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Dall'origine Della Vita Alla Comparsa Dell'uomo

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Dall'origine Della Vita Alla Comparsa Dell'uomo ALFONSO BOSELLINI - I materiali della Terra solida STORIA della TERRA Oltre il libro VI Alfonso Bosellini - Le Scienze della Terra I materiali della Terra solida Dall’origine della vita alla comparsa dell’uomo Il Precambriano L’origine della vita L’esperimento di Miller Dall’atmosfera riducente all’atmosfera ossidante I fossili più antichi Il Fanerozoico Eventi geologici del Paleozoico Eventi biologici del Paleozoico Documento Il Paleozoico in Italia Eventi geologici del Mesozoico Eventi biologici del Mesozoico Documento Il Mesozoico in Italia Documento Le estinzioni di massa Eventi geologici del Cenozoico Eventi biologici del Cenozoico Documento Il Cenozoico in Italia OLTRE IL LIBRO O-LTIRV E CILo LmIBeR sOi -coVsI trDuaislcl’oonroig lien ec adretlel ag evoitgar aaflilcah ce o-mI Ctopapryalrisogah tBd ©eol2lv’0u1o2 mIlteao lon- BCtoavpoy lreignhdtat i©Etdio2to0r1e e2 Italo Bovolenta Editore Copyright © 2012 Italo Bovolenta editore s.r.l. Ferrara Questo documento è tratto dal volume «Alfonso Bosellini - LA TERRA DINAMICA E STORIA GEOLOGICA DELL ’I TALIA , Italo Bovolenta editore, Ferra - ra, 2009» e è strettamente riservato ai docenti che adottano l’opera «Alfonso Bosellini - I MATERIALI DELLA TERRA SOLIDA , Italo Bovolenta editore, Ferrara, 2012». Disegni: Andrea Pizzirani Stesura degli esercizi: Gino Bianchi, Annarosa Baglioni, Anna Ravazzi. Copertina: – Progetto grafico: Chialab, Bologna – Realizzazione: Christian Magagni La riproduzione, la copia e la diffusione dell’intero documento, o di sue parti, è autorizzata ai soli fini dell’utilizzo nell’attività didat - tica degli studenti delle classi che hanno adottato il testo. Per segnalazioni o suggerimenti scrivere all’indirizzo: Italo Bovolenta editore Via Della Ginestra, 227/1 44020 Ferrara tel. 0532/259386 - fax 0532/259387 e-mail [email protected] sito web: www.bovolentaeditore.it VI Dall’origine della vita alla comparsa dell’uomo Se si osserva la successione delle ere geologiche, è facile notare che la loro durata diminuisce man mano che ci si avvicina alle epoche più recenti. Ciò è dovuto al fatto che la scansione delle ere è for - nita dalla traccia di eventi geologici o biologici particolarmente rilevanti. La leggibilità delle tracce la - sciate da questi eventi si affievolisce nel tempo: più si risale indietro, più gli eventi capaci di lasciare una traccia devono essere stati di grande rilievo e pertanto rari. DALLA PRIMA CROSTA ALL’ESPLOSIONE DELLA VITA VI.1 Il Precambriano scono l’eone Proterozoico (dal greco proteros = Nel Proterozoico, tra 750 primitivo e zoikos = di animale). e 580 Ma fa, si sono veri - l lunghissimo periodo di tempo che va dalla Quando si studiano le formazioni risalenti a una ficate almeno 4 glaciazio - ni. Vari dati geologici, I ge nesi della Terra fino all’esplosione della vi - determinata era, troviamo in profondità gli strati paleomagnetici e geochi - ta, avvenuta circa 545 milioni di anni fa, è chia - più antichi e in alto gli strati più recenti. Per que - mici indicano che tutto il mato informalmente Precambriano . Sebbene si sto motivo è invalso l’uso di definire «inferiore» la globo era ricoperto di tratti di un lasso di tempo superiore ai 4 miliardi parte più antica di un’era e «superiore» la parte ghiaccio. Da qui il nome di anni, quasi i nove decimi di tutta la storia del - più recente. La datazione delle rocce precambria - di Snowball Earth (Terra palla di neve) dato al no - la Terra, del Precambriano si sa poco, sia dal ne, povere o del tutto prive di fossili, è effettuata stro pianeta per quel pe - punto di vista geologico sia paleontologico. Il quasi esclusivamente con metodi radiometrici. Le riodo. Sulla «Terra palla Precambriano viene suddiviso in tre eoni: Adea - rocce precambriane sono in larga parte metamorfi - di neve» anche gli oceani no, Archeano e Proterozoico ( figura VI.1 ). L’eone che e granitiche. erano quasi completa - Adeano (dal greco hades = mondo infernale) mente gelati. La tempera - tura media globale era di comprende la parte più antica della storia della –50 °C e la temperatura Terra, dalla sua origine fino a 4÷3,8 miliardi di media all’equatore –20 anni fa. L’eone Archeano (dal greco archaios = °C. Pare che i viventi ab - antico) si fa iniziare con le rocce più antiche fi - Al periodo della storia della Terra che oggi chiamiamo Pre - cambriano furono dati i nomi, ora poco usati, di Azoico e biano superato questa nora documentate, che hanno età compresa tra drammatica fase climatica Criptozoico, che significano rispettivamente «privo di vita» grazie ai batteri rifugiati - circa 4 e 2,5 miliardi di anni. Tutte le rocce con e «con vita nascosta». si lungo le dorsali oceani - età tra 2,5 miliardi e 542 milioni di anni costitui - che dove sorgenti calde permettevano la vita. Eone Limiti Durata Eventi principali (milioni (milioni di anni) di anni) Fanerozoico 542 FIGURA VI .1 Divisione in Grandi e ripetute glaciazioni eoni e principali eventi geo - Proterozoico Comparsa delle cellule con nucleo logici e biologici del Pre - cambriano. 2.500 Sviluppo della fotosintesi Precambriano Archeano 4.058 Prime rocce sedimentarie 4.000 Le più antiche rocce conosciute Adeano Formazione degli oceani e dei continenti 4.600 3 OLTRE IL LIBRO - VI Dall’origine della vita alla comprasa dell’uomo - Copyright © 2012 Italo Bovolenta Editore ALFONSO BOSELLINI - I materiali della Terra solida Nella parte superiore, il Proterozoi co, si trovano O anche rocce sedimentarie, quali conglomerati, are - M FIGURA VI.2 narie, calcari con tracce di carboni fossili ricoperte Nello Scudo O baltico, costituito da rocce U in discordanza stratigrafica dalle rocce cambriane. ’ precambriane, affiorano i L Nel corso del Precambriano si verificarono di - resti di antichissime catene L verse orogenesi (solo in Africa ne so no state rico - montuose, le Svecocarelidi, E D nosciute sette) individuate da discordanze tra i ora profondamente erose e diversi terreni. I processi orogenetici furono ac - ridotte a modesti rilievi. A S compagnati da imponenti manifestazioni magma - R tiche e da intenso e diffuso metamorfismo. In Eu - A P ropa si formarono catene di montagne nella re - M gione russo-scandinava, cioè le Svecocarelidi e le O C Ebridi ( figura VI.2 ). In Nordamerica sono state individuate tre distinte orogenesi: Laurenziana, A L Algomanica e Huroniana. Tutte queste rocce pre - Basamento L di età sconosciuta A cambriane si trovano allo scoperto nel cuore dei Scudo Catena cratoni, cioè nelle parti più antiche e stabili dei baltico delle Svecocarelidi A T continenti. Rocce ignee I precambriane V successive all’orogenesi A delle Svecocarelidi L VI.2 L’origine della vita Rocce L post-Precambriano E D primi sistemi viventi comparvero sulla Terra prima di 3,5 miliardi di anni fa. A questa età ri - E I N salgono infatti i più antichi fossili conosciuti, resti a quelle dei viventi e il tutto sarebbe culminato I G di batteri non dissimili da quelli attuali e quindi nell’apparizione di una sostanza capace di gover - I probabilmente preceduti da altre forme ancora più nare la riproduzione e l’ereditarietà ( figura VI.4 ). R O semplici. È certamente nel Precambriano che si Secondo Oparin, questi processi si svolsero nel - ’ L originò la vita. Non sarà mai possibile documenta - l’oceano primordiale, un «brodo» di molecole or - L re direttamente quel l’evento, tuttavia possiamo ri - ganiche. La sequenza di eventi proposta da Oparin A costruire le condizioni ambientali in cui esso si prevedeva il verificarsi di fenomeni altamente im - D verificò. A partire da quelle della Terra primitiva, probabili dal punto di vista statistico. Tuttavia tali possiamo inoltre ipotizzare catene di fenomeni eventi ebbero modo di accadere ugualmente a cau - che potrebbero plausibilmente avere portato alla sa del lunghissimo lasso di tempo a disposizione e comparsa dei primi sistemi viventi e cercare prove per la grande estensione del «brodo» oceanico. FIGURA VI.3 In ambiente indirette di questi fenomeni. Attualmente le conclusioni di Oparin appaiono acquoso le proteine formano Nel 1924 un giovane geochimico russo, Alek - un po’ affrettate e devono essere riviste. Oggi, ad spontaneamente piccole sfe - sandr Ivanovic Oparin, suppose che la primordia - esempio, si attribuisce molta importanza alla for - re, le cui dimensioni sono le atmosfera terrestre mancasse di ossigeno e con - mazione di una membrana capace di separare un dello stesso ordine di gran - dezza delle più piccole cellu - tenesse molti gas riducenti quali ammoniaca, N H3, ambiente interno da quello esterno. Le membrane le procariote. idrogeno, H2, e metano, C H 4. Egli riteneva che, in biologiche sono costituite da fosfolipidi e si sa che una simile atmosfera, l’energia fornita dai raggi ul - queste molecole formano facilmente ag - travioletti o dai fulmini avesse reso possibili rea - gregati di forma laminare, con molecole zioni chimiche che avrebbero prodotto molecole disposte su doppi strati, quando sono organiche simili agli amminoacidi, i «mattoni» immerse in acqua. Si può pensare che delle proteine. nel brodo primordiale fossero presenti i Una volta formatisi, gli amminoacidi si sarebbe - fosfolipidi e che queste molecole aves - ro uniti con legami peptidici e avrebbero formato sero formato superfici continue e tal - catene polipeptidiche e proteine. Oparin aveva ve - volta sferiche. Le sfere avrebbero sepa - rificato la tendenza delle grosse molecole protei - rato un ambiente circoscritto che sareb - che a formare involucri sferoidali circondati da ac - be diventato l’interno della cellula. qua ( figura VI.3 ), che egli chiamò coacervati . I Oggi si tende sempre meno a consi - coacervati, a loro volta, avrebbero formato aggre - derare la comparsa della vita frutto di gati sempre più complessi. Alcuni aggregati avreb - una rara sequenza di eventi improbabi - bero iniziato a mostrare la proprietà di mantenersi li. Si ritiene piuttosto che, in presenza stabili e si sarebbero quindi conservati per tempi di adeguati flussi di energia, la tenden - relativamente lunghi. Tra questi aggregati alcuni za di alcuni componenti della materia a avrebbero finito con l’acquisire la capacità di ri - organizzarsi in strutture ordinate sia un prodursi e avrebbero dato origine alle più semplici fenomeno frequente.
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