Teave

Mitu korda tekkis maapealne elu ookeanist?


Evolutsiooni kujutatakse populaarkultuuris sageli ekslikult lineaarsena. Selle kujutamise üks põhijooni populaarkultuuris, aga isegi teaduse populariseerimisel, on see, et mõni ookeanis elav loom heidab kaalud ja uimed maha ning roomab maismaale.

Muidugi, see esitleb ainult üks esivanemate suguvõsa jaoks üks teatud liigid (Homo sapiens). Minu küsimus on: Kus mujal arenes elu veest maismaale?

Intuitiivselt tundub see tohutu hüpe (kohanemine põhimõtteliselt võõra keskkonnaga), kuid see pidi juhtuma ikka mitu korda (eraldi vähemalt taimede, putukate ja akordide puhul, kuna nende viimane ühine esivanem on mereelanik). Tegelikult, mida rohkem ma sellele mõtlen, seda rohkem näiteid leian.


Ma kahtlen, kas me teame täpset arvu või isegi selle läheduses. Kuid on mitmeid hästi toetatud teoreetilisi kolonisatsioone, mis võivad teile huvi pakkuda ja aidata luua pilti sellest, kui tavaline oli elu maale üleminek. Me võime kasutada ka teadaolevaid fakte selle kohta, millal erinevad evolutsioonilised liinid lahknesid, koos teadmistega maa varasemate kolonisatsioonide kohta, et mõned sündmused enda jaoks välja töötada. Olen seda siin teinud laiade taksonoomiliste kladide jaoks erinevates mõõtkavades - huvi korral võiksite sama asja uuesti teha madalamate alamklaaside puhul.

Nagu te õigesti märkisite, peab olema enne maa koloniseerimist olnud vähemalt üks koloniseerimissündmus iga maa kohta, mis erines teistest maismaa praegustest suguvõsadest. Kasutades allpool esitatud tõendeid ja põhjendusi, toimusid vähemalt järgmised 9 sõltumatut koloniseerimist:

  • bakterid
  • tsüanobakterid
  • arhaea
  • protistid
  • seened
  • vetikad
  • taimed
  • nematoodid
  • lülijalgsed
  • selgroogsed

Bakterite ja arheoloogide koloniseerimine
Esimesed tõendid elu kohta maismaal näivad pärinevat 2,6 (Watanabe jt, 2000) kuni 3,1 (Battistuzzi jt, 2004) miljardit aastat tagasi. Kuna molekulaarsed tõendid viitavad bakterite ja arheade erinevusele 3,2–3,8 miljardi aasta eest (Feng jt, 1997 - klassikaline paber) ja kuna nii baktereid kui ka arheasid leidub maismaal (nt Taketani & Tsai, 2010), peavad nad on koloniseerinud maad iseseisvalt. Ma soovitaksin, et ka seal oleks olnud palju erinevaid bakterite kolonisatsioone. Vähemalt üks on kindel - tsüanobakterid peavad olema koloniseerunud mõnest teisest vormist sõltumatult, kuna need arenesid välja pärast esimest bakterite koloniseerimist (Tomitani et al., 2006) ja neid leidub nüüd maismaal, nt. samblikes.

Protistan, seente, vetikate, taimede ja loomade koloniseerimine
Protistid on lihtsate eukarüootide polüfüleetiline rühm ja kuna seente lahknemine nendest (Wang et al., 1999 - teine ​​klassika) eelneb seente esilekerkimisele ookeanist (Taylor & Osborn, 1996), peavad need olema tekkinud eraldi. Kuna taimed ja seened lahknesid, kui seened olid veel ookeanis (Wang et al., 1999), pidid taimed olema koloniseerunud eraldi. Tegelikult on selgesõnaliselt avastatud mitmel viisil (nt molekulaarkella meetodid, Heckman jt, 2001), et taimed pidid ookeani eraldi seentesse jätma, kuid tõenäoliselt tuginesid nad sellele, et seda teha (Brundrett, 2002 - vt selle paberi all olevat märkust). Edasi, lihtsad loomad ... Lülijalgsed koloniseerisid maad iseseisvalt (Pisani jt, 2004) ja kuna nematoodid lahknesid enne lülijalgseid (Wang jt, 1999), pidid ka nemad iseseisvalt maad leidma. Seejärel tulid lõpuks metsatukad tetrapoodid (Long & Gordon, 2004).

Märkus Brundretti paberi kohta: sellel on üle 300 viite! See tüüp ilmselt lootis mingit auhinda.

Viited

  • Battistuzzi FU, Feijao A, Hedges SB. 2004. Prokarüootide evolutsiooni genoomne ajakava: arusaamad metanogeneesi, fototroofia ja maa koloniseerimise päritolust. BMC Evol Biol 4: 44.
  • Brundrett MC. 2002. Maataimede juurte ja mükoriiside koevolutsioon. Uus fütoloog 154: 275-304.
  • Feng D-F, Cho G, Doolittle RF. 1997. Valgu kellaga lahknemisaegade määramine: ajakohastamine ja ümberhindamine. Rahvusliku Teaduste Akadeemia toimetised 94: 13028 -13033.
  • Heckman DS, Geiser DM, Eidell BR, Stauffer RL, Kardos NL, Hedges SB. 2001. Molekulaarsed tõendid maa varaseks koloniseerimiseks seente ja taimede poolt. Science 293: 1129-1133.
  • Long JA, Gordon MS. 2004. Suurim samm selgroogsete ajaloos: Paleobiological Review of Fish -Tetrapod Transition. Füsioloogiline ja biokeemiline zooloogia 77: 700-719.
  • Pisani D, Poling LL, Lyons-Weiler M, Hedges SB. 2004. Maa koloniseerimine loomade poolt: molekulaarne fülogenees ja lahknemisajad lülijalgsete vahel. BMC Biol 2: 1.
  • Taketani RG, Tsai SM. 2010. Erinevate maakasutuste mõju arheoloogiliste koosluste struktuurile Amazonase antrosoolides, mis põhinevad 16S rRNA ja amoA geenidel. Microb Ecol 59: 734-743.
  • Taylor TN, Osborn JM. 1996. Seente tähtsus paleoökosüsteemi kujundamisel. Review of Paleobotany and Palynology 90: 249-262.
  • Wang DY, Kumar S, Hedges SB. 1999. Erinevuste ajahinnangud loomade füla varajase ajaloo ning taimede, loomade ja seente päritolu kohta. Proc. Biol. Sci. 266: 163-171.
  • Watanabe Y, Martini JEJ, Ohmoto H. 2000. Geokeemilised tõendid maismaa ökosüsteemide kohta 2,6 miljardit aastat tagasi. Nature 408: 574-578.

Richardi vastus on fantastiline ja ma ei hakka nii põhjalik olema. Kuid siin on veel mõned näited:

  • Kilpkonnad (kes tegid merelt maale merelt tagasi maale!)
  • Gastropoda (teod ja nälkjad)
  • Tardigrada (veekarud)
  • Onychophora (sametussid)
  • Planaria (lamedad ussid)
  • Annelida (annelid ussid)

Koos Richardi näidetega, mis sisaldavad kõiki wikipedias toodud näiteid maismaaloomadest.


Kuidas arenes mitmerakuline elu?

Teadlased avastavad viise, kuidas üksikutel rakkudel võis areneda tunnuseid, mis kinnistasid nad rühmakäitumiseks, sillutades teed mitmerakulisele elule. Need avastused võivad selgitada, kuidas võõrmaailmas võib areneda keeruline maaväline elu.

Teadlased kirjeldasid neid järeldusi ajakirja 24. oktoobri 2016. aasta numbris Teadus.

Esimesed teadaolevad üherakulised organismid ilmusid Maale umbes 3,5 miljardit aastat tagasi, umbes miljard aastat pärast Maa tekkimist. Keerulisemate eluvormide kujunemine võttis kauem aega, esimesed mitmerakulised loomad ilmusid alles umbes 600 miljonit aastat tagasi.

Mitmerakulise elu areng lihtsamatest üherakulistest mikroobidest oli Maa bioloogia ajaloos pöördeline hetk ja on planeedi ökoloogiat drastiliselt ümber kujundanud. Üks saladus mitmerakuliste organismide kohta on aga see, miks rakud ei pöördunud tagasi üherakulise elu juurde.

"Üherakulisus on selgelt edukas - üherakulisi organisme on palju rohkem kui mitmerakulisi organisme ja nad on eksisteerinud veel vähemalt 2 miljardit aastat," ütles uuringu juhtiv autor Eric Libby, New Mehhiko Santa Fe instituudi matemaatiline bioloog. "Mis on siis eelis olla mitmerakuline ja jääda selliseks?"

Sellele küsimusele vastatakse tavaliselt koostööst, kuna rakud said koos töötamisest rohkem kasu kui üksi elades. Koostöö stsenaariumides on aga pidevalt ahvatlevaid võimalusi, et „rakud saaksid oma kohustustest kõrvale hiilida, st petta,” ütles Libby.

"Võtke näiteks sipelgate koloonia, kus ainult kuninganna muneb ja töötajad, kes ei suuda paljuneda, peavad end koloonia heaks ohverdama," ütles Libby. „Mis takistab sipelgatöötajal kolooniast lahkuda ja uut kolooniat moodustada? Noh, ilmselgelt ei saa sipelgatöötaja paljuneda, seega ei saa ta oma kolooniat luua. Aga kui see saaks mutatsiooni, mis võimaldas tal seda teha, oleks see koloonia jaoks tõeline probleem. Selline võitlus on levinud mitmerakulise evolutsiooni käigus, sest esimesed mitmerakulised organismid olid vaid mutatsioonist eemal sellest, et nad oleksid üherakulised. ”

Katsed on näidanud, et mikroobide rühm, mis eritab kasulikke molekule, millest saavad kasu kõik rühma liikmed, võib kasvada kiiremini kui rühmad, kes seda ei tee. Kuid selles grupis kasvavad kõige kiiremini vabalt laadijad, kes ei kuluta ressursse ega energiat nende molekulide eraldamiseks. Teine näide rakkudest, mis kasvavad viisil, mis kahjustab nende rühmade teisi liikmeid, on vähirakud, mis on potentsiaalne probleem kõigile mitmerakulistele organismidele.

Tõepoolest, paljud primitiivsed mitmerakulised organismid kogesid tõenäoliselt nii ühe- kui ka mitmerakulisi olekuid, pakkudes võimalusi loobuda grupi elustiilist. Näiteks bakter Pseudomonas fluorescens areneb kiiresti, tekitades pindadele mitmerakulisi matte, et paremini hapnikule juurde pääseda. Kui aga matt on tekkinud, on üherakulistel petturitel stiimul mitte toota mati moodustamise eest vastutavat liimi, mis viib lõpuks mati hävimiseni.

Mitmerakulise elu püsimise saladuse lahendamiseks soovitavad teadlased seda, mida nad nimetavad "põrkemehhanismideks". Lukud on seadmed, mis võimaldavad liikumist ainult ühes suunas. Analoogia põhjal on põrkamismehhanismid tunnused, mis annavad grupi kontekstis kasu, kuid on üksildastele kahjulikud, takistades lõppkokkuvõttes üherakulisele olekule tagasipöördumist, ütles Libby ja uuringu kaasautor William Ratcliff Atlanta Georgia Tehnoloogiainstituudis.

Üldiselt, mida rohkem omadus muudab rühma rakud üksteisest sõltuvaks, seda enam toimib see põrutusena. Näiteks võivad rakurühmad jagada tööd nii, et mõned rakud kasvatavad ühte elutähtsat molekuli, teised rakud aga teist olulist ühendit, nii et need rakud toimivad paremini koos kui eraldi, seda ideed toetavad hiljutised katsed bakteritega.

Ratcheting võib seletada ka sümbioosi iidsete mikroobide vahel, mis viisid rakkudes elavate sümbiontideni, nagu mitokondrid ja kloroplastid, mis aitavad vastavalt nende peremeestel kasutada hapnikku ja päikesevalgust. Üherakulistel organismidel, mida tuntakse Paramecia nime all, läheb fotosünteesi sümbiontidest eksperimentaalselt tuletades halvasti ja sümbiontid omakorda kaotavad tavaliselt geenid, mis on vajalikud eluks väljaspool nende peremehi.

Need põrkemehhanismid võivad viia näiliselt mõttetute tulemusteni. Näiteks apoptoos või programmeeritud rakusurm on protsess, mille käigus rakk teeb sisuliselt enesetapu. Kuid katsed näitavad, et kõrgematest apoptoosi määradest võib tegelikult kasu olla. Suurtes pärmirakkude klastrites toimivad apoptootilised rakud nõrkade lülidena, mille surm võimaldab väikestel pärmirakkude tükkidel vabaneda ja levida mujale, kus neil võib olla rohkem ruumi ja toitaineid.

"See eelis ei toimi üksikute rakkude puhul, mis tähendas, et iga rühm, kes grupi hülgas, kannab ebasoodsat olukorda," ütles Libby. "See töö näitab, et rühmas elav rakk võib kogeda põhimõtteliselt teistsugust keskkonda kui omaette elav rakk. Keskkond võib olla nii erinev, et üksildase organismi jaoks hukatuslikud omadused, nagu suurenenud suremus, võivad saada kasu rühma rakkudele. ”

Kui rääkida sellest, mida need leiud tähendavad tulnukate elu otsimisel, ütles Libby, et see uuring viitab sellele, et maaväline käitumine võib tunduda veider, kuni mõistetakse paremini, et organism võib olla mõne rühma liige.

"Kogukondade organismid võivad oma käitumist, mis tunduks veider või vasturääkiv, ilma nende kogukondlikku konteksti korralikult arvesse võtmata," ütles Libby. "See on sisuliselt meeldetuletus, et pusletükk on pusle, kuni teate, kuidas see laiemasse konteksti sobib."

Libby ja tema kolleegid kavatsevad tuvastada muid põrkemehhanisme.

"Meil on töös ka mõningaid katseid, et arvutada mõningate võimalike põrkumisomaduste pakutav stabiilsus," ütles Libby.

Registreeruge, et saada NASA astrobioloogiaprogrammi uudiseid, sündmusi ja võimalusi.


Astrobioloogia ajalugu

Varsti pärast seda, kui NASA loodi 1958. aastal, alustas agentuur laiapõhjalisi jõupingutusi, et õppida otsima nii iidset kui ka praegust elu väljaspool Maad. Agentuuri inimeste ja robotite kosmoseprogrammidega liitumine bioloogiaga ei ole alati olnud lihtne ega aktsepteeritud, eriti kuna tegelikke eluproove pole mujalt leitud. Kuid praeguseks on need kaks programmi nii läbi põimunud ja üksteisest sõltuvad, et kumbki oleks ilma teiseta sügavalt kahjustatud.

Osa sellest esialgsest sidumisest tulenes juhuslikust ajastusest, kahe ajaloolise edasimineku kõrvutamisest. Kõigepealt tulid üllatavad avastused ja jätkuteooriad selle kohta, kuidas elu ise korraldab ja kuidas see võis Maal alata. Sellele järgnesid varsti pärast meie esimesed õnnestumised kosmosereisidel ja kaudne lubadus veel palju muud.

Nii et rahva võime kosmosesse jõuda tuli ajal, mil inimesed olid avatud ja isegi innukad, et rohkem teada saada elu dünaamikast ja päritolust Maal ... ja võib -olla ka väljaspool.

Seost kosmoseuuringute ja astrobioloogia (tollase nimega eksobioloogia) vahel tõi esile ja andis varajase legitiimsuse molekulaarbioloogist saanud eksobioloog Joshua Lederberg. Juba enne NASA ametlikku loomist jõudis ta kolleegideni, et leida võimalusi väljaspool Maad elu leida. Ta võitis Nobeli preemia (33 -aastaselt bakterite geneetika avastamise eest) samal aastal, kui asutati NASA.

1960. aastaks kirjutas ta ajakirja Teadus et: "Eksobioloogia pole fantastilisem kui kosmosereiside teostamine ja meil on tõsine vastutus uurida selle mõju teadusele ja inimeste heaolule oma parimate teaduslike teadmiste ja teadmistega."

Kui 1960ndad olid NASA -s määratletud peamiselt püüdlustega inimesi Kuule maandada, toetas sel perioodil agentuur ka jõulisi jõupingutusi Marsi missiooniks valmistumiseks. Selle põhieesmärk: otsida allkirju elust väljaspool Maad.

Need jõupingutused nõudsid põhjalikku uurimistööd ja vältimatut arutelu selle "elu" olemuse üle Viiking maandurid otsiksid. Veelgi enam, bioloogilistes valdkondades olevad inimesed hakkasid korralikult muretsema selle pärast, milline on mikroobide elu Viiking maandurid võivad Maalt Marsile tuua ja projitseerida edasi maavälist elu, mis võib ühel päeval meie planeedile tagasi pöörduda.

Nii et kuigi praeguse või eelmise elu jaht Marsil oli väga populaarne idee, avas see Pandora kasti äärmiselt keerulisi küsimusi elu endiselt salapärase olemuse ja päritolu kohta. Sellegipoolest jõudis võimalus tegelikult leida maavälist elu põnevuse ajal palavikus Viiking maandumine 1976. Paljud ennustasid, et Marsil leidub elu - sealhulgas Carl Sagan, kes ootas põnevusega kohtumist Viiking, nähtavad, võib -olla ujuvad olendid.

Kuid need ennustused andsid aluse piltidele süngest ja viljatu Marsi maastikust ning seejärel negatiivsetest, kuid ka segadust tekitavatest teaduslikest järeldustest selle kohta, kas elumärke või isegi orgaanilisi ühendeid on avastatud.

Kogemus oli piisavalt kainestav, et Marsi uurimine asus järsult tahaplaanile ja intresside taastumiseni kulub aastakümneid. Ja kuigi orbiidid, maandujad ja roverid naasid Marsile 1990ndatel ja 2000ndatel, algas alles 2012. aasta Curiosity maandumisel teine ​​astrobioloogia (kuigi mitte elu avastamise) missioon. Õnneks oli vahepealsetel aastatel palju õpitud.

Näiteks avastati Maalt varem tundmatud mikroobikooslused, mis säilivad - õitsevad isegi - varem surnuks peetud, elamiskõlbmatus keskkonnas. Esimene suurem "ekstremofiilne" avastus tehti Galapagose saarte lähedal asuva sügava ookeani mustuses merepõhja hüdrotermiliste ventilatsiooniavade kõrval. Vähe sellest, et mikroobid ja hilisemad torujad ussid elasid täielikus pimeduses, vaid nad elasid vees, mis muutus tuulutusavade poolt kõrvetavalt kuumaks.

See 1977. aasta avastus viis teadlased kogu maailma äärmuslikesse keskkondadesse, kus nad leidsid mikroobid, kes elasid kibedas külmas, väga happelises ja soolases vees, miilide all kaevatud kullakaevanduste kivimites, kõrgel maapinnast kõrgemal asuvas atmosfääris. radioaktiivsusest.

See sageli NASA sponsoreeritud uuringute plahvatus rääkis teadlastele palju elust Maal, kuid viitas ka üsna selgelt sellele, et elu võib eksisteerida väljaspool Maad tingimustes, mida on pikka aega peetud ületamatuteks -näiteks Jupiteri kuu Europa ookeanid.

Teadlased on leidnud ka kõik kosmoses eluks vajalikud kemikaalid ning paljud meteoriitide ja isegi komeetide peamised ehitusplokid. Aminohappeid leiti näiteks komeedi Wild 2 proovidest pärast seda, kui NASA kosmoselaev Stardust 2004. aastal komeedi tolmuse kooma läbis ning NASA teadlased avastasid meteoriitidest nukleotiide. Need astrokeemia valdkonna tulemused on teadlastele öelnud, et eeldatavalt eluks vajalikke koostisosi langeb tegelikult kõikjale planeetidele, kuudele ja asteroididele.

See, kuidas need ja muud orgaanilised ühendid võivad end korduvateks vormideks ja lõpuks organismideks korraldada, on olnud astrobioloogia üks väljakutsuvamaid valdkondi. Nii uurides elu geneetilisse infrastruktuuri kui ka proovides seda laboris uuesti luua, on teadlased tõrjunud elu päritolu saladuse tagasi varasesse RNA maailma ja isegi RNA-eelsesse maailma. Kuid protsess, mille kaudu elutud ained said elu atribuudid, jääb tabamatuks.

Maapõhised uuringud on olnud astrobioloogia jaoks hädavajalikud ning oluliselt muutnud meie arusaama Maast ja sellest, mis võib olla võimalik teistes maailmades. Kuid NASA ja Euroopa robotmissioonid ning kosmoseteleskoobid on enamasti olnud põllu liikumapanevad mootorid.

Mantrast “järgige vett” juhindudes on NASA missioonid meie päikesesüsteemis avastanud üllatavalt palju astrobioloogia sihtmärke. Kõigepealt tuli Jupiteri kuu Europa, mille jääkoore all oli ookean. Käimasolevad uuringud näitavad, et vesi on soolane, soolvesi, millel on ilmne paralleel meie ookeanidega. Ja hiljuti võis avastada selle vee lekkeid Kuult - mõnes mõttes sarnaseid Saturni kuust Enceladusest väljuvatega.

Vee lugu Marsil on olnud eriti paljutõotav, kuna on tuvastatud sügavad jõekanalid, orusüsteemid, loopealsed ja hiljuti ka järved ning ettepanekud kunagisest suurejoonelisest ookeanist. Kääbusplaneet Ceres ja Jupiteri kuu Ganymede näivad nüüd omavat ka sisemereid ning võimalused rohkemate veemaailmade leidmiseks tunduvad lõputud.

Seda seetõttu, et viimased kakskümmend aastat on olnud tunnistajaks revolutsioonile meie arusaamades eksoplaneetidest - kehadest, mis tiirlevad kaugete päikeste ümber. Teadlased on juba ammu kahtlustanud, et teised tähed toodavad päikesesüsteeme, kuid esimene avastati alles 1995. aastal. Sellest ajast alates on tuvastatud tuhandeid inimesi, eriti NASA Kepleri kosmoseteleskoobi abil, aga ka maapealsete vaatluste kaudu.

Kuna eksoplaneetide hinnanguline arv on kasvanud paljude miljarditeni, on võimalus, et mõned neist elavad elusorganismidele, muutunud usutavamaks ja põhjalikumaks uurimiseks. Teadlased on kindlaks teinud, et mõned planeedid on kivised ja Maa-sarnased ning tiirlevad ümber oma päikese hästi "elamiskõlblikus tsoonis"-kaugel, kus vesi võib planeedi pinnal vähemalt mõnda aega vedelikuks jääda . Planeedi tõeliselt elamiskõlblikuks muutmiseks on vaja palju rohkem kui kivist pinda ja aeg -ajalt vedelat vett, kuid see on oluline algus.

Tagantjärele näeme, et astrobioloogia laiaulatuslikud edusammud panid aluse sellele, mis sai kohe kõigi aegade suurimaks uudiseks, ja võimaliku avastada iidse Marsi elu märke.

1996. aasta pealkirjades räägiti NASA uurimisrühmast David McKay juhtimisel, kes oli Marsilt pärit meteoriidist leidnud kuus varasema elu näitajat. Kuulus meteoriit ALH84001, mis avastati 1984. aastal Antarktika Allan Hills'i piirkonnas, esitati kui selget märki selle kohta, et mikroobne elu eksisteeris kunagi Marsil. Seal oli isegi pilte sellest, mida tõlgendati bakterilaadse eluvormi fossiilseteks jäänusteks.

Nagu ka Viiking tulemused, aga paljud Marsi ja astrobioloogia kogukondades ei olnud selles veendunud. Kuigi autorid nii Viiking Tulemused ja Marsi meteoriiditulemused jäävad oma töö juurde, teaduslik konsensus on need suures osas tagasi lükanud ja järeldanud, et tulemusi saab seletada ilma bioloogia olemasoluta.

Sellegipoolest andsid Marsi meteoriit ja seda ümbritsev põnevus hüppelaua NASA uuendatud elu otsingutele väljaspool Maad. NASA Astrobioloogia Instituut asutati kaks aastat pärast Marsi meteoriidipaberi avaldamist, selle direktoriks sai Nobeli preemia laureaat Baruch Blumberg ja organisatsioon on sellest ajast alates rahastanud laiaulatuslikke uuringuid.

Osa tööst hõlmab Maa keskkondade uurimist, et paremini mõista potentsiaalselt sarnaseid keskkondi väljaspool Maad (nn analoogkeskkonnad). Muu töö hõlmab tehnoloogia väljatöötamist kasutamiseks teistel planeetidel ja kuudel, samas kui teised uuringud uurivad meie planeedi elu päritolu ja varajast arengut.


Evolutsioon: merest väljas

Neljapäeval, 26. juulil käivitati uus ingliskeelne teadusblogide võrgustik SciLogs.com. Loodusvõrgustiku blogijate uhiuus kodu SciLogs.com on osa rahvusvahelisest SciLogsi ajaveebikogust, mis on juba olemas saksa, hispaania ja hollandi keeles. Selle NPG teadusblogimise perekonna täiendamise tähistamiseks avaldavad mõned NPG ajaveebid postitusi, mis keskenduvad algusele.

Sellel võrgustikeülesel ajaveebifestivalil osalevad nature.com’i seebikateaduse ajaveeb, Scitable’s Student Voices ajaveeb ja blogijad SciLogs.com, SciLogs.de, Scitable ja Scientific American Blog Network. Liituge meiega, kui uurime alguse erinevaid tõlgendusi - alates teaduslikest näidetest, nagu tüvirakud, kuni esmakordsete kogemusteni, näiteks oma esimese töö avaldamiseni. Samuti saate sotsiaalmeedias vestlusi jälgida ja neile kaasa aidata, kasutades hashtagi #BeginScights. - Bora

Alguses oli maa ilma vormita ning tühjus ja pimedus olid sügavuse pinnal, kuna meie päikesesüsteemi moodustamiseks varises kokku hiiglaslik gaasi- ja tolmupilv. Planeedid tekkisid, kui udukogu keerles, lähedalasuv supernoova liikuma pani ja keskel süttis kõige kiirem osakeste kokkusurumine, et saada meie päike. Umbes 4,5 miljardit aastat tagasi hakkas sula maa jahtuma. Vägivaldsed kokkupõrked komeetide ja asteroididega tõid kaasa eluvedeliku - vee - ning pilved ja ookeanid hakkasid ilmet võtma. Alles miljard aastat hiljem toodi esile esimene elu, täites atmosfääri hapnikuga.

Järgmise paari miljardi aasta jooksul sulasid üherakulised organismid kokku ja muutusid mitmerakulisteks kehaplaanideks, mis mitmekesistusid ja kiirgasid, plahvatades selgrootute hulka. Ometi piirdus kogu see küllus ja elu vaid meredega ning suur ja külluslik maa jäi kasutamata. Umbes 530 miljonit aastat tagasi on tõendeid selle kohta, et sajajalgsed loomad hakkasid vee kohal maailma uurima. Umbes 430 miljonit aastat tagasi taimed ja koloniseerisid palja maa, luues toidu- ja ressursirikka maa, samal ajal kui kalad arenesid esivanemate selgroogsetest meres. Kulus veel 30 miljonit aastat, enne kui need eelajaloolised kalad veest välja ronisid ja alustasid evolutsioonilist suguvõsa, mille kohal me täna istume. Et mõista elu sellisena, nagu me seda teame, peame vaatama tagasi, kust me tulime, ja mõistma, kuidas meie esivanemad trotsisid lainete kohal uhiuut maailma.

Kalade jaoks oli see väike samm, kuid loomade jaoks hiiglaslik hüpe. Kuigi kaasaegseid kalaliike vaadates ei ole nii raske ette kujutada aeglast kohanemist eluga merest. Just teisel päeval toitsin oma lemmiklooma skorpionkala Stumpyt ja ta üllatas mind selle aeglase ja tahtliku roomamisega oma toidu poole:

Paljudel kaladel on iseloomulikud tunnused, mis ei erine esimeste neljajalgsete omadest: neljajalgsed selgroogsed, kes esimest korda maa peal julgesid elada, iidsete kalade otsesed järeltulijad. Paljud Stumpy sugulased, sealhulgas kurjategijad, on tuntud oma "kõndiva" käitumise poolest. Samamoodi on porilapsed anatoomiliselt ja käitumuslikult kohanenud, et maal püsida. Nad ei saa mitte ainult uimedega ühest kohast teise liikuda, vaid ka hingata läbi naha nagu kahepaiksed, võimaldades neil madalatest basseinidest lahkudes ellu jääda. Kõndivad sägad on oma hingamissüsteemi nii palju muutnud, et suudavad veest väljas viibitud päevad üle elada. Kuid kõik need on vaid pilgud esimeste tetrapoodide alguse kohta, kuna ükski neist loomadest ei ole täielikult kohanenud eluga maismaal. Et mõista, kuidas tetrapoodid sellise saavutuse saavutasid, peame kõigepealt mõistma tõkkeid, mis seisavad nende elu all mere all ja neid ees ootaval maal.

Vee asemel õhus elamine on täis raskusi. Liikumine on üks probleem, kuigi nagu mitmete suguvõsade areng on näidanud, pole see nii suur probleem, kui võite arvata. Sellegipoolest, kuigi mudakiskurid ja säga näivad kergelt kõndivat, ei saa seda öelda meie esivanemate kohta. Mõned esimesed tetrapoodid, näiteks Ihtüostega olid maal üsna tülikad ja veetsid tõenäoliselt suurema osa ajast veemugavuses. Need esimesed tetrapoodid pärinesid iidsetest kalapüükidest, mida kutsuti Sarcopterygii või Lobe-Finned Fish, millest tänapäeval säilivad vaid vähesed. Nagu nimigi ütleb, on neil loomadel enamiku tänapäevaste kalaliikide õhukeste kiirte asemel lihavad, mõlalaadsed uimed. Need viljalihaga kaetud sagarad olid küpsed jäsemetega kohanemiseks.

Kuid need varajased tetrapoodid pidid välja töötama rohkem kui uue kõndimisviisi - kogu nende luustik pidi muutuma, et kanda rohkem kaalu, kuna vesi toetab massi viisil, mida õhk lihtsalt ei tee. Iga selgroolüli pidi toeks tugevamaks muutuma. Roided ja selgroolülid muutsid kuju ning arenesid täiendava toe ja kaalu paremaks jaotamiseks. Koljud on lahti ühendatud ja kaelad arenevad, et võimaldada pea paremat liikuvust ja neelata kõndimisšokki. Luud olid kadunud ja nihkunud, lihtsustades jäsemeid ja luues viiekohalise mustri, mis peegeldub siiani meie endi kätes ja jalgades. Liigendid liigendatud liikumiseks ja pööratud ettepoole, et võimaldada neljajalgset roomamist. Üldiselt kulus maismaal kõndimiseks sobiva kehaplaani väljatöötamiseks umbes 30 miljonit aastat.

Samal ajal seisid need kohmakad tahavad olla maaelanikud veel ühe takistusega: õhk ise. Kuna lõpused oskasid veest hapnikku ammutada, olid varajased tetrapoodid õhu hingamiseks halvasti varustatud. Kuigi paljud arvavad, et varased tetrapoodid muutsid oma lõpused kopsudeks, pole see tegelikult tõsi - selle asemel kohandus kalade seedesüsteem kopsude moodustamiseks. Esimesed veest lahkunud tetrapoodid hingasid õhku alla neelates ja soolestikus hapnikku imades. Aja jooksul tekkis spetsiaalne tasku, mis võimaldas paremat gaasivahetust. Paljudel kaladel on sarnane struktuur - nn ujumispõis -, mis võimaldab neil vees ujuvust reguleerida, ja seega on paljud oletanud, et tetrapood -kopsud on ujumispõis. Tegelikult on ebaselge, millal tetrapoodidel kopsud tekkisid. Kuigi ainsatel varasematel tetrapoodidel - kopsukaladel - säilinud sugulastel on ka kopsud (kui nende nimi seda ei ütleks), ei tundu paljudel fossiilsetel tetrapoodidel neid olevat, mis viitab sellele, et kopsukalad arendasid iseseisvalt oma võimet õhku hingata. Me teame, et alles umbes 360 miljonit aastat tagasi hingasid tetrapoodid tõeliselt nagu nende kaasaegsed järeltulijad.

Teine häda õhuga on see, et see kipub asju kuivama. Võib-olla olete kuulnud statistikat, et meie kehad on 98% ulatuses veest, kuid hästi arenenud maismaaorganismidena on meil kõrgelt arenenud struktuurid, mis tagavad, et kogu see vesi lihtsalt ei aurustuks. Varased tetrapoodid vajasid nende iseseisvat arendamist. Alguses, nagu ka neist pärit kahepaiksed, jäid paljud tetrapoodid tõenäoliselt veekao vältimiseks niiskete elupaikade juurde. Kuid lõpuks pidid loomad kuivade maade ja kõrbete vallutamiseks leidma teise viisi, kuidas end kuivada. Tõenäoliselt hakkasid paljud varajased tetrapoodid katsetama oma naha veekindluse võimalusi. Veelgi olulisem oli kuivade munade teema. Kahepaiksed lahendavad kuivuse probleemi, pannes oma munad vette, kuid maad vallutanud tetrapoodidel polnud seda luksust.

Maa kuiva olemuse lahendus oli munade sulgemine mitmesse membraanikihti, mida praegu tuntakse amnionimuna all. Isegi meie oma lapsed peegeldavad seda, sest inimlapsed kasvavad endiselt lootekotti, mis ümbritseb looteid, kuigi me enam ei mune. See ülioluline kohanemine võimaldas loomadel katkestada sidemed vesiste elupaikadega ning eristab kahepaiksetest tetrapoodide, sealhulgas roomajate, lindude ja imetajate peamist suguvõsa.

Need üliolulised kohandused tetrapodi luustikele ja anatoomiale võimaldasid neil vallutada lainetest kõrgema maailma. Ilma nende evolutsioonilise leidlikkuseta poleks mitmekesine loomade komplekt, sealhulgas kõik imetajad, seal, kus nad praegu on. Ometi ei mõista me vaevalt ökoloogilisi tingimusi, mis need varajased loomad merest välja ajasid. Kas kuiv maa pakkus lõputut toitu, mida ei tohi mööda lasta? Võib -olla, kuid on tõendeid selle kohta, et meie esivanemad trotsisid kuiva maailma väga varakult, isegi enne enamikku maismaataimi või putukaid, seega on võimalik, et maa oli viljatu. Kas nad pääsesid konkurentsi ja röövloomade eest sügaval? Või oli maa mingil veel määramata põhjusel oluline? Me ei pruugi kunagi teada. Kuid kui me mõtleme oma algusele, peame tunnustama julgeid loomi, kes alustasid mitmekesist evolutsioonilist suguvõsa, millest me oleme osa. Kuigi me ei pruugi kunagi aru saada, miks nad veest lahkusid, oleme tänulikud, et nad seda tegid.

Muud postitused Evolution sarjas:

Foto: mudel Tiktaalik rosea, üks esimesi tetrapoodide esivanemaid. Foto viisakalt Tyler Keillor.


Mitu korda tekkis maapealne elu ookeanist? - bioloogia

Elusolendid (isegi iidsed organismid nagu bakterid) on tohutult keerulised. Kuid kogu see keerukus ei hüpanud ürgsupist täielikult vormituna. Selle asemel sai elu peaaegu kindlasti alguse väikestest sammudest, millest igaüks tugines varem välja kujunenud keerukusele:

    Tekkisid lihtsad orgaanilised molekulid.
    Lihtsad orgaanilised molekulid, mis on sarnased allpool näidatud nukleotiidiga, on elu alustalad ja peavad olema selle päritoluga seotud. Katsed näitavad, et orgaanilisi molekule oleks võinud sünteesida Maa varajases atmosfääris ja sadada ookeanidesse. RNA ja DNA molekulid ning geneetiline materjal kogu eluks ja#151 on lihtsate nukleotiidide pikad ahelad.

Paljud bioloogid oletavad, et see samm viis "RNA maailma", kus RNA tegi palju töid, salvestas geneetilist teavet, kopeeris ennast ja täitis metaboolseid põhifunktsioone. Tänapäeval täidavad neid töid paljud erinevad molekulid (enamasti DNA, RNA ja valgud), kuid RNA maailmas tegi seda kõike RNA.

Self-replication opened the door for natural selection. Once a self-replicating molecule formed, some variants of these early replicators would have done a better job of copying themselves than others, producing more "offspring." These super-replicators would have become more common — that is, until one of them was accidentally built in a way that allowed it to be a super-super-replicator — and then, seda variant would take over. Through this process of continuous natural selection, small changes in replicating molecules eventually accumulated until a stable, efficient replicating system evolved.


Sisu

The term terrestrial is typically applied for species that live primarily on the ground, in contrast to arboreal species, which live primarily in trees.

There are other less common terms that apply to specific groups of terrestrial animals:

  • Saxicolous creatures are rock dwelling. Saxicolous is derived from the Latin word "saxum," meaning a rock.
  • Arenicolous creatures live in the sand.
  • Troglofauna predominantly live in caves.

Terrestrial invasion is one of the most important events in the history of life. [1] [2] [3] Terrestrial lineages evolved in several animal phyla, among which vertebrates, arthropods, and mollusks are representatives of more successful groups of terrestrial animals.

Terrestrial animals do not form a unified clade rather, they share only the fact that they live on land. The transition from an aquatic to terrestrial life has evolved independently and successfully many times by various groups of animals. [3] Most terrestrial lineages originated under a mild or tropical climate during the Paleozoic and Mesozoic, whereas few animals became fully terrestrial during the Cenozoic.

When excluding internal parasites, free living species in terrestrial environments are represented by the following eleven phyla:

    (hairy-backs) live in transient terrestrial water and go dormant during desiccation (wheel animals) live in transient terrestrial water and go dormant during desiccation (roundworms) by going dormant during desiccation (water bears) live in transient terrestrial water and go dormant during desiccation (land planarians) require moist habitats and have restricted range (ribbon worms in Monostilifera) require moist habitats and have restricted range (velvet worms) require moist habitats and have restricted range, the only solely terrestrial phylum (Clitellates) require moist habitats, highly diverse and derived from their marine relatives (fully terrestrial members: Insects, Arachnids, Myriapods, Woodlice, Sandhoppers, and Terrestrial crabs, semi-terrestrial members include Water Fleas, Copepods, and Seed Shrimp) (Gastropods: land snails and slugs) (Tetrapods)

Roundworms, gastrotrichs, tardigrades, rotifers and some smaller species of arthropods and annelids are microscopic animals that require a film of water to live in, and are therefore considered semi-terrestrial. [4] Flatworms, ribbon worms, velvet worms and annelids all depend on more or less moist habitats. The three remaining phyla, arthropods, mollusks, and chordates, all contain species that have adapted totally to dry terrestrial environments, and which have no aquatic phase in their life cycles.

Difficulties Edit

Labeling an animal species "terrestrial" or "aquatic" is often obscure and becomes a matter of judgment. Many animals considered terrestrial have a life-cycle that is partly dependent on being in water. Penguins, seals, and walruses sleep on land and feed in the ocean, yet they are all considered terrestrial. Many insects, e.g. mosquitos, and all terrestrial crabs, as well as other clades, have an aquatic life cycle stage: their eggs need to be laid in and to hatch in water after hatching, there is an early aquatic form, either a nymph or larva.

There are crab species that are completely aquatic, crab species that are amphibious, and crab species that are terrestrial. Fiddler crabs are called "semi-terrestrial" since they make burrows in the muddy substrate, to which they retreat during high tides. When the tide is out, fiddler crabs search the beach for food. The same is true in the mollusca. Many hundreds of gastropod genera and species live in intermediate situations, such as for example, Truncatella. Some gastropods with gills live on land, and others with a lung live in the water.

As well as the purely terrestrial and the purely aquatic animals, there are many borderline species. There are no universally accepted criteria for deciding how to label these species, thus some assignments are disputed.

Fossil evidence has shown that sea creatures, likely related to arthropods, first began to make forays on to land around 530 million years ago. There is little reason to believe, however, that animals first began living reliably on land around this same time period. A more likely hypothesis is that these early arthropods' motivation for venturing on to dry land was to mate (as modern horseshoe crabs do) or lay eggs out of the reach of predators. [5] As time went on, evidence suggests that by approximately 375 million years ago [3] the bony fish best adapted to life in shallow coastal/swampy waters (such as Tiktaalik roseae), were much more viable as amphibians than were their arthropod predecessors. Thanks to relatively strong, muscular limbs (which were likely weight-bearing, thus making them a preferable alternative to traditional fins in extremely shallow water), [6] and lungs which existed in conjunction with gills, Tiktaalik and animals like it were able to establish a strong foothold on land by the end of the Devonian period. As such, they are likely the most recent common ancestor of all modern tetrapods.

Gastropod mollusks are one of the most successful animals that have diversified in the fully terrestrial habitat. [7] They have evolved terrestrial taxa in more than nine lineages. [7] They are commonly referred to as land snails and slugs.

Terrestrial invasion of gastropod mollusks has occurred in Neritopsina, Cyclophoroidea, Littorinoidea, Rissooidea, Ellobioidea, Onchidioidea, Veronicelloidea, Succineoidea, and Stylommatophora, and in particular, each of Neritopsina, Rissooidea and Ellobioidea has likely achieved land invasion more than once. [7]

Most terrestrialization events have occurred during the Paleozoic or Mesozoic. [7] Gastropods are especially unique due to several fully terrestrial and epifaunal lineages that evolved during the Cenozoic. [7] Some members of rissooidean families Truncatellidae, Assimineidae, and Pomatiopsidae are considered to have colonized to land during the Cenozoic. [7] Most truncatellid and assimineid snails amphibiously live in intertidal and supratidal zones from brackish water to pelagic areas. [7] Terrestrial lineages likely evolved from such ancestors. [7] The rissooidean gastropod family Pomatiopsidae is one of the few groups that have evolved fully terrestrial taxa during the late Cenozoic in the Japanese Archipelago only. [7] Shifts from aquatic to terrestrial life occurred at least twice within two Japanese endemic lineages in Japanese Pomatiopsidae and it started in the Late Miocene. [7]

About one-third of gastropod species are terrestrial. [8] In terrestrial habitats they are subjected to daily and seasonal variation in temperature and water availability. [8] Their success in colonizing different habitats is due to physiological, behavioral, and morphological adaptations to water availability, as well as ionic and thermal balance. [8] They are adapted to most of the habitats on Earth. [8] The shell of a snail is constructed of calcium carbonate, but even in acidic soils one can find various species of shell-less slugs. [8] Land-snails, such as Xerocrassa seetzeni and Sphincterochila boissieri, also live in deserts, where they must contend with heat and aridity. [8] Terrestrial gastropods are primarily herbivores and only a few groups are carnivorous. [9] Carnivorous gastropods usually feed on other gastropod species or on weak individuals of the same species some feed on insect larvae or earthworms. [9]

Terrestrial arthropods come from many distinct lineages: both sister Panarthropod phyla Velvet Worms and Water Bears have some degree of terrestrialization, with Velvet Worms being solely terrestrial. Among Euarthropods Myriapods, Arachnids, and Insects all independently adapted to terrestrial life and diversified in very ancient times. More recently three groups of Crustaceans have also independently adapted to terrestrial life: Woodlice Sandhoppers and Terrestrial Crabs, according to the Pancrustacea hypothesis insects are crustaceans, but of a very distant group from either of those groups

Additionally many microscopic crustacean groups like copepods and amphipods (of which Sandhoppers are members) and Seed Shrimp are known to go dormant when dry and live in transient bodies of water

Semi-terrestrial animals are macroscopic animals that rely on very moist environments to thrive, they may be considered a transitional point between true terrestrial animals and aquatic animals. Among vertebrates Amphibians have this characteristic relying on a moist environment and breathing through their moist skin.

Many other animal groups solely have terrestrial animals that live like this: Land Planarians, Land Ribbon Worms, Nematodes, and Land Annelids breathe like this.

Land Annelids are primarily of the group Clitellata and demonstrate many unique terrestrial adaptations especially in their methods of reproduction, they tend towards being simpler than their marine relatives, laking many of the complex appendages polychaetes have.

Velvet worms are prone to desiccation not due to breathing through their skin but due to their spiracles being inefficient at protecting from desiccation, like Clitellates they demonstrate extensive terrestrial adaptations and differences from their marine relatives including live birth. During the Carboniferous the marine relatives of Velvet Worms went extinct, making them the only solely terrestrial phylum.

Many animals live in terrestrial environments by thriving in transient often microscopic bodies of water and moisture, these include Rotifers and Gastrotrichs which lay resilient eggs capable of surviving years in dry environments, and some of which can go dormant themselves. Nematodes are usually microscopic with this lifestyle. Water Bears although only having lifespans of a few months, famously can enter suspended animation during dry or hostile conditions and survive for decades, this allows them to be ubiquitous in terrestrial environments despite needing water to grow and reproduce. Many microscopic crustacean groups like copepods and amphipods (of which Sandhoppers are members) and Seed Shrimp are known to go dormant when dry and live in transient bodies of water too [4]

    (2002). Gaining ground: the origin and evolution of tetrapods. Indiana University Press, 369 pp., ISBN978-0-253-34054-2.
  • Cloudsley-Thompson J. L. (1988). Evolution and adaptation of terrestrial arthropods. Springer, 141 pp., 978-3-540-18188-0.
  • Dejours P. et al. (1987). Comparative physiology: life in water and on land. Liviana Editrice, Italy, 556 pp., 978-0-387-96515-4.
  • Gordon M. S. & Olson E. C. (1995). Invasions of the land: the transitions of organisms from aquatic to terrestrial life. Columbia University Press, 312 pp., 978-0-231-06876-5.
  • Little C. (1983). The colonisation of land: Origins and adaptations of terrestrial animals. Cambridge University Press, Cambridge. 290 pp., 978-0-521-25218-8.
  • Little C. (1990). The terrestrial invasion. An ecophysiological approach to the origin of land animals. Cambridge University Press, Cambridge. 304 pp. 978-0-521-33669-7.
  • Zimmer, Carl (1999). At the Water's Edge : Fish with Fingers, Whales with Legs, and How Life Came Ashore but Then Went Back to Sea. New York: Touchstone. ISBN0684856239 .

This article incorporates CC-BY-2.0 text from the reference [7] and CC-BY-2.5 text from the reference [8] and CC-BY-3.0 text from the reference [9]


Disciplinary divide

Looking closer, the divide between those who support a terrestrial and those supporting an oceanic origin is split between disciplines. Synthetic chemists generally favour a continental origin and geologists and biologist mostly deep-sea hydrothermal vents. Chemists argue it&rsquos impossible to do the chemistry in hydrothermal vents, while biologists argue that the terrestrial chemistry proposed just isn&rsquot like anything seen in biochemistry and doesn&rsquot narrow the gap between geochemistry and biochemistry.

So is there a way to unite the disciplines? &lsquoAt the moment there is not much common ground between these ideas,&rsquo Lane says. Deamer agrees. &lsquoAt this point, all we can say is that everyone has the right to do a plausibility judgement on the basis of their ideas but then they also must do experimental and observational tests.&rsquo

The smaller problems will be solvable &ndash that&rsquos what gets me out of bed in the morning

What is needed is that killer piece of evidence or experiment that could join the dots together and explain how and where life began from a prebiotic world. &lsquoIt would really be a big breakthrough if we can find a ribozyme among all of these trillions of random polymers that we are making,&rsquo suggests Deamer. Ribozymes are RNA catalysts that are part of the cell&rsquos protein-synthesis machinery, but are candidates for the first self-replicating molecules.

Further evidence to support the origins of life in deep sea hydrothermal vents centres on showing a plausible set of metabolic steps leading to complex molecules. At JPL, they are looking at how amino acid behave in their chemical gardens, according to Barge. &lsquoWe are working on making an amino acid, and then seeing whether [amino acids] get stuck in the chimneys and whether you can concentrate them and maybe make some peptides.&rsquo

&lsquoThere are problems and difficulties,&rsquo Lane acknowledges. &lsquoCan we really make carbon dioxide react with hydrogen to make more complex molecules like amino acids and nucleotides? I&rsquom fairly confident we can do that, but I am aware we have not demonstrated that yet.&rsquo Other difficult questions include whether lipid membranes can be stabilised in seawater with its high calcium and magnesium ion concentrations. But says Lane the big problem of the thermodynamic driving force is solved by hydrothermal vents. &lsquoWhich gives me confidence that the smaller problems will be solvable in that context too, even if they look difficult now &ndash that&rsquos what gets me out of bed in the morning.&rsquo

Of course there is one other possibility &ndash that life did not start on earth at all. Panspermia &ndash the theory that life was seeded from space, seems eccentric, but not everybody counts it out. &lsquoAn argument can be made that life actually began on Mars,&rsquo according to Deamer, because it was first to cool down to a temperatures that could support life.

Whether this is the case or not, life elsewhere is certainly feasible. Jupiter&rsquos moon Europa and Saturn&rsquos moon Enceladus are candidates because they both have oceans beneath icy shells. In the next five years, Nasa is planning to send a spaceprobe to both these moons to look for signs of life. Understanding our own origin story could help us work out where to look.

Viited

1 M J Russell, R M Daniel and A J Hall, Terra Nova, 1993, 5, 343 (DOI: 10.1111/j.1365-3121.1993.tb00267.x)

2 W Martin and M J Russell, Philos. Trans. R. Soc. B: Biol. Sci., 2003, 358, 59 (DOI: 10.1098/rstb.2002.1183)

3 L M Barge et al, Angew. Chem. Int. Ed. Engl., 2015, 54, 8184 (DOI: 10.1002/anie.201501663)

4 B Herschy et al, J. Mol. Evol., 2014, 79, 213 (DOI: 10.1007/s00239-014-9658-4)

5 F Klein et al, Proc. Natl Acad. Sci. USA, 2015, 112, 12036 (DOI: 10.1073/pnas.1504674112)

6 L Da Silva, M C Maurel and D Deamer, J. Mol. Evol., 2015, 80, 86 (DOI: 10.1007/s00239-014-9661-9)

7 M W Powner, B Gerland and J D Sutherland, Loodus, 2009, 459, 239 (DOI: 10.1038/nature08013)

8 B H Patel et al, Nat. Chem., 2015, 7, 301 (DOI: 10.1038/nchem.2202)

This article is reproduced with permission from Chemistry World. The article was first published on April 16, 2017.


Crocodile Eyes

MacIver had an intriguing hypothesis, but he needed evidence. He teamed up with Schmitz, who had expertise in interpreting the eye sockets of four-legged “tetrapod” fossils (of which Tiktaalik was one), and the two scientists pondered how best to test MacIver’s idea.

MacIver and Schmitz first made a careful review of the fossil record to track changes in the size of eye sockets, which would indicate corresponding changes in eyes, since they are proportional to socket size. The pair collected 59 early tetrapod skulls spanning the water-to-land transition period that were sufficiently intact to allow them to measure both the eye orbit and the length of the skull. Then they fed those data into a computer model to simulate how eye socket size changed over many generations, so as to gain a sense of the evolutionary genetic drift of that trait.

They found that there was indeed a marked increase in eye size — a tripling, in fact — during the transitional period. The average eye socket size before transition was 13 millimeters, compared to 36 millimeters after. Furthermore, in those creatures that went from water to land and back to the water — like the Mexican cave fish Astyanax mexicanus — the mean orbit size shrank back to 14 millimeters, nearly the same as it had been before.

There was just one problem with these results. Originally, MacIver had assumed that the increase occurred after animals became fully terrestrial, since the evolutionary benefits of being able to see farther on land would have led to the increase in eye socket size. But the shift occurred before the water-to-land transition was complete, even before creatures developed rudimentary digits on their fishlike appendages. So how could being on land have driven the gradual increase in eye socket size.

Early tetrapods probably hunted like crocodiles, waiting with eyes out of the water.

In that case, “it looks like hunting like a crocodile was the gateway drug to terrestriality,” MacIver said. “Just as data comes before action, coming up on land was likely about how the huge gain in visual performance from poking eyes above the water to see an unexploited source of prey gradually selected for limbs.”

This insight is consistent with the work of Jennifer Clack, a paleontologist at the University of Cambridge, on a fossil known as Pederpes finneyae, which had the oldest known foot for walking on land, yet was not a truly terrestrial creature. While early tetrapods were primarily aquatic, and later tetrapods were clearly terrestrial, paleontologists believe this creature likely spent time in water and on land.

After determining how much eye sizes increased, MacIver set out to calculate how much farther the animals could see with bigger eyes. He adapted an existing ecological model that takes into account not just the anatomy of the eye, but other factors such as the surrounding environment. In water, a larger eye only increases the visual range from just over six meters to nearly seven meters. But increase the eye size in air, and the improvement in range goes from 200 meters to 600 meters.

MacIver and Schmitz ran the same simulation under many different conditions: daylight, a moonless night, starlight, clear water and murky water. “It doesn’t matter,” MacIver said. “In all cases, the increment [in air] is huge. Even if they were hunting in broad daylight in the water and only came out on moonless nights, it’s still advantageous for them, vision-wise.”

Using quantitative tools to help explain patterns in the fossil record is something of a novel approach to the problem, but a growing number of paleontologists and evolutionary biologists, like Schmitz, are embracing these methods.

“So much of paleontology is looking at fossils and then making up narratives on how the fossils might have fit into a particular environment,” said John Long, a paleobiologist at Flinders University in Australia who studies how fish evolved into tetrapods. “This paper has very good hard experimental data, testing vision in different environments. And that data does fit the patterns that we see in these fish.”

Schmitz identified two key developments in the quantitative approach over the past decade. First, more scientists have been adapting methods from modern comparative biology to fossil record analysis, studying how animals are related to each other. Second, there is a lot of interest in modeling the biomechanics of ancient creatures in a way that is actually testable — to determine how fast dinosaurs could run, for instance. Such a model-based approach to interpreting fossils can be applied not only to biomechanics but to sensory function — in this case, it explained how coming out of the water affected the vision of the early tetrapods.

A model of Tiktaalik roseae, a 375-million-year-old transitional fossil that had a neck — unheard of for a fish — and both lungs and gills.

“Both approaches bring something unique, so they should go hand in hand,” Schmitz said. “If I had done the [eye socket size] analysis just by itself, I would be lacking what it could actually mean. Eyes do get bigger, but why?” Sensory modeling can answer this kind of question in a quantitative, rather than qualitative, way.

Schmitz plans to examine other water-to-land transitions in the fossil record — not just that of the early tetrapods — to see if he can find a corresponding increase in eye size. “If you look at other transitions between water and land, and land back to water, you see similar patterns that would potentially corroborate this hypothesis,” he said. For example, the fossil record for marine reptiles, which rely heavily on vision, should also show evidence for an increase in eye socket size as they moved from water to land.


From Fins to Limbs and Water to Land: Evolution of Terrestrial Movement in Early Tetrapods

The aerial scene depicts two Late Devonian early tetrapods — Ichthyostega and Acanthostega — coming out of the water to move on land. Footprints trail behind the animals to show a sense of movement. Credit: Davide Bonadonna

The water-to-land transition is one of the most important and inspiring major transitions in vertebrate evolution. And the question of how and when tetrapods transitioned from water to land has long been a source of wonder and scientific debate.

Early ideas posited that drying-up-pools of water stranded fish on land and that being out of water provided the selective pressure to evolve more limb-like appendages to walk back to water. In the 1990s newly discovered specimens suggested that the first tetrapods retained many aquatic features, like gills and a tail fin, and that limbs may have evolved in the water before tetrapods adapted to life on land. There is, however, still uncertainty about when the water-to-land transition took place and how terrestrial early tetrapods really were.

A paper published today (November 25, 2020) in Loodus addresses these questions using high-resolution fossil data and shows that although these early tetrapods were still tied to water and had aquatic features, they also had adaptations that indicate some ability to move on land. Although, they may not have been very good at doing it, at least by today’s standards.

Lead author Blake Dickson, PhD 󈧘 in the Department of Organismic and Evolutionary Biology at Harvard University, and senior author Stephanie Pierce, Thomas D. Cabot Associate Professor in the Department of Organismic and Evolutionary Biology and curator of vertebrate paleontology in the Museum of Comparative Zoology at Harvard University, examined 40 three-dimensional models of fossil humeri (upper arm bone) from extinct animals that bridge the water-to-land transition.

Three major stages of humerus shape evolution: from the blocky humerus of aquatic fish, to the L-shape humerus of transitional tetrapods, and the twisted humerus of terrestrial tetrapods. Columns (left to right) = aquatic fish, transitional tetrapod, and terrestrial tetrapod. Rows = Top: extinct animal silhouettes Middle: 3D humerus fossils Bottom: landmarks used to quantified shape. Credit: Courtesy of Blake Dickson

“Because the fossil record of the transition to land in tetrapods is so poor we went to a source of fossils that could better represent the entirety of the transition all the way from being a completely aquatic fish to a fully terrestrial tetrapod,” said Dickson.

Two thirds of the fossils came from the historical collections housed at Harvard’s Museum of Comparative Zoology, which are sourced from all over the world. To fill in the missing gaps, Pierce reached out to colleagues with key specimens from Canada, Scotland, and Australia. Of importance to the study were new fossils recently discovered by co-authors Dr. Tim Smithson and Professor Jennifer Clack, University of Cambridge, UK, as part of the TW:eed project, an initiative designed to understand the early evolution of land-going tetrapods.

The researchers chose the humerus bone because it is not only abundant and well preserved in the fossil record, but it is also present in all sarcopterygians — a group of animals which includes coelacanth fish, lungfish, and all tetrapods, including all of their fossil representatives. “We expected the humerus would carry a strong functional signal as the animals transitioned from being a fully functional fish to being fully terrestrial tetrapods, and that we could use that to predict when tetrapods started to move on land,” said Pierce. “We found that terrestrial ability appears to coincide with the origin of limbs, which is really exciting.”

The evolutionary pathway and shape change from an aquatic fish humerus to a terrestrial tetrapod humerus. Credit: Courtesy of Blake Dickson

The humerus anchors the front leg onto the body, hosts many muscles, and must resist a lot of stress during limb-based motion. Because of this, it holds a great deal of critical functional information related to an animal’s movement and ecology. Researchers have suggested that evolutionary changes in the shape of the humerus bone, from short and squat in fish to more elongate and featured in tetrapods, had important functional implications related to the transition to land locomotion. This idea has rarely been investigated from a quantitative perspective — that is, until now.

When Dickson was a second-year graduate student, he became fascinated with applying the theory of quantitative trait modeling to understanding functional evolution, a technique pioneered in a 2016 study led by a team of paleontologists and co-authored by Pierce. Central to quantitative trait modeling is paleontologist George Gaylord Simpson’s 1944 concept of the adaptive landscape, a rugged three-dimensional surface with peaks and valleys, like a mountain range. On this landscape, increasing height represents better functional performance and adaptive fitness, and over time it is expected that natural selection will drive populations uphill towards an adaptive peak.

Dickson and Pierce thought they could use this approach to model the tetrapod transition from water to land. They hypothesized that as the humerus changed shape, the adaptive landscape would change too. For instance, fish would have an adaptive peak where functional performance was maximized for swimming and terrestrial tetrapods would have an adaptive peak where functional performance was maximized for walking on land. “We could then use these landscapes to see if the humerus shape of earlier tetrapods was better adapted for performing in water or on land” said Pierce.

“We started to think about what functional traits would be important to glean from the humerus,” said Dickson. “Which wasn’t an easy task as fish fins are very different from tetrapod limbs.” In the end, they narrowed their focus on six traits that could be reliably measured on all of the fossils including simple measurements like the relative length of the bone as a proxy for stride length and more sophisticated analyses that simulated mechanical stress under different weight bearing scenarios to estimate humerus strength.

“If you have an equal representation of all the functional traits you can map out how the performance changes as you go from one adaptive peak to another,” Dickson explained. Using computational optimization the team was able to reveal the exact combination of functional traits that maximized performance for aquatic fish, terrestrial tetrapods, and the earliest tetrapods. Their results showed that the earliest tetrapods had a unique combination of functional traits, but did not conform to their own adaptive peak.

“What we found was that the humeri of the earliest tetrapods clustered at the base of the terrestrial landscape,” said Pierce. “indicating increasing performance for moving on land. But these animals had only evolved a limited set of functional traits for effective terrestrial walking.”

The researchers suggest that the ability to move on land may have been limited due to selection on other traits, like feeding in water, that tied early tetrapods to their ancestral aquatic habitat. Once tetrapods broke free of this constraint, the humerus was free to evolve morphologies and functions that enhanced limb-based locomotion and the eventual invasion of terrestrial ecosystems

“Our study provides the first quantitative, high-resolution insight into the evolution of terrestrial locomotion across the water-land transition,” said Dickson. “It also provides a prediction of when and how [the transition] happened and what functions were important in the transition, at least in the humerus.”

“Moving forward, we are interested in extending our research to other parts of the tetrapod skeleton,” Pierce said. “For instance, it has been suggested that the forelimbs became terrestrially capable before the hindlimbs and our novel methodology can be used to help test that hypothesis.”

Dickson recently started as a Postdoctoral Researcher in the Animal Locomotion lab at Duke University, but continues to collaborate with Pierce and her lab members on further studies involving the use of these methods on other parts of the skeleton and fossil record.

Reference: “Functional Adaptive Landscapes Predict Terrestrial Capacity at the Origin of Limbs” by Blake V. Dickson, Jennifer A. Clack, Timothy R. Smithson and Stephanie E. Pierce, 25 November 2020. Loodus.
DOI: 10.1038/s41586-020-2974-5


How life may have first emerged on Earth: Foldable proteins in a high-salt environment

A structural biologist at the Florida State University College of Medicine has made discoveries that could lead scientists a step closer to understanding how life first emerged on Earth billions of years ago.

Professor Michael Blaber and his team produced data supporting the idea that 10 amino acids believed to exist on Earth around 4 billion years ago were capable of forming foldable proteins in a high-salt (halophile) environment. Such proteins would have been capable of providing metabolic activity for the first living organisms to emerge on the planet between 3.5 and 3.9 billion years ago.

The results of Blaber's three-year study, which was built around investigative techniques that took more than 17 years to develop, are published in the journal Rahvusliku Teaduste Akadeemia toimetised.

The first living organisms would have been microscopic, cell-like organizations capable of replicating and adapting to environmental conditions -- a humble beginning to life on Earth.

"The current paradigm on the emergence of life is that RNA came first and in a high-temperature environment," Blaber said. "The data we are generating are much more in favor of a protein-first view in a halophile environment."

The widely accepted view among scientists is that RNA, found in all living cells, would have likely represented the first molecules of life, hypothesizing an "RNA-first" view of the origin of living systems from non-living molecules. Blaber's results indicate that the set of amino acids produced by simple chemical processes contains the requisite information to produce complex folded proteins, which supports an opposing "protein-first" view.

Another prevailing view holds that a high-temperature (thermophile) environment, such as deep-ocean thermal vents, may have been the breeding ground for the origin of life. "The halophile, or salt-loving, environment has typically been considered one that life adapted into, not started in," Blaber said. "Our study of the prebiotic amino acids and protein design and folding suggests the opposite."

Without the ability to fold, proteins would not be able to form the precise structures essential for functions that sustain life as we know it. Folding allows proteins to take on a globular shape through which they can interact with other proteins, perform specific chemical reactions, and adapt to enable organisms to exploit a given environment.

"There are numerous niches that life can evolve into," Blaber said. "For example, extremophiles are organisms that exist in high temperatures, high acidity, extreme cold, extreme pressure and extreme salt and so on. For life to exist in such environments it is essential that proteins are able to adapt in those conditions. In other words, they have to be able to fold."

Comet and meteorite fragments, like those that recently struck in the Urals region of Russia, have provided evidence regarding the arrival of amino acids on Earth. Such fragments predate Earth and would have been responsible for delivering a set of 10 prebiotic (before life) amino acids, whose origins are in the formation of our solar system.

Today the human body uses 20 common amino acids to make all its proteins. Ten of those emerged through biosynthetic pathways -- the way living systems evolve. Ten -- the prebiotic set -- can be made by chemical reactions without requiring any living system or biosynthetic pathway.

Scientific evidence exists to support many elements in theories of abiogenesis (the emergence of life), including the time frame (around 3.5 to 3.9 billion years ago) and the conditions on Earth and in its atmosphere at that time. Earth would have been made up of volcanic land masses (the beginning of the formation of continents), salty oceans and fresh-water ponds, along with a hot (around 80 degrees Celsius) and steamy atmosphere comprising carbon dioxide and nitrogen. Oxygen would have come later as a by-product of green plant life and bacteria that emerged.

Using a technique called top-down symmetric deconstruction, Blaber's lab has been able to identify small peptide building blocks capable of spontaneous assembly into specific and complex protein architectures. His recent work explored whether such building blocks can be composed of only the 10 prebiotic amino acids and still fold.

His team has achieved foldability in proteins down to 12 amino acids -- about 80 percent of the way to proving his hypothesis.

If Blaber's theory holds, scientists may refocus where they look for evidence in the quest to understand where, and how, life began.

"Rather than a curious niche that life evolved into, the halophile environment now may take center stage as the likely location for key aspects of abiogenesis," he said.

"Likewise, the role of the formation of proteins takes on additional importance in the earliest steps in the beginnings of life on Earth."


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