disappearance of lineages, is the fundamental complement to evolution. Clearly, a species goes extinct when the number of individuals and geographic range drops to nil. The fossil record reveals that extinction is inevitable and, for lineages, usually terminal; few species go extinct by evolving into descendants. Among different styles of extinction, there is no evidence that cetaceans have been involved in mass extinction comparable to that affecting dinosaurs. Taxonomic extinction (the disappearance of a clade) has occurred, as shown by the fossil record of such well-defined clades as the odontocetes Eurhinodelphinidae and Squalodontidae, and the Cetotherium group of mysticetes. Environmental change might explain extinction through loss of habitat or food or through climate change. Most extinction has probably involved the piecemeal extinction of single species, but the cetacean fossil record is too patchy to expect pattern or cause to be clear. Species susceptible to extinction are those in low-diversity clades, e. g., one or two species in a genus, with no close relatives, occurring in geographically limited physical settings that are unstable over geological time. For Cetacea, this means particularly the “river dolphins. ” Conversely, widely distributed oceanic species would seem resistant to extinction. Patterns of extinction beg a question that ecologists might consider unthinkable: Are there vacant modern niches that formerly were occupied by Cetacea? For example, stem Platanistidae lived in shallow marine settings until about 10 Ma, but Platanista now occurs only in freshwaters, and species of Squalodontidae and Eurhinodelphinidae were widely distributed before their demise in the later Miocene. Judging from the functional complexes seen in the latter fossils, there are no modern equivalents to these groups: some morphotypes and lifestyles have disappeared. Pharmacology of Fear Extinction M. Davis, K. Myers, in Encyclopedia of Neuroscience, 2009 Transcription Extinction has been associated with an increase in the expression of immediate-early genes in mPFC and BLA and an upregulation of gephyrin and BDNF mRNA in the BLA. A persistent impairment of extinction of step-down avoidance has been found when the transcription inhibitors a-amanitin or 5, 6-dichloro-1-b-D-ribofuranosyl benzimidazole (DRB) were infused into the CA1 region of hippocampus 15  min before, but not 1 or 3 h after, the first of several extinction exposures to the apparatus. The lack of effect of post-extinction training administration is somewhat surprising given the likelihood that mRNA transcription is involved in consolidation of extinction memory, although it is possible that the onset of transcription occurs fairly rapidly (<1 h) following extinction training. Facilitation of extinction of fear-potentiated startle by pre-extinction training DCS was blocked by pre-administration of the transcription inhibitor actinomycin D into the BLA, an effect not due to state dependency. Clinical Implications of Network Principles 3–12 Warren W. Tryon, in Cognitive Neuroscience and Psychotherapy, 2014 Extinction Extinction produces a temporary increase in response strength followed by a gradual decrease in response strength. This gradual decrease gave rise to the hypothesis that the therapeutic effects of systematic desensitization were caused by extinction. For example, Waters et al. (1972) explained that the ability of systematic desensitization to treat phobias was based on extinction. Extinction requires that a specific target response not be followed by reinforcement; i. e., the absence of onset or offset of stimuli with positive or negative reinforcing properties contingent upon either the emission or omission of a response. No specific ‘response is obvious during systematic desensitization. Hence, extinction cannot account for why systematic desensitization works. Response decrement is explained in terms of the lack of reinforcement. Exposure or exposure plus non-avoidance are only partially consistent with an extinction explanation, because a complete extinction explanation needs to: 1) define the target behavior; 2) define the reinforcer; and (3) show that no onset or offset of the reinforcer occurred contingent upon either the emission or omission of the target behavior. The empirical literature on systematic desensitization does not strongly support the third criteria and only partially supports the other two. More importantly, extinction is a functional concept that lacks mechanism information. Behaviorism cannot explain why failure to reinforce a behavior causes its frequency to decrease. Extinction is a behavioral concept. Hence, extinction entails no mechanism information capable of explaining how or why systematic desensitization works. Wolpe (1995) criticized the extinction explanation on the basis that it lacked mechanism information and that it was falsified by his clinical experience; i. e., anxiety did not naturally extinguish. McGlynn et al. (1981) noted that ‘… exposure theory is not an explanation of therapeutic desensitization effects. Rather, it is simply a hypothesis concerning the necessary and sufficient procedural ingredients within the technique. The therapeutic effects of the exposure remain to be explained (e. g., as extinction, as counterconditioning, as habituation) p. 154. We have already seen that there is little supportive evidence for these explanations. Self-injurious Behavior: Nonhuman Primate Models for the Human Condition Corrine K. Lutz, Jerrold S. Meyer, in Primate Models of Children's Health and Developmental Disabilities, 2008 Extinction Extinction involves the elimination of reinforcement that previously followed SIB. For example, if the reinforcement consisted of attention from caretakers, the caretakers would be instructed to ignore the self-injurious act and continue as if the behavior did not occur. In some situations, the reinforcement may be a result of the behavior itself, such as producing sensory stimulation. In this case, extinction may be brought about by placing equipment such as gloves on the subject to reduce the sensory experience gained by this type of SIB ( Roscoe et al., 1998. In general, extinction has been found to reduce the incidence of SIB ( Repp et al., 1988. However, one problem with extinction is the occurrence of extinction bursts, an immediate but transient increase in the unwanted behavior ( Zarcone et al., 1993; Vollmer et al., 1998; Lerman et al., 1999. Animal Extinctions Samia R. Toukhsati, in Animals and Human Society, 2018 Summary Extinctions refer to the death of a single or multiple species (or taxon) and are common in the history of life on this planet. Using the fossil record, it has been estimated that 99. 9% of the species that existed on earth is now extinct. Extinctions occur when a species fails to meet or adapt to changing environmental forces (such as global warming or cooling, habitat loss, destruction, or fragmentation) or when species origination is low, creating ecological niches for new, better adapted, species. This process of “background” extinction and new species evolution is natural, occurs continuously, and describes the way life diversified and radiated on this planet. However, when extinctions involve vast numbers of species and appear to occur around the same time in many different regions, as may be the case in modern times, they are termed “mass extinctions”; these are much less common, but greatly reduce species diversity. There is much debate and little consensus as to the cause and timescale of mass extinctions, generally referred to as the “Big Five” extinction events, which mark the point of transition to new geological epochs. This chapter will focus on the modern-day Holocene–Anthropocene extinction, which attributes the possible loss of up to 58, 000 species per year to human activities.