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Emir Pasanovic
stripovi.com suradnik
Bosnia and Herzegovina
6221 Posts
Member since 10/06/2002 |
 Posted - 27/07/2010 : 10:46:40
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quote:
Originally posted by anto
Nauka i religija se savršeno slažu, ali teorija evolucija i hrišcanstvo baš i ne
Koje ti aspekte evolucije vidiš da se ne slažu sa kršcanstvom? Imaj na umu da su tu teoriju postavili i razvili kršcani i da ti nisi autoritet za to šta kršcanstvo kao cjelina podrazumjeva za svakog kršcanina.
S druge strane, religija i nauka se slažu kad im to odgovara, ali otpocetka su ljuti neprijatelji jer religija mrzi promjene, a nauka živi na njima. |
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kitaric
Average Member
Serbia
975 Posts
Member since 20/11/2004 |
Posted - 27/07/2010 : 11:09:35
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quote:
Originally posted by Emir Pasanovic
quote:
Originally posted by anto
Samo jedno pitanje pre nego što zapocnemo diskusiju - veruješ li ti u Boga ?
Ako ne veruješ, bolje da ovde završimo pricu.
Pa tu i nema neke diskusije - parafrazirajuci Jossa Whedona, religiozni ljudi vjeruju u nešto svim dokazima za suprotno usprkos, dok ljudi razuma, naucnici, traže dokaze za nešto prije nego što pristanu da vjeruju da je to istina.
Ne veruj zato što je to mudrac rekao,
Ne veruj zato što je to uobicajeno mišljenje,
Ne veruj zato što je to zapisano,
Ne veruj zato što je to proreceno,
Ne veruj zato što neko drugi to veruje,
Ali veruj onome što si sam zakljucio da je istina.
Buda |
You tell anyone about this and I'll fucking kill you. I'm kidding, I'm kidding, we'll have him home by midnight. |
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Emir Pasanovic
stripovi.com suradnik
Bosnia and Herzegovina
6221 Posts
Member since 10/06/2002 |
Posted - 27/07/2010 : 11:15:12
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Eh, ali malo koji vjerski autoritet ovdje i šire može citirati i prihvata rijeci Budine. To je ja mislim gore nego da citiraju nekog naucnika.  |
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anto
Advanced Member
Serbia
21081 Posts
Member since 29/06/2008 |
Posted - 27/07/2010 : 13:08:21
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quote:
Originally posted by Emir Pasanovic
quote:
Originally posted by anto
Nauka i religija se savršeno slažu, ali teorija evolucija i hrišcanstvo baš i ne
Koje ti aspekte evolucije vidiš da se ne slažu sa kršcanstvom? Imaj na umu da su tu teoriju postavili i razvili kršcani i da ti nisi autoritet za to šta kršcanstvo kao cjelina podrazumjeva za svakog kršcanina.
S druge strane, religija i nauka se slažu kad im to odgovara, ali otpocetka su ljuti neprijatelji jer religija mrzi promjene, a nauka živi na njima.
Stvar je krajnje prosta.
Kao prvo, teorija evolucije negira postojanje Boga, govori da je život nastao iz neživota,samim tim živi svet nije nastao Božjom voljom vec pukom koincidencijom.
Teorijom evolucije Darvin i ljudi koji su progurali ovu stvar su pljunuli na hrišcanstvo, tako da Emire ne pricaj kako su ovu teoriju razvili hrišcani.
Što se tice odnosa nauke i religije, da ne dubim mnogo, navešcu samo primer Nikole Tesle, iskrenog vernika i coveka koji je promenio svet svojim izumima - izumima koji su pomogli ljudskoj vrsti.
A primer neslaganja, je normalno neslaganje sa Bogohulnom teorijom evolucije.
Ovo je moj zadnji post u ovom topiku, jer stvarno je glupo da bilo ko od nas gubi vreme na diskusije koje nikome nec ništa doneti osim price u krug i ukrug.Svako neka veruje u šta hoce.
Živi bili! |
Dajte nam Ralph Azhama;može i mekokoricen (ali u boji obavezno)! |
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Mali_Mate
stripovi.com suradnik
Croatia
2065 Posts
Member since 18/03/2006 |
Posted - 27/07/2010 : 13:39:17
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quote:
Stvar je krajnje prosta.
Kao prvo, teorija evolucije negira postojanje Boga, govori da je život nastao iz neživota,samim tim živi svet nije nastao Božjom voljom vec pukom koincidencijom.
pa i ne bas...naime evolucija objašnjava mehanizam nastanka kako ti to zoves nezivota u zivot. O prvoj slucajnosti koja je bila uzrok svega evolucija nema sta da kaze. Sto se nje tice, ta prva slucajnost mogao je biti i bog. Evolucija doduse negira autoritet Biblije, Kur'ana, Tore, Veda i kojecega...al to je vec nesto drugo. Nije njih samo evolucija dovela u pitanje.
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http://matanovogumno.blogspot.com |
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Mr. Bushido
stripovi.com suradnik
Croatia
12917 Posts
Member since 23/09/2005 |
Posted - 27/07/2010 : 14:43:48
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| Na stranu prica o tome je li teorija evolucije stvarna i dokaziva ili nije, mene više zanima zašto ljudi vjeruju u postojanje boga, ili opcenito nekog višeg bica ili sile? |
Comics is any art you can read. -- Sean T. Collins |
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Caleb
Advanced Member
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Greece
16386 Posts
Member since 01/07/2008 |
Posted - 27/07/2010 : 14:45:26
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quote:
Originally posted by Mr. Bushido
Na stranu prica o tome je li teorija evolucije stvarna i dokaziva ili nije, mene više zanima zašto ljudi vjeruju u postojanje boga, ili opcenito nekog višeg bica ili sile?
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Preveo: Goran Krickovic, Tekst interpretirali: Ljubiša Bacic, Nada Blam, Miroslav Bijelic, Vlastimir Ðuza Stoiljkovic i Nikola Simic, Magnetoskop: Miroslav Nikolov, Ton majstor: Rihard Merc, Realizacija: Slavko Tatic
https://www.youtube.com/watch?v=TkE5VYffiR0
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Mali_Mate
stripovi.com suradnik
Croatia
2065 Posts
Member since 18/03/2006 |
Posted - 27/07/2010 : 15:49:15
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quote:
Originally posted by Mr. Bushido
Na stranu prica o tome je li teorija evolucije stvarna i dokaziva ili nije, mene više zanima zašto ljudi vjeruju u postojanje boga, ili opcenito nekog višeg bica ili sile?
linija manjeg otpora?
stajaznam, ima milijon mogucih odgovora.... |
http://matanovogumno.blogspot.com |
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lwood
Advanced Member
Colombia
47132 Posts
Member since 09/12/2005 |
Posted - 27/07/2010 : 15:55:07
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evo jedne zanimljive vjesti glede ovih "neslaganja".
Sveta Stolica: Darwinova teorija evolucije je u skladu s Biblijom
SVETA Stolica priznala je da je "teorija evolucije u skladu s Biblijom", ali iz Vatikana su porucili da se ne namjeravaju posthumno ispricati njezinom tvorcu Charlesu Darwinu. Osim toga, vatikanski ministar kulture Gianfranco Ravasi je najavio organiziranje konferencije na kojoj ce se okupiti znanstvenici, teolozi i filozofi povodom 150 godišnjice objavljivanja Darwinove knjige "O porijeklu vrsta", kojom je položio temelje evolucionisticke teorije.
Kršcanske crkve dugo su vremena bile u sukobu s Darwinovima ucenjem zato što njegove razmišljanja proturjece kreacionizmu - doslovnom citanju biblijskog opisa postanka prema kojem je Bog tvorac svijeta.
Anglikanci se ispricali Darwinu na 126. godišnjicu njegove smrti
Podsjecamo, Anglikanska crkva se prije tri dana, tocno 126 godina nakon Darwinove smrti, službeno ispricala ocu teorije o evoluciji, nakon cega se cijela britanska javnost samo grohotom nasmijala, dok je Darwinov praunuk ispriku nazvao besmislicom.
"Charlese Darwinu, 200 godina nakon tvoga rodenja, Engleska crkva duguje ti ispriku jer te pogrešno shvatila te stoga što je naša prva reakcija bila pogrešna, podupiruci druge da te još uvijek pogrešno shvacaju", naveli su anglikanci na svojim internetskim stranicama. Nastavili su da je potrebno ispraviti pogreške predaka, koji su kritizirali Darwina 1860-tih zato što je iznio teoriju da se covjek razvio od majmuna.
Nadalje, još je 1950. papa Pio XII ocijenio da je evolucija valjani znanstveni pristup nastavku ljudi, a to je ponovio i papa Ivan Pavao II 1996. Ravanasi je objasnio da se Katolicka crkva ne namjerava ispricavati jer Darwinovu teoriju nikad nije osudila, niti je njegovu knjigu zabranila.
Najveci protivnici Darwina još uvijek su americki konzervativni protestanti
S novim katolickim mišljenjem ne slažu se americki konzervativni protestanti, koji Knjigu postanka shvacaju doslovce, pa se i protive nastavi biologije koja "propovijeda" Darwinovo ucenje. Katolicka crkva, s druge strane, biblijski postanak, prema kojem je Bog stvorio zemlju za šest dana i koje stoji u osnovi kreacionizma, doživljava kao alegoriju o nacinu na koji je Bog stvorio svijet.
Zalaže se za "teisticku evoluciju" po kojem se evolucija prihvaca kao znanstvena teorija i prema kojoj ne postoji razlog zašto Bog nije mogao iskoristiti evolucijske procese u stvaranju ljudske vrste. Na ovom tragu je i govor pape Benedikta XVI kojeg je održao u Parizu prošlog tjedna |
depresivni iskompleksirani primitivac i nadrkana budala kojeg financira stari i koji sa skoro 50 godina nema ni žene ni posla.Pa naravno da je ljut na sve, a narocito na one koji su uspješni.
Jbga Lwoode nisam ti ja kriv kaj nisi uspio u životu |
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lwood
Advanced Member
Colombia
47132 Posts
Member since 09/12/2005 |
Posted - 27/07/2010 : 15:58:01
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ne znan kakvo je stajalište pravoslavnih crkvi glede mede ,ali cisto sumnjan da je fundamentalisticko.
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depresivni iskompleksirani primitivac i nadrkana budala kojeg financira stari i koji sa skoro 50 godina nema ni žene ni posla.Pa naravno da je ljut na sve, a narocito na one koji su uspješni.
Jbga Lwoode nisam ti ja kriv kaj nisi uspio u životu |
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lwood
Advanced Member
Colombia
47132 Posts
Member since 09/12/2005 |
Posted - 27/07/2010 : 15:59:46
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ovaj dokument objavljen 1996. ima san priliku procitat ,ža mi je da ga ne mogu nac po netu.
al zanimljivo koliko se strukture brže prilagodavaju od "obicnih laika".
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depresivni iskompleksirani primitivac i nadrkana budala kojeg financira stari i koji sa skoro 50 godina nema ni žene ni posla.Pa naravno da je ljut na sve, a narocito na one koji su uspješni.
Jbga Lwoode nisam ti ja kriv kaj nisi uspio u životu |
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Emir Pasanovic
stripovi.com suradnik
Bosnia and Herzegovina
6221 Posts
Member since 10/06/2002 |
Posted - 27/07/2010 : 16:46:46
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quote:
Originally posted by anto
Stvar je krajnje prosta.
Kao prvo, teorija evolucije negira postojanje Boga, govori da je život nastao iz neživota,samim tim živi svet nije nastao Božjom voljom vec pukom koincidencijom.
Kao što Mali Mate rece, nije tacno. U principu, meni i dalje nije jasno koji je tvoj problem sa 'neživotom' i šta to tacno znaci. Šta je živo? Jesu li spermiji živi kao npr. žirafa prije nego što oplode jajnu celiju? (Nisu, jer se ne razmnožavaju kao živa bica nego nastaju u testisima sa jednom jedinom zadacom, da unesu svoj genetski materijal u jaje. Jednom kada neuspiju u tome, 'umiru'.) Kakav tacno život zamišljaš u svom bogu? Je li stvarno ideš do te mjere u antropomorfikaciju da zamišljaš da je neko covjekoliko bice uzelo gline i izvajalo prvog covjeka i udahnulo mu život sa svim svojim prednostima i nedostacima? Zašto onda i druge životinje imaju te prednosti i nedostatke, neke na gotovo istovjetan nacin?
quote:
Teorijom evolucije Darvin i ljudi koji su progurali ovu stvar su pljunuli na hrišcanstvo, tako da Emire ne pricaj kako su ovu teoriju razvili hrišcani.
Darwin je 20 godina pisao svoju teoriju jer se mucio. Ne sa dokazivanjem, jednom kad je uocio uzorak, vidio je dokaze evolucije na svakom koraku. Ne, njegov otac je bio jako religiozan, a i Charles je ucio na teologiji prije nego je krenuo u svijet da istražuje nova kopna jer je tada geologija i biologija bila pomodna novotarija kojom su se bavili mnogi sinovi dobrostojecih porodica, pa je tako i on prekinuo studij (a prije je izbacen sa studija medicine koji je pohadao paralelno s teologijom, no to nije toliko bitno).
Dakle, nemoj ti pricati o vjeroispovijesti ljudi i njihovim uvjerenjima prije nego što provjeriš cinjenice.
quote:
Što se tice odnosa nauke i religije, da ne dubim mnogo, navešcu samo primer Nikole Tesle, iskrenog vernika i coveka koji je promenio svet svojim izumima - izumima koji su pomogli ljudskoj vrsti.
A primer neslaganja, je normalno neslaganje sa Bogohulnom teorijom evolucije.
Jesi li pogledao kakav je bio stav Tesle o evoluciji vrsta? Baš bi me zanimalo vidjeti ako je išta rekao o tome, jer je
Pokaži mi jednog vjernika koji se ne slaže sa teorijom evolucijom i ja cu ti parirati sa jednim koji se slaže. Zašto imaš potrebu da generaliziraš stvari o kojima ne znaš ili znaš jako malo? Znam da si pravoslavac, ali cisto poredenja radi, Vatikan ima odjeljenja koja su posvecena pracenju naucnih dostignuca. A to znam jer sam gledao intervju jednom sa visoko pozicioniranim kardinalom koji je jasno u intervjuu dao do znanja da naravno da vjeruje u evoluciju vrsta.
quote:
Ovo je moj zadnji post u ovom topiku, jer stvarno je glupo da bilo ko od nas gubi vreme na diskusije koje nikome nec ništa doneti osim price u krug i ukrug.Svako neka veruje u šta hoce.
Živi bili!
Ti si prvi poceo. Ali evo, nakon mog posta makar znaš da je Darwin bio do kraja života iskreni kršcanin, i opet nije mogao ostati slijep pred naucnim dokazima koji su ga boli u oko. Naucnih dokaza, primjetit cu, od kojih ti nijedan nisi spomenuo niti pokušao oboriti, koje ti kategoricki želiš da nikako ne postoje, što bi bilo maltene kao da se sve što je ikada uradeno zadnjih 2000 godina nije dogodilo. Što je jednostavno glupo. |
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acestroke
stripovi.com suradnik
USA
11652 Posts
Member since 16/04/2009 |
Posted - 27/07/2010 : 16:47:11
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quote:
Originally posted by Mr. Bushido
Na stranu prica o tome je li teorija evolucije stvarna i dokaziva ili nije, mene više zanima zašto ljudi vjeruju u postojanje boga, ili opcenito nekog višeg bica ili sile?
Treba im neko ko ce im redit kako da zive, sta da rade, kako da se ponasaju, sta smiju da rade a sta ne, i onda da ih 'kazni' kad grijese
'Najbolja' (i najsmijesnija) stvar koju sam ikad vidio je ovdje u Americi, u drzavi Kentucky. Ima jedan ogroman muzej, koji je pun veliki lutki dinosaurusa i ljudi, i sve te izlozbe koje imaju u muzeju pokazuju da su covjek i dinosaurusi zivjeli zajedno. Imaju puno primjerka gdje covjek jase dinosarusua, sa sedlom, i slicnih stvari, i tako to.
Ti konzervativci sve to uce svoju djecu, i tvrde drugim ljudima da je sve to stvarno bilo ovako, i da je citava planeta samo par hiljada godina stara. To oni vjeruju, i ponavljam, to uce svoju djecu. Nevjerovatno.
Ako ikad viditie onaj Bill Maherov film "Religiolous", sve to mozete vidjeti. Nije los film, dokumentarac o razlicitim vjerama sirom svijeta, u neku ruku kao komedija. |
http://www.acestroke.blogspot.com |
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supermark
stripovi.com suradnik
(1).jpg)
Croatia
29618 Posts
Member since 06/02/2007 |
Posted - 27/07/2010 : 17:34:54
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quote:
Originally posted by anto
Stvar je krajnje prosta.
Kao prvo, teorija evolucije negira postojanje Boga, govori da je život nastao iz neživota,samim tim živi svet nije nastao Božjom voljom vec pukom koincidencijom.
uopce ne negira 
pa jesi ti rocitao moj post par postova iznad?!  |
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Mali_Mate
stripovi.com suradnik
Croatia
2065 Posts
Member since 18/03/2006 |
Posted - 27/07/2010 : 19:00:39
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quote:
Ti konzervativci sve to uce svoju djecu, i tvrde drugim ljudima da je sve to stvarno bilo ovako, i da je citava planeta samo par hiljada godina stara. To oni vjeruju, i ponavljam, to uce svoju djecu. Nevjerovatno.
Nastranu apsurd svega toga, al stvarno nema neke velike stete od toga da to uce svoju djecu. Nije da je ljudski rod ista sretniji zbog toga što zna da se životinje prilagodavaju svojoj okolini. Jebiga, kad pomislis da upravo zbog evolucije znamo da ljudski mozak skoro da postoji u drugom time-lineu, tj. evolucijski je prilagodjen zivotu kakav je izgledao nekoliko milenija unazad dodje ti da se smrznes...Umjesto Boga svemogucega dobili smo socijalni darwinizam sta je jos gore jer je taj, kao, naucno dokazan...dok ti klinci ne pocnu palit sve oko sebe i bacat bombe da bi druge uvjerili u pricu s dinosaurima sve je dobro... |
http://matanovogumno.blogspot.com |
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dsormaz1
Advanced Member
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13147 Posts
Member since 28/04/2002 |
Posted - 27/07/2010 : 20:07:57
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quote:
Originally posted by Emir Pasanovic
quote:
Originally posted by dsormaz1
Sigurno lijepo i uvjerljivo izgleda ta konstrukcija.
Konstrukcija zasnovana na naucnim dokazima i istraživanjima, za razliku od filozofskih konstrukcija o postanku svijeta i vrsta sa malo ili nimalo utemeljenja u stvarnosti.
Ostavimo religiju malo po strani. To je pitanje vjere, dakle netko vjeruje a netko ne. Vjerniku ne trebaju nikakvi dokazi koji bi potvrdili njegovu vjeru. On jednostavno vjeruje.
Ali ovdje se razglaba o teoriji koja nikad nije znanstveno dokazana, dakle bila je i ostala samo teorija koja se namece cijelom svijetu kao neprikosnovena istina. Dakle kad govorimo o znanosti, znanstvenim istraživanjima i cinjenicama, onda bi trebalo podastrijeti neke cvrste okaze za neku teoriju, jer u suprotnom ona ostaje samo teorija i ništa više od toga. A ovdje ipak nedostaje jedna (ili više) karika. |
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Elderane84
Advanced Member
Bosnia and Herzegovina
3247 Posts
Member since 30/10/2008 |
Posted - 27/07/2010 : 20:30:42
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| Odakle tebi to da nije dokazana? Dokazi su pruzeni, pitanje je samo ko je u stanju da ih vidi. I ja se slazem da vjeru ne treba mijesati, za vjeru dokazi nisu potrebni. Za naucne teorije koje objasnjavaju kako svijet funkcionise dokazi su potrebni, i po danasnjim saznanjima, tvrditi da je zemlja nastala prije 6000 godina nije smijesno, vec tuzno. Sramota je koliko ljudi koji se ne bave direktno naukom nemaju pojma koliko je stvari do danas otkriveno. |
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acestroke
stripovi.com suradnik
USA
11652 Posts
Member since 16/04/2009 |
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Elderane84
Advanced Member
Bosnia and Herzegovina
3247 Posts
Member since 30/10/2008 |
Posted - 27/07/2010 : 21:52:37
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Pa, sta ces kad jeste :( Jednostavno, ljudi vrse hiljade eksperimenata, dovijaju se na milion nacina da shvate kako nesto funkcionise, zasto Sunce sija, zasto smo mi ovdje gdje jesmo, kakva nam je neposredna okolina, kakva je nasa galaksija, kakve su ostale galaksije, kako smo nastali, ima li zivota van Zemlje ili, iduci u drugu krajnost, dokle mozemo dijeliti materiju, a da dalje ne moze, sta su "prave elementarne" cestice. A Teorija Evolucije je samo jedan djelic cjelokupnog mozaika, koji je danas prilicno dobro slozen. Danas se otkrivaju samo detalji, a gruba slika je napravljena i trenutno se samo nadogradjuje novim i novim saznanjima. Vjeru stvarno ne bih mjesao u svu pricu, posto bi osnovno pitanje bilo vjerujemo li zaista da je sve to sto se desavalo i danas se desava samo dio bozanskog plana, ili se desilo slucajno. To je vec sasvim drugacije pitanje od prvobitno postavljenog, "Je li Zemlja ravna ploca", pa do ovog aktuelnog pitanja o tome je li "Teorija Evolucije" tacna.
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Lord Vader89
stripovi.com suradnik
France
9616 Posts
Member since 05/07/2007 |
Posted - 27/07/2010 : 22:21:07
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Mene zanima anto, ko si ti, ili ja ili bilo ko, da kazemo ko je i sta je Bog
I ako toliko slepo ne verujes u evoluciju, a kunes se u Teslu, trebao bi malo da se preispitas, jer evolucija nije dokazana (ili potvrdjena, ne moze se nista definitivno dokazati nikad) samo filozofiranjem i pricama, nego primenjivanjem bezbroj drugih nauka i tehnologija koje ti i ja i svi svakodnevno koristimo i bezuslovno prihvatamo njihove ucinke.
I reci mi, sta kad se nadje jedan anto negde u Egiptu ili Iraku i vas dvojica se lepo nadjete na kafi. Kako biste se vi dogovorili koji od vas je pravi potomak onog prvog coveka kojeg je Bog stvorio Posto ne mozete obojica biti u pravu
Sta to znaci, jedan ce u pakao, drugi nece
aj nije ni bitno....izbacice me zbog gluposti.... |
Potpis u dva reda, prema pravilniku:
Vader je, kao i uvek, u pravu. ; john connor ; 2022 A.D. |
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morrison
Advanced Member
Serbia
5142 Posts
Member since 29/01/2008 |
Posted - 27/07/2010 : 22:47:52
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@elderane84
Vidis kako je topik lepo evoluirao :)
Ja sam vernik, ali nisam fundamentalista. Prihvatam evoluciju, i ta cinjenica nikako ne utice na moje poimanje religije, niti je dovodi u sumnju. Iskreno ne vidim potrebu za daljom konfrontacijom darvinizma i pojedinih hriscanskih konfesija, koji cesto svojom tvrdoglavoscu na zao glas dovode sve verujuce ljude, prikazujuci nas kao nepokolebljive fanatike koji se protive i golim cinjenicama.
S druge strane, razgovori izmedju vernika i ateista se i mogu odvijati na cisto filozofskom nivou, jer empirijski dokazi o postojanju ili nepostojanju Boga ne postoje. Nema matematicke formule ili opita koji to moze dokazati ili opovrgnuti. |
There ain't no grave gonna hold my body down. |
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lwood
Advanced Member
Colombia
47132 Posts
Member since 09/12/2005 |
Posted - 27/07/2010 : 23:13:26
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Scientific understanding requires both facts and theories that can explain those facts in a coherent manner. Evolution, in this context, is both a fact and a theory. It is an incontrovertible fact that organisms have changed, or evolved, during the history of life on Earth. And biologists have identified and investigated mechanisms that can explain the major patterns of change.
Patterns in Nature
The field of evolutionary biology seeks to provide explanations for four conspicuous patterns that are manifest in nature. The first three concern living species, whereas the fourth relates to fossils.
Genetic variation. There is tremendous genetic diversity within almost all species, including humans. No two individuals have the same DNA sequence, with the exception of identical twins or clones. This genetic variation contributes to phenotypic variation — that is, diversity in the outward appearance and behavior of individuals of the same species.
•Adaptation. Living organisms have morphological, biochemical, and behavioral features that make them well adapted for life in the environments in which they are usually found. For example, consider the hollow bones and feathers of birds that enable them to fly, or the cryptic coloration that allows many organisms to hide from their predators. These features may give the superficial appearance that organisms were designed by a creator (or engineer) to live in a particular environment. Evolutionary biology has demonstrated that adaptations arise through selection acting on genetic variation.
Divergence. All living species differ from one another. In some cases, these differences are subtle, while in other cases the differences are dramatic. Carl Linnaeus (1707-1778) proposed a classification that is still used today with slight changes. In the modern scheme, similar species are grouped into genera, similar genera into families, and so on. This hierarchical pattern of relationship produces a tree-like pattern, which implies a process of splitting and divergence from a common ancestor.
Fossil species . Fossils are the mineralized remnants or impressions of once-living organisms. Many fossils, such as trilobites and dinosaurs, belong to groups that no longer exist on the face of the Earth. Conversely, many modern species appear similar to other fossils, yet fossils of the modern species are absent from rocks of corresponding ages. The age of the Earth is estimated to be about 4.5 billion years, with the earliest bacterial fossils about 3.5 billion years old. Fossils from around 550 million years ago (the Cambrian period) show a diverse assemblage of multicellular animals.
Evolutionary biology provides a scientific framework for understanding the changes that have occurred since the first life forms arose on Earth several billion years ago. Biochemists, geologists, and physicists seek natural explanations for the origin of life on Earth. While progress has been made in this area, the origin of life remains an interesting, but unanswered, question.
Mechanisms of Evolution
Biological evolution results from changes over time in the genetic constitution of species. Genetic changes often, but not always, produce noticeable changes in the appearance or behavior of organisms. Evolution requires both the production of variation and the spread of some variants that replace others.
•Genetic variation arises through two processes, mutation and recombination. Mutation occurs when DNA is imperfectly copied during replication, leading to a difference between a parent’s gene and that of its offspring. Some mutations affect only one bit in the DNA; others produce rearrangements of large blocks of DNA.
•Recombination occurs when genes from two parents are shuffled to produce an offspring, as happens regularly in sexual reproduction. Usually the two parents belong to the same species, but sometimes (especially in bacteria) genes move between more distantly related organisms.
•The fate of any particular genetic variant depends on two processes, drift and selection. Drift refers to random fluctuations in gene frequency, and its effects are usually seen at the level of DNA. Ten flips of a coin do not always produce exactly five heads and five tails; drift refers to the same statistical issue applied to the transmission of genetic variants across generations.
•The principle of natural selection was discovered by Charles Darwin (1809-1882), and it is the process by which organisms become adapted to their environments. Selection occurs when some individual organisms have genes that encode physical or behavioral features that allow them to better harvest resources, avoid predators, and such relative to other individuals that do not carry the same genes. The individuals that have these useful features will tend to leave more offspring than other individuals, so the responsible genes will become more common over time, leading the population as a whole to become better adapted.
•The process that many people find most confusing about evolution is speciation, which is not a separate mechanism at all, but rather a consequence of the preceding mechanisms played out in time and space. Speciation occurs when a population changes sufficiently over time that it becomes convenient to refer to the early and late forms by different names. Speciation also occurs when one population splits into two distinct forms that can no longer interbreed. Reproductive isolation does not generally happen in one generation; it may require many thousands of generations when, for example, one part of a population becomes geographically separated from the rest and adapts to a new environment. Given time, it is inevitable that two populations that live apart will diverge by mutation, drift, and selection until eventually their genes are no longer compatible for successful reproduction
Evidence for Evolution, and its Significance in our Lives
It is impossible to review all the evidence for evolution in a short article such as this. However, the following offers a sample of the kinds of evidence that have been discovered and confirmed repeatedly by scientists. These examples also illustrate the importance of this evidence for science and society more generally.
Evidence from fossils. Based on myriad similarities and differences between living species, evolutionary biology makes predictions about the features of ancestral forms. For example, numerous features indicate that birds are derived from reptilian ancestors. By contrast, these data reject the possibility that birds were derived from other groups, such as flying insects. Scientists have discovered fossil birds with feathers and legs like modern birds, but which also have teeth, clawed digits on their forelimbs, and a tailbone like their reptilian ancestors. Fossils are especially important evidence for evolution because, with little effort, each of us can use our eyes and minds to observe and interpret the dinosaur and other ancient fossils in public museums.
Evidence from genetics. The genomes of all organisms contain overwhelming evidence for evolution. All living species share the same basic mechanism of heredity using DNA (or RNA in some viruses) to encode genes that are passed from parent to offspring, and which are transcribed and translated into proteins during each organism’s life. Using DNA sequences, biologists quantify the genetic similarities and differences among species, in order to determine which species are more closely related to one another and which are more distantly related. In doing so, biologists use essentially the same evidence and logic used to determine paternity in lawsuits. The pattern of genetic relatedness between all species indicates a branching tree that implies divergence from a common ancestor. Within this tree of life, there are also occasional reticulations where two branches fuse, rather than separate. (For example, mitochondria are organelles found in the cells of plants and animals. Mitochondria have their own genes, which are more similar to genes in bacteria than to genes on the chromosomes in the cell nucleus. Thus, one of our distant ancestors arose from a symbiosis of two different cell types.) The genetic similarity between species, which exists by virtue of evolution from the same ancestral form, is an essential fact that underlies biomedical research. This similarity allows us to begin to understand the effects of our own genes by conducting research on genes from other species. For example, genes that control the process of DNA repair in bacteria, flies, and mice have been discovered to influence certain cancers in humans. These findings also suggest strategies for intervention that can be explored in other species before testing on humans.
Evolution in action. Evolutionary change continues to this day, and it will proceed so long as life itself exists. In recent years, many bacterial pathogens have evolved resistance to antibiotics used to cure infections, thereby requiring the development of new and more costly treatments. In some frightening cases, bacteria have evolved resistance to every available antibiotic, so there is no longer any effective treatment. In the case of HIV, which causes AIDS, significant viral evolution occurs within the course of infection of a single patient, and this rapid evolution enables the virus to evade the immune system. Many agricultural pests have evolved resistance to chemicals that farmers have used for only a few decades. As we work to control diseases and pests, the responsible organisms have been evolving to escape our controls. Moreover, scientists can perform experiments to study evolution in real time, just as experiments are used to observe dynamic processes in physics, chemistry, and other branches of biology. To study evolution in action, scientists use organisms like bacteria and fruitflies that reproduce quickly, so they can see changes that require many generations.
Conclusions
Evolutionary biology is a strong and vigorous field of science. A theoretical framework that encompasses several basic mechanisms is consistent with the patterns seen in nature; and there is abundant evidence demonstrating the action of these mechanisms as well as their contributions to nature. Hence, evolution is both a theory and a set of established facts that the theory explains.
Like every other science, there is scientific debate about some aspects of evolution, but none of these debates appear likely to shake the foundations of this field. There exists no other scientific explanation that can account for all the patterns in nature, only non-scientific explanations that require a miraculous force, like a creator. Such super-natural explanations lie outside of science, which can neither prove nor disprove miracles. Science provides us with a compelling account and explanation of the changing life on Earth. It should also remind us of our good fortune to have come into being and our great responsibility to ensure the continuity of life.
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depresivni iskompleksirani primitivac i nadrkana budala kojeg financira stari i koji sa skoro 50 godina nema ni žene ni posla.Pa naravno da je ljut na sve, a narocito na one koji su uspješni.
Jbga Lwoode nisam ti ja kriv kaj nisi uspio u životu |
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Lord Vader89
stripovi.com suradnik
France
9616 Posts
Member since 05/07/2007 |
Posted - 27/07/2010 : 23:14:15
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aj brate izrazavaj se ko covek, nemoj kopirati romane ovde vise  |
Potpis u dva reda, prema pravilniku:
Vader je, kao i uvek, u pravu. ; john connor ; 2022 A.D. |
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lwood
Advanced Member
Colombia
47132 Posts
Member since 09/12/2005 |
Posted - 27/07/2010 : 23:18:09
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1. Introducing LUCA
LUCA is short for Last Universal Common Ancestor, and it is from this organism that every living cell on the planet has descended. LUCA does not represent the earliest stage in the evolution of life — it is widely accepted that before the evolution of proteins and DNA (which are common to all cellular life) there was a period where RNA carried out the roles now performed by proteins and DNA [Jeffares & Poole 2000]. There are a lot of uncertainties when we go this far back in evolutionary time and perhaps all we can be certain of is that, at a point in Earth’s history (probably over 3 billion years ago), cells emerged which stored recipes for making both proteins and RNA on a third molecule, DNA.
Nevertheless, studying LUCA is not science fiction. In the same way as humans and chimpanzees shared a common history until less than 10 million years ago, all modern lifeforms shared a common history back as far as the split that gave rise to the three ‘domains’ of life we now know of as archaea, bacteria and eukaryotes; that is, back as far as LUCA. That there are three domains was first established by Carl Woese and colleagues, who found that the group called prokaryotes was actually two groups, the archaea and bacteria [Woese & Fox 1977; Woese et al. 1990]. Amazingly, this work has largely stood the test of time, and though it is argued that there has been extensive gene swapping between these two groups [Pennisi 1998, 1999; Doolittle 1999; Eisen 2000], recent analyses using complete genomes supports Woese and colleagues’ decision to split prokaryotes into archaea and bacteria [Snel et al. 1999; Sicheritz-Pontén & Andersson 2001; Brown et al. 2001].
Woese and colleagues’ discovery rested on an even more astounding one — all life stores its genetic information on DNA, using a common code which we call the genetic code. The information is stored as packets, called genes — recipes for making RNA, and proteins [see Appendix A: Making Protein]. The languages of DNA and RNA are so similar they may as well be called dialects, but both are markedly different from the language of protein. For RNA and DNA, the information-carrying part of both molecules is made up of four bases (analogous to letters in an alphabet) read in a linear fashion, as with written human languages. In RNA, the four bases are A, G, C and U. In DNA, A, G and C are also used, while T is used instead of U. Establishing the evolutionary basis for this change from U to T is not a trivial exercise, and is an interesting problem in itself [Poole et al. 2001]; but in terms of the actual language, the difference is as minor as the variant spelling of English words, e.g., civilisation and civilization.
The unearthing of the genetic code, and the subsequent demonstration that it is common to all life (a gene from a human can be read by the translation machinery of a bacterium) is without a doubt a key piece of evidence in establishing that there was a LUCA. But what else can we find out about LUCA? Knowing that the genetic code had arisen tells us there was probably a LUCA, but gives us very little information on the nature of LUCA.
In a nutshell, the study of LUCA broadly revolves around two questions:
•What features are common to all cellular life?
•What sets the three domains — archaea, bacteria and eukaryotes — apart from one another?
At first sight, building a list of LUCA features might seem a fairly straightforward process, especially now that advances in technology allow all the genes possessed by an organism to be identified by sequencing its genome. A sensible approach would perhaps be to compare all the genes from representative genomes of archaea, bacteria and eukaryotes. Those genes that are common to all three domains were in the LUCA and those that aren’t must have been added later. Unfortunately, it’s not that straightforward, for two main reasons:
•Some genes appear to have moved from organism to organism like genetic gypsies, confounding our ability to distinguish between features that are universal and date back to LUCA, and features that are universal because of genes moving about.
•Some genes which were found in LUCA may no longer be universal. That means it may be impossible to distinguish some LUCA features from genes that arose later, say in the evolution of eukaryotes.
Ask any two researchers to give an overview of what they think the LUCA was like, and you will no doubt get different answers. With such a tricky scientific endeavour as this — working out what an organism that lived billions of years ago was like — this is hardly surprising. Some of my own views on one aspect of the LUCA question are to be found in an earlier piece posted on Actionbioscience.org which I co-wrote with Dan Jeffares [Jeffares & Poole 2000], but what follows is a broader overview of the fast-growing field of ‘LUCA biology.’
2. The minimal genome project
One hands-on approach to trying to uncover the biology of the LUCA has been to look for genes that are universal — that is, genes that all life forms possess. Once a list of these genes has been made, they also lead to another possibility: perhaps this list encapsulates the essence of cellular life — the minimum number of genes required to make a cell. In 1996, with the sequences of the first two bacterial genomes (Mycoplasma genitalium & Haemophilus influenzae) in hand, Arcady Mushegian & Eugene Koonin [Mushegian & Koonin 1996] tried exactly this. The most striking features of their minimal genome were:
•A mere 256 genes
•No biosynthetic machinery for making the building blocks of DNA
From this they tentatively concluded that LUCA stored its genetic information in RNA, not DNA, and made suggestions on how to further reduce the number of genes in their minimal genome. The work heralded the arrival of comparative genome studies, and there is no doubt that a good number of the genes in their 256-strong list do date back to the LUCA. However, the work was squarely criticised because of the omission of DNA [Becerra et al. 1997]. Both these bacteria are human parasites and it seems most likely that they did away with parts of the machinery for making their own DNA because they can steal from the host (i.e., the organism infected with these pathogens). Indeed, why put in the effort to make your own when it’s there for the taking?
Regardless of whether or not DNA was a part of the LUCA (I think it was [Poole et al. 2000], but there are plenty of researchers that beg to differ [e.g., Leipe et al. 1999]), this omission highlighted a wider problem with the minimal genome. Namely, the genomes you begin with probably affect the final set of genes. This is a problem for the following reasons:
•How many genomes must be compared before we are confident we aren’t leaving anything out?
•‘Lifestyle’ can affect the final list (in Mushegian & Koonin’s work, the minimal gene set may in fact be a generic set required for parasitism in humans, and has little in common with what was required for a free-living cell to go about its business billions of years ago)
•Gene losses: if a gene was in the LUCA, but now remains in only one of the three domains, this method would consistently leave it off the list of LUCA genes.
•Finally, if genes can move from organism to organism (so-called horizontal or lateral gene transfer), certain genes may have done such a good job of spreading that they sometimes appear to date back to the time of LUCA, whereas in actual fact, they arose more recently.
Despite its limitations, the minimal genome concept is probably the best attempt to put money where mouth is and come up with a hard list of genes that may have been a feature of the LUCA. It is also the only sensible framework we currently have. That said, if gene transfer is extreme, genes will have moved about so often that this and related methods are rendered futile [Doolittle 1999].
Koonin has recently published an updated minimal gene set, using 21 complete genomes [Koonin 2000]. Surprisingly, of the 256 genes in the original set, only 81 remain, and this list is clearly insufficient to describe either the minimum number of genes required for a cell to function, or the genetic makeup of LUCA.
While working out which genes were part of LUCA is no easy task, the various attempts [Mushegian & Koonin 1996; Kyrpides et al. 1999; Hutchison et al. 1999] are a good starting point, and have served to highlight important problems that must be dealt with in the field of ‘LUCA biology.”
3. LUCA genomics
As the minimal genome work demonstrates, the major issues are:
•How much does reliance on universal features underestimate the genetic makeup of LUCA?
•How much gene swapping has gone on during the evolution of life from LUCA to the present?
The magic number of universal features is likely to shift about, and there have been plenty of criticisms of all the attempts to reconstruct the LUCA. Nevertheless, universal features are important because they describe a lower limit from which to build upon, and importantly, all these attempts converge on agreement insofar as concluding that LUCA was quite complex. Some of the gaps will be relatively simple to fill, but others may be close to impossible.
LUCA biologists are aware that universal features may underestimate the complexity of LUCA to some extent, but another concern is emerging that could cause even more headaches — the spectre of ‘horizontal gene transfer’ (also called lateral gene transfer):
•If there is lots of gene swapping between organisms, the tree of life becomes more like a web, and it may not be possible to disentangle the branches.
•If genes are extremely nomadic, truly universal features cannot be distinguished from genes that have successfully spread themselves by gene transfer.
The problem of gene transfer was made apparent in a landmark study of two bacterial genomes, Escherichia coli and Salmonella. Jeffery Lawrence & Howard Ochman [Lawrence & Ochman 1998] concluded that, since diverging from a shared ancestor 100 million years ago, at least 10% of the E. coli genome has been acquired in somewhere in excess of 200 horizontal gene transfer events.
In an equally insightful commentary, William Martin [Martin 1999] has discussed the implications of this work for our ability to reconstruct phylogenetic trees:
•The further back in time an evolutionary divergence, the greater the likelihood that any given gene in a genome has been transferred.
•Indeed, it may be the case that all bacterial genes have been subject to horizontal gene transfer at some point in their evolutionary history.
•This could undermine the utility of phylogenetic tree reconstruction for deep divergences.
Currently, there is a lot of debate over whether gene transfer is so rampant that evolutionary trees cannot be built, or whether the levels of gene transfer are negligible. Both extremes are currently championed in the literature, and ironically, when it comes to the LUCA, Carl Woese’s work is central to both — many of those that view Woese’s three domains as correct have been arguing for little or insignificant levels of transfer, whereas Woese has recently suggested that very early in evolution, gene transfer between organisms was more important than inheritance from generation to generation [Woese 1998].
While it sounds like Woese is being inconsistent, his more recent claim is limited to the earliest periods in the evolution of life, and arose from concerns that LUCA was beginning to appear totipotent, a crazy notion that would have LUCA as the ultimate source for all life’s diversity:
•A fertilised egg is totipotent — from a single cell it will develop into all the different cells and tissues that make up an adult human being.
•If genes move between organisms, LUCA might mistakenly appear totipotent because many features would be incorrectly counted as universal.
Extrapolating Lawrence & Ochman’s result back billions of years may not be realistic. But what if horizontal transfer was the default state? This is the idea Woese has developed. His argument is that genes were so free to exchange that there were no distinct lineages — genes moved more through horizontal transfer than by vertical inheritance. As the genetic system becomes more accurate and as the complexity increases, more genes become interdependent, and transfer gives way to vertical inheritance. Woese argues that translation (and therefore the genetic code) was the first thing to be fixed or crystallised, with other cellular functions following later. From this horizontal transfer dominated system, the three domains (archaea, bacteria and eukaryotes) each emerged independently as lineages.
This is certainly food for thought, but there are several issues:
•Gathering evidence to support it is not exactly easy since there’s no real way to establish that horizontal transfer was the initial state.
•Koonin’s shrinking minimal gene set (Koonin 2000), shows that the minimal genome approach is not creating a totipotent LUCA. Instead, the number of genes ascribable to LUCA is becoming smaller as more genomes are added.
•Another problem has to do with the switch from horizontal transfer to vertical inheritance. How many genes would have been able to partake in global transfers before becoming crystallised and therefore unable to transfer? Would it really have been complex enough for the ancestors of the three domains of life to have emerged independently as distinct lineages?
Gene transfer is going to be a hotly debated topic for a while, and will continue to confound the reconstruction of the LUCA. The problem is a complex one:
•Horizontal gene transfer has been demonstrated — e.g., the spread of antibiotic resistance.
•Limitations of the methods for building evolutionary trees can give false evidence for gene transfer.
•Methods that don’t make use of evolutionary information are being used to examine genetic relationships and, in many cases, the data that have been used to argue for horizontal gene transfer are weak.
•There is little consensus on the reliability of methods for detecting horizontal gene transfer.
•What data are required to demonstrate ancient horizontal gene transfer events?
•If natural selection is considered, most horizontal gene transfers will probably result in the gene being lost — by analogy, the organism needs a new gene like a fish needs a bicycle! For instance, antibiotic resistance genes won’t spread and be maintained by selection unless the organisms with the genes are being assaulted with the antibiotic.
This last point has been too often ignored, and there has been little attempt to establish patterns (e.g., are all genes equally nomadic?). So how should LUCA biologists deal with horizontal gene transfer?
•If we accept that there is or has been massive unbridled horizontal gene transfer between the three domains [e.g., Doolittle 1999], we must conclude that all our tools for looking into the evolutionary past are invalidated, which means we might as well give up on the question of the LUCA. We know that there are demonstrated cases of horizontal gene transfer, but this extreme position is like throwing the baby out with the bathwater.
•If we take as our starting point the opposite extreme, that the effect of horizontal gene transfer has been negligible, we are in a much better position — we still have our tools in place, and any suggestions of horizontal gene transfer will need to be backed up with good evidence.
There is no doubt a middle ground can be found, but amidst the furore over horizontal gene transfer, a number of researchers, making use of whole genome sequences, have reported results suggesting gene transfers have minimal effect on the ability to recover evolutionary trees [e.g., Snel et al. 1999; Sicheritz-Pontén & Andersson 2001]. These results suggest that it is possible to reconstruct the tree of life, and moreover, conclude that the 3 domain structure of the tree, as first reported by Woese and his colleagues, is supported by whole genomes.
In a timely article, Chuck Kurland has firmly criticised the eagerness of many to attribute horizontal gene transfer [Kurland 2000]. One particularly interesting aspect of his exposition is that he suggests a number of non-scientific factors that have contributed to the hype around horizontal gene transfer, and is as much a comment on how science currently operates as it is about gene transfer.
The root of the tree of life is hard to pin down [Pennisi 1999], and unbridled horizontal transfers early in the evolution of life can’t easily be distinguished from the limits of the sensitivity of our phylogenetic tools — that researchers have failed to reach a consensus on the shape of the tree does not mean that there must therefore have been horizontal transfer.
Indeed, there is another issue here — the reliability of the methods used for building evolutionary trees. Many researchers are very confident of the reliability of these methods, yet it is well known that these are based on mathematical algorithms which are convenient, but which do not necessarily accurately model real biological sequence evolution. These methods are likely to be robust for recent evolutionary events, and are definitely the most robust of the methods for detecting gene transfers. The problem is that near the root of the tree of life, they may be just too inaccurate to be useful for scrutinizing the very earliest events in evolution. In a worst-case scenario, the situation might in fact be a bit like timing the 100M sprint at the Olympics with a sundial!
David Penny, Bennet McComish and their coworkers have recently tried to address this question by investigating how far back in time the standard models used in evolutionary tree building can go before they start to go wrong. Their overall conclusion is that the models used do seem to do a little better than might be expected from theory, but that the models still do poorly for very early evolutionary events. Penny and colleagues also criticise the recent trend in reporting conflicting trees as evidence for horizontal gene transfer — given how hard it seems to accurately reconstruct the tree of life, it is hard to say whether conflicting answers are evidence for gene transfer, or just reflect the limitations of the methods for building the trees. Their testing of the models suggests that it is just not reasonable to say that there is horizontal gene transfer just because two trees made with two different genes don’t come back with the same relationships between organisms. They make the following comment, which sums up the problem very succinctly:
“… there are major difficulties between data sets for ancient divergences. It is difficult to see why researchers are so confident in their results when the relatively recent divergences within mammals, birds, or flowering plants are only now being resolved.”
This work of Penny et al. [2001] and the picture coming from evolutionary trees of whole genomes [Snel et al. 1999; Sicheritz-Pontén & Andersson 2001] seems to bolster Kurland’s provocative assertion that horizontal transfer is ‘an ideology that is begging for deconstruction.’
Nevertheless, horizontal gene transfer does occur to some extent — Lawrence & Ochman’s 1998 paper is but one of many demonstrating this. Moreover, many of the technologies biologists use for inserting genes are simply human exploitation of what has been described as natural genetic engineering. The following naturally occurring mechanisms of ‘genetic engineering’ are routinely used in molecular biology laboratories:
•Plasmids: small, usually circular, pieces of DNA that often carry genes that enable them to move from one bacterium to another.
•Viruses: many will naturally insert themselves into the DNA of the organism they are infecting, and can be engineered to carry extra pieces of DNA.
•Natural or assisted DNA uptake by bacterial cells.
•Restriction endonucleases: molecular scissors that allow precise ‘cutting’ of DNA.
We also know it is possible to identify ancient gene transfers that may have occurred as far back as 2 billion years ago. Biologists can readily identify genes in the eukaryote repertoire that have come in via the mitochondrion, a compartment in the eukaryote cell which is bacterial in origin. Indeed, the handful of genes remaining in this compartment have been shown to be bacterial in origin, as have some that have since taken up residence in the eukaryote nucleus [Lang et al. 1999].
Returning for a moment to the biology of nomadic genes, the consensus emerging from studies of bacteria is that we should indeed start thinking of bacterial (and perhaps archaeal) genomes as being an ever changing collection of genes, but only to a degree [Hacker & Carniel 2001]:
•Genes which are central to the running of any cell — these are often referred to as housekeeping genes — make up the ‘core’ genome.
•Genes which come and go make up the flexible genome.
The flexible genome might be a window into the nomadic gene pool of bacteria — Lan & Reeves [2000] point out that closely related strains of bacteria differ in the genes they carry by as much as 20%, and this requires we reevaluate how we categorise species of bacteria. The hope is that the flexible genome can tell us how a particular bug is currently making its living. Losing genes isn’t in itself particularly surprising for organisms that are in competition to reproduce as fast as they can [Jeffares & Poole 2000] — genes that aren’t being used aren’t kept. ‘Use it or lose it’ is the maxim of natural selection, and there are plenty of examples wherever you look in biology (our appendix and tail bone both appear to have headed in that direction for instance).
If this picture of core and flexible genomes is correct, it is good news for LUCA research because many universal features can in theory be recovered. This goes too for the ancient horizontal transfers seen with the mitochondrion. We should be optimistic that some patterns of horizontal gene transfer can be analyzed, though we still need to exercise care when looking so far back in time.
4. Fusion
When it comes to horizontal gene transfer, the hype about archaea and bacteria is arguably a case of squabbling over crumbs when compared to what seems to have happened in the early evolution of the eukaryotic cell. The now popular idea that eukaryotes emerged from a massive fusion (the ultimate gene transfer) event between a bacterium and an archaeon is also raising problems for LUCA biology.
While fusion is all about the origin of the eukaryotes, it is also about LUCA:
•Fusion scenarios challenge Woese’s division of the living world into three domains.
•Rather than a tree with three branches all tracing back to the LUCA, fusion has two lineages, with the eukaryotes emerging by fusion.
•Fusion is in conflict with the emerging picture of the direct link between eukaryote biology and the RNA world.
A number of researchers have argued that the genes in the average eukaryote look to be a mixture of bacterial-like and archaeal-like. That is to say, at the genetic level, eukaryotes look to be some sort of genetic fusion between archaea and bacteria [Ribeiro & Golding 1998; Rivera et al. 1998; Horiike et al. 2001].
In understanding this, it has been helpful to divide genes into two categories: informational or operational [Rivera et al. 1998]. Informational genes are those which are involved with the copying, storing and regulation of genetic information, while operational genes are the recipes for making proteins for synthesis and breakdown of molecules in the cell, and are largely involved in energy metabolism.
Consistent with earlier research [see Gupta & Golding 1996] Rivera and colleagues found that there was rhyme and reason to the mixture of bacterial and archaeal genes in eukaryotes:
•For informational genes — archaea and eukaryotes share more in common
•For operational genes — bacteria and eukaryotes share more in common
Mark Ridley [2000] has suggested a good analogy for what many think has happened — a business merger. Instead of doubling up and having two departments for every aspect of the new company (Eukaryote Inc.), only one of each was kept, with the result being that the informational department came from Archaea Inc. and the ‘operational department’ from Bacteria Inc.
With the ongoing debate on how much horizontal gene transfer there is between organisms, the most exciting contribution to this picture looks not at the genes, but at gene networks. Taking a page from the study of complex networks such as the Internet, Eörs Szathmáry and colleagues [Podani et al. 2001] have recently shown that, while eukaryotic operational genes appear bacterial in origin, the structure of the metabolic network that these genes make up is in fact much much more like what is observed in archaea. In keeping with the business merger analogy, this is perhaps equivalent to keeping the management structures of Archaea Inc. in place.
This is an exciting picture, and there is no question that modern-day eukaryotes are the product of some sort of fusion [Ribeiro & Golding 1998; Horiike et al. 2001]. However, the tricky thing is working out what it all means for the origin of the eukaryotic cell. These are some of the outstanding issues:
•Why has such a merger apparently only happened once?
•No one has ever observed modern bacteria and archaea fusing.
•Why is it we don’t see ‘anti-eukaryotes’ (that is, organisms which have the operational genes of archaea and the informational genes of bacteria)?
•A number of features found exclusively in eukaryotes are tricky to explain by a fusion event.
Indeed, there are a number of ways of explaining the fusion data, and consequently, there are quite a few different opinions on how the eukaryotes came to be [Minkel 2001].
If eukaryotes are the result of a fusion between a bacterium and an archaeon, then the 3 domain picture that Carl Woese’s work supports would be wrong. Fusion would imply that everything in eukaryote biology is either a recent innovation specific to this domain, or an offshoot of the biology of archaea and bacteria. In other words, if you want to know about LUCA, archaea and bacteria are the only two domains worth looking at. This is an assumption that is often made, regardless of fusion, and a point against which some researchers, myself included, have argued [see Jeffares & Poole 2000; also Forterre & Philippe 1999; Poole et al. 1999].
To make sense of the motivation behind the many emerging fusion scenarios for the origin of the eukaryote cell and how these might impact on LUCA biology, it helps to concentrate on the big picture, rather than wading through the details of the various scenarios. Laura Katz [1998] has written a good overview of the various fusion scenarios, though several new scenarios have been published since then [e.g., Margulis, et al., 2000; Horiike et al. 2001; Bell 2001; Hartman & Fedorov 2002]. Fusion theories have developed out of the endosymbiotic theory for the origin of the mitochondrion:
•The endosymbiotic theory was first formulated by Mereschkowsky at the beginning of the 20th century, but reintroduced and updated by Lynn Margulis in the 1970s [Martin et al. 2001].
•This theory argues that the mitochondrion, sometimes called the powerhouse of the cell, was originally a bacterial cell that took up residence in the ancestor of modern eukaryotes.
•Both structural and genetic similarities have shown without a shadow of a doubt that the endosymbiotic theory is correct — the DNA in the mitochondrion is more closely related to bacteria than to the DNA stored in the eukaryotic cell nucleus.
•It is now widely accepted that this event happened once only.
Despite much agreement, there is ongoing debate surrounding the endosymbiotic theory:
1.How was this partnership founded (e.g., oxygen-based or hydrogen-based metabolism)?
2.Was the host that ultimately engulfed the bacterium a eukaryote or an archaeon?
The first question opens up a whole can of worms (which we’ll avoid here), and is a current source of intense debate [Andersson & Kurland 1999; Rotte et al. 2000]. The second question is what has the major impact on LUCA biology, but these two questions have been unnecessarily muddled. The bottom line is that the genomic & gene network data supporting fusion between an archaeon and a bacterium can as easily be made to fit a fusion between a eukaryote and a bacterium.
The state of the field is as follows:
•Everyone agrees that the mitochondrion evolved from a bacterial ancestor (though there is current debate as to what the bacterial ancestor was like, and how it interacted with its host).
•There is disagreement as to whether the host was a eukaryote, or an archaeon.
•Archaea-Bacteria fusion hypotheses require all genes found only in eukaryotes to have arisen post-LUCA, post-fusion — that is, they are indirectly descended from LUCA.
•This comes into conflict with the picture of LUCA from RNA [Jeffares & Poole 2000], and Woese’s tree of life [Woese et al. 1990] — both require that eukaryotes were directly descended from LUCA.
So how do we distinguish between an archaeal and a eukaryotic host? The key is in two parts — one is historical and the other requires careful thought about how archaea and eukaryotes might be related:
1.The historical aspect centres around understanding the shift in thinking from the original picture of an ancient eukaryote playing host to the now largely agreed-upon picture of an archaeon playing host. This shift largely revolves around the changing branches in the eukaryote evolutionary tree [Dacks 2002].
2.The relationship between archaea and eukaryotes cuts to the heart of how researchers view the evolution of cells.
Archaezoa - missing links lost
So why is it that fusion hypotheses have become so popular? Indeed, this goes against the classical interpretation, most thoroughly espoused by Tom Cavalier-Smith [1987], who identified a disparate group of eukaryotes that appeared to him to be missing links — the so-called Archaezoa, which look like eukaryotes but lack mitochondria. His hypothesis, that the Archaezoa evolved before the introduction of mitochondria into the eukaryote lineage, held sway for many years, though has recently been dropped in favour of fusion:
•There is growing evidence that all eukaryotes once harboured mitochondria
•Thus, the Archaezoa have probably all lost their mitochondria, rather than never having had them [Embley & Hirt 1998].
•For instance, one group of the Archaezoa called the microsporidia are now widely accepted to have been incorrectly placed very deep on the eukaryotic tree. Indeed, probably most of the Archaezoa, if not all, are incorrecly placed on the tree. Rather than being missing links leading back to the origin of eukaryotes, they probably arose more recently [see Dacks & Doolittle 2001; Keeling 1998; Dacks 2002].
•If the Archaezoa aren’t a series of missing links, the origin of eukaryotes may have been concurrent with the endosymbiosis that gave rise to mitochondria.
The conclusion from the above is that all eukaryotes probably had a mitochondrion, and without the Archaezoa, the only ancestor of eukaryotes is archaea. Voil#224;! We have fusion.
A case of throwing the baby out with the bathwater?
The important point to keep in mind about the picture for fusion is that it is a partial one, based largely on gene data. There are a large number of differences in the general structure of eukaryotic and prokaryotic (archaeal & bacterial) cells that aren’t explained by fusion [Poole & Penny 2001]. However, the major inconsistency is that the picture provided from trees is not the same for the relationship between archaea and eukaryotes, and that of bacteria and eukaryotes [Poole & Penny 2002, submitted]:
•Margulis’ hypothesis is evidenced from trees. There is now overwhelming agreement that the mitochondria branch is within the bacterial tree, specifically within a subgroup called the alpha-proteobacteria [Lang et al. 1999], and a bacterial origin is also observed for chloroplasts (where photosynthesis takes place in plants and other photosynthetic eukaryotes).
•Comparisons of relevant genes from eukaryotes and archaea should give this picture also, yet the evidence points to archaea and eukaryotes being very distinct domains.
•This has strong parallels to the way the Archaezoa case is being treated — if there are no modern groups of archaea that appear to have split from the trunk of the tree of life before the appearance of eukaryotes, should we accept fusion? Stronger evidence was certainly required in testing the origin of the mitochondrion!
•Another issue has to do with missing links. If the disappearance of the missing links (the Archaezoa) is used to suggest fusion, it is surely just as reasonable to argue against fusion on exactly the same grounds — there are no intermediates between eukaryotes with mitochondria and the archaea. For example, we don’t see examples of archaea with mitochondria in them, or archaea with nucleus-like structures.
With perhaps a couple of billion years separating the divergence of archaea and eukaryotes, it would be incorrect to require that the archaeon in the fusion must have been just like modern archaea. This cuts right to the heart of the problem — there is no inherent requirement that evolution leaves behind a series of intermediates for us to use to piece together the different evolutionary trajectories of archaea and eukaryotes. As with Chinese whispers, the end point may be very different from the starting phrase, but with evolution, all we have to look at is a number of different endpoints, from which we can only guess at the starting phrase!
While the specifics of the Archaezoan hypothesis are most probably wrong, it should not be thrown out completely. Explanations of eukaryote origins by fusion or via Margulis’ original scenario each suffers from the disappearance of intermediate forms, but this is expected. As Stephen Jay Gould had often said, evolution results in bushes, not ladders.
A number of researchers [Forterre & Philippe 1999; Andersson & Kurland 1999; Penny & Poole 1999] maintain that the data for fusion can be reconciled with Lynn Margulis’ endosymbiotic theory and Carl Woese’s three-domain tree. Indeed, David Penny and I have argued that fusion does the worst job of explaining the available data [Penny & Poole 1999; Poole & Penny 2001]. For instance, fusion doesn’t fit with the hypothesis that some eukaryote features, which have since been lost in archaea and bacteria, actually date back to the LUCA (see Jeffares & Poole 2000).
What it very tentatively implies is that archaea and eukaryotes may have shared a more recent ancestor than either shares with bacteria, as is often shown in textbooks, but this too is not certain, since the relationships between these three groups is also a point of controversy [see Forterre & Philippe 1999; Pennisi 1999]!
5. Conclusions
We are now entering a very exciting period in uncovering the history of the LUCA — the field has been given a major boost from a broader range of ideas being applied to the problem:
•Acknowledging the technical challenges with building the tree of life is an important step in the right direction. So is the idea of using the RNA world period in the origin of life for establishing aspects of the nature of the LUCA (see Jeffares & Poole 2000).
•Despite the deluge of genome data available, it is hard to say whether we will ever actually manage to get a complete list of genes which LUCA possessed. We may pick out certain characteristics, but each needs to be evaluated with extreme care. The minimal genome study of Mushegian & Koonin [1996] demonstrates this and, in its failure, it stands as a strong caveat. Most researchers have toyed with this idea, and many were probably disappointed (then later relieved) that Mushegian & Koonin beat them to it!
•Horizontal gene transfer is likely to be a factor in confounding such efforts, but it is better to err on the side of caution with respect to how pervasive this is in the history of life. The emerging picture from genome research suggests that not all genes transfer equally easily, and that there may be an ecological underpinning to the nature of gene transfer.
•The fusion hypothesis has important consequences for the LUCA — if correct, the LUCA must have been like bacteria and/or archaea, because those unique features of the biology of eukaryotes had not yet evolved.
•The three domain tree that emerged from Woese’s original work permits features of all three domains to trace back to the LUCA while the fusion hypothesis, in its strictest form, does not. Unless an argument for loss of a feature in all modern archaea can be made, it is diametrically opposed to the nature of the LUCA as suggested from RNA world fossils [Jeffares & Poole 2000].
Currently, many major assumptions are being questioned:
•Were there three domains or two, with the third arising by fusion?
•Was LUCA prokaryote-like or eukaryote-like or even a mixture?
•Is the genetic code the only one possible?
•Was early evolution more reliant on horizontal gene transfer than inheritance?
•Was there one or more LUCAs? |
depresivni iskompleksirani primitivac i nadrkana budala kojeg financira stari i koji sa skoro 50 godina nema ni žene ni posla.Pa naravno da je ljut na sve, a narocito na one koji su uspješni.
Jbga Lwoode nisam ti ja kriv kaj nisi uspio u životu |
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lwood
Advanced Member
Colombia
47132 Posts
Member since 09/12/2005 |
Posted - 27/07/2010 : 23:19:05
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quote:
Originally posted by Lord Vader89
aj brate izrazavaj se ko covek, nemoj kopirati romane ovde vise
imali smo ovu temu prije par godina.ne da mi se ponovo natizat .
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depresivni iskompleksirani primitivac i nadrkana budala kojeg financira stari i koji sa skoro 50 godina nema ni žene ni posla.Pa naravno da je ljut na sve, a narocito na one koji su uspješni.
Jbga Lwoode nisam ti ja kriv kaj nisi uspio u životu |
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