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Pet velikih masovnih izumiranja na Zemlji, počelo i šesto

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Pet velikih masovnih izumiranja na Zemlji, počelo i šesto

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Brojni naučnici procenjuju da se Zemlja nalazi na početku novog „masovnog izumiranja“, fenomena koji karakteriše nestanak vrsta u alarmantnom broju za relativno kratko vreme, ovog puta izazvanog ljudskim aktivnostima.

U proteklih 500 miliona godina, planeta je pet puta bila svedok perioda tokom kojih je u treptaju oka geološke istorije nestalo više od polovine živih bića.

Sveukupno, više od 90 odsto vrsta koje su nekada hodale, plivale, letele ili puzale Zemljom sada je izumrlo.

Prvo masovno izumiranje odigralo se pre oko 445 miliona godina. U takozvanom „Ordovicijsko-silurijskom izumiranju“ nestalo je izmedju 60 i 70 odsto vrsta.

Veruje se da je do tog istrebljenja dovelo kratko ali veoma snažno ledeno doba.

U to vreme se živi svet najvećim delom nalazio u okeanima. Naučnici smatraju da je brzo formiranje glečera zamrzlo najveći deo vode na planeti i dovelo do drastičnog pada nivoa mora. Morski organizmi poput sundjera i algi su najviše stradali, kao i školjke, primitivni glavonošci i ribe bez vilica zvane ostrakodermi.

Zatim je pre oko 360 do 375 miliona godina usledilo tzv. „Kasnodevonsko izumiranje“, u kom je nestalo do tri četvrtine živog sveta. Ponovo su najviše stradali morski organizmi, usled smanjenja količine kiseonika u okeanima.

Kao najverovatniji uzroci takvog razvoja dogadjaja navode se variranje nivoa okeana, klimatske promene ili čak udar asteroida. Prema jednoj teoriji širenje kopnene vegetacije dovelo je do manjka kiseonika u vodama. Verovatno najpoznatija žrtva „Kasnodevonskog izumiranja“ bili su trilobiti, artropodi koji su živeli na okeanskom dnu.

Posebno razorno bilo je „Permsko-trijasko izumiranje“, pre oko 252 miliona godina. Taj period, nazvan i „majka svih izumiranja“, odneo je 95 odsto vrsta.

Izazvana po svoj prilici udarima asteroida i vulkanskom aktivnošću, ta biološka kriza je opustošila i okeane i kopno. Neki naučnici veruju da je trajala milionima godina, dok drugi smatraju da je trajala samo 200.000 godina.

To je bio jedini put da su masovno nestali insekti. Nestale su i poslednje preživele vrste trilobita, kao i neke vrste ajkula i riba. Na kopnu, istrebljeni supored ostalih i mošopi, više metara dugi gmizavci biljojedi.

Naredno je usledilo „Trijasko-jursko izumiranje“ i nestanak 70 do 80 odsto vrsta, zbog brojnih i ne do kraja razjašnjenih razloga. Prema jednoj teoriji, okidač su bile masivne erupcije lave usled razbijanja superkontinenta Pangee, što je pratilo i oslobadjanje velikih količina ugljen-dioksida koje je dovelo do brzih klimatskih promena. Iako neki naučnici sumnjaju i na asteroide, do sada nije pronadjen nijedan krater.

Na prelazu iz perioda trijasa u juru nestale su brojne vrste velikih kopnenih životinja, medju kojima i najveći broj arhosaurusa, predaka dinosaurusa ali i današnjih ptica i krokodila. Istrebljen je i najveći deo vrsta džinovskih vodozemaca.

Za sada poslednje, i verovatno najpoznatije masovno izumiranje desilo se na kraju perioda krede, pre 66 miliona godina, po svoj prilici zaslugom asteroida koji je udario područje današnjeg meksičkog poluostrva Jukatan i ostavio ogroman krater. Izumrli su svi neleteći dinosaurusi, ali su na kopnu preživeli najveći broj vrsta sisara, kornjače, krokodili, žabe i ptice. U morima su opstale ajkule, morske zvezde i ježevi.

Nestanak dinosaurusa je omogućio širenje sisara, koje je na kraju dovelo do nastanka homosapijensa, vrste odgovorne za izvesno šesto masovno izumiranje.

U Parizu je trenutno u toku skup naučnika i diplomata iz više od 130 zemalja koji posle 15 godina rade globalnu procenu ekosistema.

Autor/Izvor Beta

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