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Wat is 'n paar voorbeelde van kruising tussen spesies wat met 5 miljoen jaar of meer geskei word?

Wat is 'n paar voorbeelde van kruising tussen spesies wat met 5 miljoen jaar of meer geskei word?


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Interteling word beperk deurdat die nageslag die genepoel van die ouers deel. Wat is 'n paar voorbeelde van kruisteling wat plaasgevind het, waar die twee spesies met 5 miljoen jaar of meer geskei is?


Ek hang 'n bietjie af van wat jy bedoel met kruising. My antwoord sal in elk geval nie heeltemal bevredigend wees nie.

Horisontale geenoordrag

As jy verwys na enige uitruil van genetiese materiaal as, kan jy 'n blik op Salzberg et al. (2001) wat 40 gene rapporteer wat horisontaal van bakterieë oorgedra is.

Elysia chlorotica is seeslak wat in staat is tot fotosintese danksy 'n horisontale geenoordrag (Schwartz et al., 2014)

Tipiese kruising

As jy aan 'n soort van meer algemene tipe inteling dink, sal jy dalk wil oorweeg

muil wat die baster is tussen 'n manlike donkie en 'n vroulike perd. Donkie en perd deel 'n gemeenskaplike voorouer wat ongeveer 4,5 miljoen jaar oud is (volgens Live Science). Muile is dikwels steriel, maar sommige van hulle was vrugbaar (Rong et al. 1988; sien ook hierdie artikel uit die Denver-pos; dankie @iayork)

Platanus × acerifolia is 'n baster tussen Platanus orientalis en Platanus occidentalis. Platanus is 'n taamlik ou genera met fossiele van 115 miljoen jaar oud gevind (van wiki). Ek weet egter nie hoe oud die gemeenskaplike voorouer tussen is nie P. orientalis en P. occidentalis

Daar is baie ander voorbeelde in wikipedia > baster, maar ek kon nie 'n goeie skatting van gemeenskaplike afkoms vind vir diegene wat 'n gemeenskaplike voorouer ouer as 5 miljoen jaar gehad het nie, maar dit bestaan ​​waarskynlik gegewe dat die muilvoorbeeld reeds 4,5 miljoen jaar oud is.


Hier is 'n bietjie navorsing oor 'n varingbaster wat na 60 miljoen jaar konvergeer:

http://www.iflscience.com/plants-and-animals/distant-species-produce-hybrid-60-million-years-after-their-split/

Sommige giste in 'n brouery blykbaar na 10-20 miljoen jaar: http://www.iflscience.com/plants-and-animals/distant-species-produce-hybrid-60-million-years-after-their-split/

'n Bok Skaap Geep na 7 miljoen jaar: https://whyevolutionistrue.wordpress.com/2014/08/03/a-new-geep-a-sheepgoat-hybrid/

vir tydsberekening van evolusie http://www.timetree.org/


Sjimpansees wat met bonobo's gekruis is, onthul verrassende studie

Net soos vroeë mense met Neanderdalmense gekruis het, blyk dit dat ons naaste familie ook 'n paar prettige tye met mekaar gehad het. Volgens 'n nuwe genetiese ontleding kom een ​​persent van die sjimpansee-genoom van bonobo's.

Sjimpansees en bonobo's is die enigste twee spesies in die genus Pan en hulle verteenwoordig ons naaste genetiese familie. Beide spesies bewoon die Kongo-oerwoud in Afrika suid van die Sahara, en in sommige gebiede is hul habitatte baie naby aan mekaar – alhoewel geskei deur die Kongo-rivier. Ten spyte daarvan dat hulle intelligent is en verskeie menslike eienskappe vertoon, word hulle bedreig en word hulle dikwels gejag of as gevangenes aangehou. Dit is die hoofrede waarom die studie uitgevoer is – nie om verbande tussen die twee spesies te identifiseer nie, maar om die sjimpansees te help bewaar.

“Dit is die grootste ontleding van sjimpansee-genome tot nog toe en toon dat genetika gebruik kan word om redelik presies op te spoor waar in die natuur 'n sjimpansee vandaan kom,” het dr Chris Tyler Smith, van die Wellcome Trust Sanger Institute, gesê.

“Dit kan die vrylating van onwettig gevange sjimpansees help terug na die regte plek in die natuur en sleutelbewyse verskaf vir optrede teen die ontvoerders.”

Tog het die navorsing interessante biologiese inligting opgelewer. Navorsers het bevind dat die twee spesies tussen 1,5 en 2 miljoen jaar gelede van 'n gemeenskaplike voorouer afgewyk het. Maar sommige sjimpanseebevolkings het 'n verrassing gehad: bonobo-DNS wat in hul eie gene ingebed is.

“Ons het gevind dat sentrale en oostelike sjimpansees aansienlik meer genetiese materiaal met bonobo's deel as die ander sjimpansee-subspesies. Hierdie sjimpansees het ten minste 1% van hul genome afkomstig van bonobos. Dit wys dat daar nie 'n skoon skeiding was nie, maar dat die aanvanklike verskil gevolg is deur af en toe episodes van vermenging tussen die spesies.

Baie bioloë het’ nie eens oorweeg om kruising tussen die twee spesies so dit was nogal 'n verrassing, maar die resultate is redelik duidelik. Daar is 'n duidelike ooreenkoms tussen wat studie oor ons eie spesie gevind het — vroeë mense en Neanderdalmense het van dieselfde voorouer afgewyk, maar hulle het ook lank gekruis. Nie-Afrikaanse mense dra 'n beduidende deel van Neanderdal-DNS in hulle. Dr Tomàs Marquès-Bonet, leier van die studie van die Instituut vir Biologiese Evolusie (Universiteit Pompeu Fabra en CSIC), Barcelona, ​​het gesê:

“Dit is die eerste studie wat aan die lig gebring het dat antieke genevloei-gebeure plaasgevind het onder die lewende spesies naaste aan mense — die bonobo's en sjimpansees. Dit impliseer dat suksesvolle teling tussen nabye spesies eintlik wydverspreid kon gewees het in die voorouers van mense en lewende ape.”


Vroegste kruisteelgebeurtenis tussen antieke menslike bevolkings ontdek

BEELD: 'n Evolusionêre boom wat vier voorgestelde episodes van geenvloei insluit. Die voorheen onbekende gebeurtenis 744 372 jaar gelede (oranje) dui daarop dat kruisteling tussen super-argaïese en Neanderdal-Denisovan-voorouers in Eurasië plaasgevind het. sien meer

Krediet: Aangepas deur Alan Rogers

Vir drie jaar het die antropoloog Alan Rogers probeer om 'n evolusionêre legkaart op te los. Sy navorsing ontknoop miljoene jare van menslike evolusie deur DNS-stringe van antieke menslike spesies bekend as hominiene te ontleed. Soos baie evolusionêre genetici, vergelyk Rogers hominiengenome op soek na genetiese patrone soos mutasies en gedeelde gene. Hy ontwikkel statistiese metodes wat die geskiedenis van antieke menslike bevolkings aflei.

In 2017 het Rogers 'n studie gelei wat bevind het dat twee afstammelinge van antieke mense, Neanderdalmense en Denisovans, baie vroeër geskei het as wat voorheen gedink is en 'n bottelnek bevolkingsgrootte voorgestel het. Dit het 'n mate van kontroversie veroorsaak - antropoloë Mafessoni en Prüfer het aangevoer dat hul metode om die DNA te ontleed verskillende resultate opgelewer het. Rogers het ingestem, maar het besef dat nie een van die metodes die genetiese data baie goed verduidelik het nie.

"Albei ons metodes onder bespreking het iets gemis, maar wat?" vra Rogers, professor in antropologie aan die Universiteit van Utah.

Die nuwe studie het daardie legkaart opgelos en sodoende het dit die vroegste bekende kruisteelgebeurtenis tussen antieke menslike bevolkings gedokumenteer - 'n groep bekend as die "super-argaïeke" in Eurasië wat ongeveer 700 000 jaar gelede met 'n Neanderdal-Denisovan-voorouer gekruis is. Die gebeurtenis was tussen twee bevolkings wat meer ver verwant was as enige ander wat aangeteken is. Die skrywers het ook 'n hersiene tydlyn vir menslike migrasie uit Afrika en Eurasië voorgestel. Die metode vir die ontleding van antieke DNS bied 'n nuwe manier om verder terug te kyk na die menslike geslag as ooit tevore.

“Ons het nog nooit geweet van hierdie episode van kruisteling nie en ons kon nog nooit die grootte van die super-argaïese bevolking skat nie,” het Rogers, hoofskrywer van die studie, gesê. "Ons werp net lig op 'n interval op menslike evolusionêre geskiedenis wat voorheen heeltemal donker was."

Die referaat is op 20 Februarie 2020 in die joernaal gepubliseer Wetenskap vooruitgang.

Uit Afrika en kruisteling

Rogers het die maniere waarop mutasies onder moderne Afrikane en Europeërs, en antieke Neanderdalmense en Denisovans gedeel word, bestudeer. Die patroon van deel het vyf episodes van kruisteling geïmpliseer, insluitend een wat voorheen onbekend was. Die nuut ontdekte episode behels die kruising van meer as 700 000 jaar gelede tussen 'n ver-verwante "super-argaïese" bevolking wat ongeveer twee miljoen jaar gelede van alle ander mense geskei het, en die voorouers van Neanderdalmense en Denisovans.

Die super-argaïese en Neanderdal-Denisovan-voorouerbevolkings was meer verwant as enige ander paar menslike bevolkings wat voorheen bekend was om te kruis. Moderne mense en Neanderdalmense was byvoorbeeld vir ongeveer 750 000 jaar geskei toe hulle gekruis het. Die super-argaïese en Neanderdal-Denisovan-voorouers was vir meer as 'n miljoen jaar geskei.

"Hierdie bevindinge oor die tydsberekening waarop kruisteling in die menslike geslag plaasgevind het, sê iets oor hoe lank dit neem vir reproduktiewe isolasie om te ontwikkel," het Rogers gesê.

Die skrywers het ander leidrade in die genome gebruik om te skat wanneer die antieke menslike bevolkings geskei het en hul effektiewe bevolkingsgrootte. Hulle het geskat die super-argaïese het sowat twee miljoen jaar gelede in sy eie spesie geskei. Dit stem ooreen met menslike fossielbewyse in Eurasië wat 1,85 miljoen jaar oud is.

Die navorsers het ook voorgestel dat daar drie golwe van menslike migrasie na Eurasië was. Die eerste was twee miljoen jaar gelede toe die super-argaïese na Eurasië migreer en uitgebrei het tot 'n groot bevolking. Toe 700 000 jaar gelede, het Neanderdal-Denisovan-voorouers na Eurasië migreer en vinnig met die afstammelinge van die super-argaïeë gekruis. Uiteindelik het moderne mense uitgebrei na Eurasië 50 000 jaar gelede waar ons weet dat hulle met ander antieke mense gekruis het, insluitend met die Neanderdalmense.

"Ek het die afgelope paar jaar gewerk aan hierdie ander manier om genetiese data te ontleed om meer oor geskiedenis uit te vind," het Rogers gesê. "Dit is net verblydend dat jy met 'n ander manier vorendag kom om na die data te kyk en jy uiteindelik dinge ontdek wat mense nie met ander metodes kon sien nie."

Nathan S. Harris en Alan A. Achenbach van die Departement Antropologie aan die Universiteit van Utah het ook bygedra tot die studie.

Vrywaring: AAAS en EurekAlert! is nie verantwoordelik vir die akkuraatheid van nuusvrystellings wat op EurekAlert geplaas is nie! deur bydraende instellings of vir die gebruik van enige inligting deur die EurekAlert-stelsel.


Voorouers van Neanderdallers en Denisovans wat met 'Superargaïese' Hominin verweef is

'n Nuwe studie deur navorsers van die Departement Antropologie aan die Universiteit van Utah toon dat meer as 700 000 jaar gelede die voorouers van Neanderdallers en Denisovans met hul Eurasiese voorgangers — lede van 'n 'superargaïese' bevolking geskei het van ander mense ongeveer 2 miljoen jaar gelede.

Vroeë Neanderdallers wat by Sima de los Huesos, 'n grotplek in Atapuerca-berge, Spanje, gewoon het. Beeldkrediet: © Kennis & Kennis / Madrid Scientific Films.

"Ons het nog nooit geweet van hierdie episode van kruising nie en ons kon nog nooit die grootte van die superargaïese bevolking skat nie," het professor Alan Rogers, die hoofskrywer van die studie, gesê.

"Ons werp net lig op 'n interval op menslike evolusionêre geskiedenis wat voorheen heeltemal donker was."

Professor Rogers en kollegas het die maniere waarop mutasies gedeel word tussen moderne Afrikane en Europeërs, en antieke Neanderdalmense en Denisovane bestudeer.

Die patroon van deel het vyf episodes van kruisteling geïmpliseer, insluitend een wat voorheen onbekend was.

Die nuut ontdekte episode behels die kruising van meer as 700 000 jaar gelede tussen 'n ver-verwante 'superargaïese' bevolking wat ongeveer 2 miljoen jaar gelede van alle ander mense geskei het, en die voorvaders van Neanderdalmense en Denisovans.

Die superargaïese en Neanderdal-Denisovan-voorouerbevolkings was verder verwant as enige ander paar menslike bevolkings wat voorheen bekend was om te kruis. Moderne mense en Neanderdalmense was byvoorbeeld vir ongeveer 750 000 jaar geskei toe hulle gekruis het.

Die superargaïese en Neanderdal-Denisovan-voorouers was vir meer as 'n miljoen jaar geskei.

"Hierdie bevindinge oor die tydsberekening waarop kruisteling in die menslike geslag plaasgevind het, sê iets oor hoe lank dit neem vir reproduktiewe isolasie om te ontwikkel," het professor Rogers gesê.

Die navorsers het ander leidrade in die genome gebruik om te skat wanneer die antieke menslike bevolkings geskei het en hul effektiewe bevolkingsgrootte.

Hulle het geskat die superargaïese het sowat 2 miljoen jaar gelede in sy eie spesie geskei. Dit stem ooreen met menslike fossielbewyse in Eurasië wat 1,85 miljoen jaar oud is.

'n Evolusionêre boom wat vier voorgestelde episodes van geenvloei insluit, die voorheen onbekende gebeurtenis 744 372 jaar gelede (oranje) dui daarop dat kruisteling tussen superargaë en Neanderdal-Denisovan-voorouers in Eurasië plaasgevind het. Beeldkrediet: Rogers et al, doi: 10.1126/sciadv.aay5483.

Die wetenskaplikes het ook voorgestel dat daar drie golwe van menslike migrasie na Eurasië was.

Die eerste was 2 miljoen jaar gelede toe die superargaë na Eurasië migreer en uitgebrei het tot 'n groot bevolking.

Toe 700 000 jaar gelede, het Neanderdal-Denisovan-voorouers na Eurasië migreer en vinnig met die afstammelinge van die superargaïeë gekruis.

Uiteindelik het moderne mense uitgebrei na Eurasië 50 000 jaar gelede waar ons weet dat hulle met ander antieke mense gekruis het, insluitend met die Neanderdalmense.

"Ek het die afgelope paar jaar gewerk aan hierdie ander manier om genetiese data te ontleed om uit te vind oor geskiedenis," het professor Rogers gesê.

"Dit is net verblydend dat jy met 'n ander manier vorendag kom om na die data te kyk en jy uiteindelik dinge ontdek wat mense nie met ander metodes kon sien nie."


Kon kruisteling tussen mense en Neanderdalmense tot 'n verbeterde menslike brein gelei het?

'n Nuwe studie dui daarop dat menslike evolusie nie net 'n kwessie was van spontane voordelige mutasies wat binne die menslike geslag ontstaan ​​het nie.

Het paring tussen 'n antieke mens en 'n Neanderdal - miskien net in 'n enkele geval - 'n geenvariant in die menslike bevolking ingebring wat die menslike breinfunksie verbeter het? Dié vraag is die kern van 'n nuwe studie deur navorsers by die Howard Hughes Mediese Instituut en die Universiteit van Chicago.

Die nuwe navorsing, wat aanlyn gepubliseer is gedurende die week van 6 November 2006, in die vroeë uitgawe van die Verrigtinge van die National Academy of Sciences (PNAS), dui daarop dat menslike evolusie nie net 'n kwessie was van spontane voordelige mutasies wat binne die menslike geslag ontstaan ​​het nie. Menslike evolusie is moontlik ook beïnvloed deur kruising met ander Homo spesies, wat geenvariante bekendgestel het, bekend as allele, wat voordelig is vir menslike voortplantingsfiksheid, het gesê die studie se senior skrywer Bruce T. Lahn, 'n Howard Hughes Mediese Instituut navorser aan die Universiteit van Chicago.

Hierdie bevindinge is geensins 'n definitiewe bewys dat 'n Neanderdaller die bron van die oorspronklike kopie van die D-alleel was nie. Ons bewyse toon egter dat dit een van die beste kandidate is.

Die wetenskaplikes het gesê dat hulle die mees robuuste genetiese bewyse tot nog toe ontwikkel het wat daarop dui dat mense en Neanderdalmense deurmekaar geteel is toe hulle duisende jare gelede saam bestaan ​​het. Die kruisteling-hipotese staan ​​in kontras met ten minste een prominente teorie wat beweer dat geen kruising plaasgevind het toe die twee spesies mekaar teëgekom het nie.

Lahn het aan die studies saamgewerk met Patrick D. Evans, Nitzan Mekel-Bobrov, Eric J. Vallender en Richard R. Hudson, almal van die Universiteit van Chicago.

Lahn en sy kollegas het in hul studies 'n gedetailleerde statistiese ontleding van die DNA-volgordestruktuur van die geen gedoen. mikrokefalien, wat bekend is dat dit 'n rol speel in die regulering van breingrootte by mense. Mutasies in die menslike geen veroorsaak die ontwikkeling van 'n baie kleiner brein, 'n toestand wat mikrokefalie genoem word.

Vroeëre studies deur Lahn se groep het bewyse gelewer dat die mikrokefalien geen het twee afsonderlike klasse allele. Een klas, genoem die D-allele, bestaan ​​uit 'n groep allele met redelik soortgelyke DNS-volgordes. Die ander klas word die nie-D-allele genoem. Lahn en kollegas het voorheen getoon dat alle moderne kopieë van die D-allele ongeveer 37 000 jaar gelede uit 'n enkele stamvader-kopie ontstaan ​​het, wat toe vinnig in frekwensie toegeneem het en nou in ongeveer 70 persent van die wêreld se bevolking teenwoordig is. Hierdie vinnige toename in frekwensie dui daarop dat die D-allele positiewe seleksie ondergaan het in die onlangse geskiedenis van mense. Dit beteken dat hierdie allele 'n fiksheidsvoordeel verleen het aan diegene wat een van hulle besit, sodat hierdie mense effens groter reproduktiewe sukses behaal het as mense wat nie die allele gehad het nie, het Lahn gesê.

Die skatting dat alle moderne kopieë van die D-allele ongeveer 37 000 jaar gelede van 'n enkele stamvader-kopie afstam, is gebaseer op die meting van volgordeverskil tussen verskillende kopieë van die D-allele. Soos 'n kopie van 'n geen van een generasie na die volgende oorgedra word, word mutasies teen 'n bestendige tempo ingebring, sodat 'n sekere aantal generasies later die afstammelinge van die geen gemiddeld van mekaar in DNS-volgorde sal verskil met 'n sekere bedrag. Hoe groter die aantal generasies, hoe meer DNS-volgorde verskil sal daar tussen twee afstammelinge kopieë wees, het Lahn gesê. Die hoeveelheid volgordeverskil tussen verskillende kopieë van 'n geen kan dus gebruik word om die hoeveelheid evolusionêre tyd wat verloop het sedert die twee kopieë van hul gemeenskaplike stamvader afstam, te skat.

In die nuwe studies berig in PNAS, het die navorsers gedetailleerde volgordevergelykings tussen die D-allele en die nie-D-allele van uitgevoer mikrokefalien. Die wetenskaplikes het vasgestel dat hierdie twee klasse allele waarskynlik vir ongeveer 1,1 miljoen jaar in twee afsonderlike afstammelinge ontwikkel het - met die nie-D-allele wat in die Homo sapiens afkoms en die D-allele het ontwikkel in 'n argaïese, en nou uitgesterf, Homo afkoms. Toe, ongeveer 37 000 jaar gelede, het 'n kopie van die D-alleel gekruis van die argaïese Homo afkoms na mense, moontlik deur kruising tussen lede van die twee bevolkings. Hierdie kopie het daarna in mense versprei vanaf 'n enkele kopie toe dit die eerste keer in mense gekruis het na 'n alleel wat vandag in 'n geskatte 70 persent van die bevolking wêreldwyd teenwoordig is.

Die skatting van 1,1 miljoen jaar wat die twee geslagte skei, is gebaseer op die hoeveelheid volgordeverskil tussen die D- en die nie-D-allele. Alhoewel die identiteit van hierdie argaïese Homo afkoms moet nog bepaal word, die navorsers argumenteer dat 'n waarskynlike kandidaat die Neanderdalmense is. Die skeiding van 1,1 miljoen jaar tussen mense en hierdie argaïese Homo spesie is min of meer in ooreenstemming met vorige skattings van die hoeveelheid evolusionêre tyd wat die skei Homo sapiens geslag en die Neanderdal-lyn, het Lahn gesê. Verder is die tyd van introgressie van die D-alleel in mense - ongeveer 37 000 jaar gelede - toe mense en Neanderdalmense in baie dele van die wêreld saambestaan ​​het.

Lahn het gesê die groep se data dui daarop dat die kruising waarskynlik nie 'n deeglike genetiese vermenging sou wees nie, maar eerder 'n seldsame - en miskien selfs 'n enkele - gebeurtenis wat die voorvaderlike D-alleel bekendgestel het wat voorheen in hierdie ander teenwoordig was. Homo spesies in die menslike lyn.

"Hierdie bevindinge is geensins 'n definitiewe bewys dat 'n Neanderdaller die bron van die oorspronklike kopie van die D-alleel was nie," het Lahn gesê. “Ons bewyse toon egter dat dit een van die beste kandidate is. Die tydlyn – insluitend die introgressie van die alleel in mense 37 000 jaar gelede en die oorsprong daarvan in ’n afstamming wat 1,1 miljoen jaar gelede met die menslike lyn geskei het – stem ooreen met die kontak tussen, en die evolusionêre geskiedenis van, Neanderdalmense en mense.

"En 'n derde lyn van bewyse, alhoewel swakker, is dat die D-allele baie meer algemeen voorkom in Eurasië en laer in Afrika suid van die Sahara, wat ooreenstem met 'n oorsprong in eersgenoemde gebied. En ons weet dat Neanderdalmense buite Afrika ontwikkel het,” het Lahn gesê.

Lahn het ook gesê dat hoewel die ontwrigting van die mikrokefalien geen in mense lei tot kleiner breine, die rol van die D-allele in breinevolusie bly onbekend. "Die D-allele mag nie eers breingrootte verander nie, hulle kan die brein net 'n bietjie meer doeltreffend maak as dit inderdaad breinfunksie beïnvloed," het hy gesê. "Iemand wat byvoorbeeld die D-alleel erf, kan gemiddeld net 'n effens meer doeltreffende brein hê. Alhoewel daardie verbetering slegs 'n subtiele evolusionêre voordeel aan daardie persoon kan verleen, wanneer daardie effek oor 'n duisend generasies van natuurlike seleksie gepropageer word, sal die resultaat wees om die D-allele tot 'n baie hoë voorkoms te dryf."

Lahn en sy kollegas glo dat ander gene moontlik soortgelyke tekens kan toon van 'n oorsprong in argaïese Homo geslagte soos Neanderdalmense. Hulle gebruik tans hul analitiese instrument om bewyse van daardie oorsprong vir ander gene in die menslike genoom te soek.

Sulke bevindings kan wyer implikasies hê vir die begrip van menslike evolusie as om net die moontlikheid van mens-Neanderdal-kruising te openbaar, het hy gesê. "Benewens die feit dat dit miskien die mees robuuste genetiese bewyse is vir introgressie van gene van argaïese Homo spesies in mense, ek dink hierdie bevinding demonstreer dat die evolusie van ons spesie diep beïnvloed is deur genevloei van ons relatiewe spesie,” het Lahn gesê.

“Om bewyse van vermenging te vind, is nie so verbasend nie. Maar ons studie demonstreer die moontlikheid dat kruisteling voordelige variante bygedra het in die menslike genepoel wat daarna versprei het. Dit impliseer dat die evolusie van menslike biologie beïnvloed is deur die bydrae van voordelige genetiese variante van argaïese familielede wat ons vervang of selfs doodgemaak het,” het hy gesê.

Tot nou toe, het Lahn gesê, die wetenskaplike debat oor genetiese uitruiling tussen mense en ander Homo spesies het gelei tot twee prominente mededingende teorieë. Een is van mening dat anatomies moderne mense argaïese spesies vervang het, met geen kruising nie. En die ander stel dat uitgebreide kruising wel plaasgevind het en dat moderne mense uit daardie kruising in baie streke van die wêreld ontwikkel het.

Genetiese en fossielbewyse vir laasgenoemde "veelstreekse" teorie was onbeslis, het Lahn gesê, sodat die teorie grootliks gediskrediteer is. Maar, het hy gesê, die nuwer bewyse van geenuitruiling - sowel as ander genetiese bewyse wat kan volg - kan aanleiding gee tot 'n meer gematigde weergawe wat beweer dat een of ander genetiese uitruiling wel plaasgevind het. Verder sal dit toenemend besef word dat sulke genetiese uitruiling dalk ons ​​spesie baie meer fiks gemaak het.


Inhoud

Genetika wysig

Proporsie van vermenging Edit

Op 7 Mei 2010, na die genoomvolgordebepaling van drie Vindija Neanderdallers, is 'n konsepvolgorde van die Neanderdal-genoom gepubliseer en het aan die lig gebring dat Neanderdallers meer allele met Eurasiese bevolkings (bv. Frans, Han-Chinees en Papoea-Nieu-Guinees) gedeel het as met sub-Sahara Afrika-bevolkings (bv. Yoruba en San). [8] Volgens Green et al. (2010), die skrywers, word die waargenome oormaat van genetiese ooreenkomste die beste verklaar deur onlangse geenvloei van Neanderdalmense na moderne mense na die migrasie uit Afrika. [8] Hulle het geskat dat die proporsie van die Neanderdal-afkoms 1–4% van die Eurasiese genoom is. [8] Prüfer et al. (2013) beraam die proporsie is 1,5–2,1% vir nie-Afrikane, [9] Lohse en Frantz (2014) lei 'n hoër koers van 3,4–7,3% in Eurasië af. [10] In 2017 het Prüfer et al. hul skatting hersien na 1,8–2,6% vir nie-Afrikaners buite Oseanië. [11]

Volgens 'n latere studie deur Chen et al. (2020), Afrikane (spesifiek die 1000 Genome Afrika-bevolkings) het ook Neanderdal-byvoeging, [12] met hierdie Neanderdal-mengsel in Afrika-individue wat verantwoordelik is vir 17 megabasisse, [12] wat 0,3% van hul genoom is. [3] Volgens die skrywers het Afrikane hul Neanderdal-vermenging hoofsaaklik verkry uit 'n terug-migrasie deur mense (moderne mense wat Neanderdal-mengsel dra) wat afgewyk het van voorvaderlike Europeërs (nadat die skeuring tussen Oos-Asiërs en Europeërs). [12] Daar word voorgestel dat hierdie terugmigrasie ongeveer 20 000 jaar gelede plaasgevind het. [3] Sommige wetenskaplikes, soos die genetikus David Reich, betwis egter die studie se gevolgtrekkings wat daarop dui dat Neanderdal-vermenging in Afrikane suid van die Sahara voorkom. [13]

Introgressed genoom Edit

Ongeveer 20% van die Neanderdal-genoom is in die moderne menslike bevolking ingetrek of geassimileer gevind (deur Oos-Asiërs en Europeërs te ontleed), [14] maar die syfer is ook op ongeveer 'n derde geraam. [15]

Subpopulasie vermenging koers Wysig

’n Hoër Neanderdal-vermenging is gevind in Oos-Asiërs as in Europeërs, [14] [16] [17] [18] [19] wat na raming sowat 20% meer introgressie in Oos-Asiërs is. [14] [16] [19] Dit kan moontlik verklaar word deur die voorkoms van verdere vermengingsgebeure in die vroeë voorouers van Oos-Asiërs na die skeiding van Europeërs en Oos-Asiërs, [4] [14] [16] [17] [ 19] verdunning van Neanderdal-afkoms by Europeërs deur bevolkings met ’n lae Neanderdal-afkoms van latere migrasies, [4] [16] [19] of natuurlike seleksie wat moontlik relatief laer in Oos-Asiërs as in Europeërs was. [4] [18] [19] Studies wat vermengingsmodelle simuleer, dui daarop dat 'n verminderde doeltreffendheid van suiwerende seleksie teen Neanderdal-allele in Oos-Asiërs nie die groter proporsie Neanderdal-afkoms van Oos-Asiërs kan verantwoord nie, en dus meer komplekse modelle bevoordeel wat bykomende pulse behels van Neanderdal-introgressie in Oos-Asiërs. [20] [21] Sulke modelle wys 'n polsslag aan voorvaderlike Eurasiërs, gevolg deur skeiding en 'n bykomende polsslag aan voorvaderlike Oos-Asiërs. [4] Daar word waargeneem dat daar 'n klein maar beduidende variasie van Neanderdal-vermengingskoerse binne Europese bevolkings is, maar geen noemenswaardige variasie binne Oos-Asiatiese bevolkings nie. [14] Prüfer et al. (2017) het opgemerk dat Oos-Asiërs meer Neanderdal-DNS (2,3–2,6%) dra as Wes-Eurasiërs (1,8–2,4%). [11]

Dit is later deur Chen et al. (2020) dat Oos-Asiërs 8% meer Neanderdal-afkoms het, hersien van die vorige verslae van 20% meer Neanderdal-afkoms, in vergelyking met Europeërs. [12] Dit spruit uit die feit dat Neanderdal-afkoms wat met Afrikane gedeel is, gemasker is, omdat daar gedink is dat Afrikane geen Neanderdal-byvoeging het nie en daarom as verwysingsmonsters gebruik is. [12] So het enige oorvleueling in Neanderdal-vermenging met Afrikane gelei tot 'n onderskatting van Neanderdal-vermenging by nie-Afrikane en veral in Europeërs. [12] Die skrywers gee 'n enkele puls van Neanderdal-vermenging na die buite-Afrika-verspreiding as die mees spaarsamige verklaring vir die verryking in Oos-Asiërs, maar hulle voeg by dat variasie in Neanderdal-herkoms ook aan verdunning toegeskryf kan word om die nou-meer-beskeie verskille gevind. [12] As 'n proporsie van die totale hoeveelheid Neanderdal-volgorde vir elke bevolking, word 7,2% van die volgorde in Europeërs uitsluitlik met Afrikane gedeel, terwyl 2% van die volgorde in Oos-Asiërs uitsluitlik met Afrikane gedeel word. [12]

Genomiese analise dui daarop dat daar 'n globale verdeling in Neanderdal-introgressie tussen Afrika-bevolkings suid van die Sahara en ander moderne menslike groepe (insluitend Noord-Afrikaners) eerder as tussen Afrika- en nie-Afrika-bevolkings is. [22] Noord-Afrikaanse groepe deel 'n soortgelyke oormaat afgeleide allele met Neanderdalmense as nie-Afrikaanse bevolkings, terwyl Afrika-groepe suid van die Sahara die enigste moderne menslike bevolkings is wat oor die algemeen nie Neanderdal-vermenging ervaar het nie. [23] Daar is gevind dat die Neanderdal-genetiese sein onder Noord-Afrikaanse bevolkings verskil na gelang van die relatiewe hoeveelheid autochtone Noord-Afrikaanse, Europese, Nabye Oosterse en sub-Sahara voorgeslagte. Met behulp van statistiese ontleding van f4-afkomsverhoudings, is die Neanderdal-afgeleide mengsel waargeneem as: hoogste onder die Noord-Afrikaanse bevolkings met maksimum outogtone Noord-Afrikaanse afkoms soos Tunisiese Berbers, waar dit op dieselfde vlak of selfs hoër was as dié van Eurasiese bevolkings (100) –138%) hoog onder Noord-Afrikaanse bevolkings wat groter Europese of Nabye Oosterse vermenging dra, soos groepe in Noord-Marokko en Egipte (∼60–70%) en die laagste onder Noord-Afrikaanse bevolkings met groter Sub-Sahara vermenging, soos in Suid-Marokko (20%). [24] Quinto et al. (2012) postuleer dus dat die teenwoordigheid van hierdie Neanderdal-genetiese sein in Afrika nie te wyte is aan onlangse geenvloei van Nabye Oosterse of Europese bevolkings nie, aangesien dit hoër is onder bevolkings wat inheemse pre-Neolitiese Noord-Afrikaanse voorgeslagte dra. [25] Lae maar beduidende dosisse van Neanderdal-vermenging is ook waargeneem vir die Masai van Oos-Afrika. [26] Nadat Afrika- en nie-Afrikaanse voorgeslagte onder die Masai geïdentifiseer is, kan die gevolgtrekking gemaak word dat onlangse nie-Afrika moderne menslike (post-Neanderdal) geenvloei die bron van die bydrae was aangesien ongeveer 30% van die Masai genoom kan herlei word na nie-Afrikaanse introgressie van ongeveer 100 generasies gelede. [17]

Afstand na geslagte Wysig

Met 'n hoë-gehalte genoomvolgorde van 'n vroulike Altai Neanderdal, is gevind dat die Neanderdal komponent in nie-Afrika moderne mense meer verwant is aan die Mezmaiskaya Neanderdal (Kaukasus) as aan die Altai Neanderdal (Siberië) of die Vindija Neanderdal ( Kroasië). [9] Deur 'n hoë-dekking volgordebepaling van die genoom van 'n 50 000 jaar oue vroulike Vindija Neanderdal fragment, is later gevind dat die Vindija en Mezmaiskaya Neanderdal nie skynbaar verskil in die omvang van hul alleeldeling met moderne mense nie. [11] In hierdie geval is daar ook gevind dat die Neanderdal-komponent by nie-Afrikaanse moderne mense nader verwant is aan die Vindija- en Mezmaiskaya-Neanderdallers as aan die Altai-Neanderdallers. [11] Hierdie resultate dui daarop dat 'n meerderheid van die vermenging in moderne mense afkomstig was van Neanderdal-bevolkings wat (ongeveer 80–100kya) van die Vindija- en Mezmaiskaya Neanderdal-afstammelinge afgewyk het voordat laasgenoemde twee van mekaar afgewyk het. [11]

Deur chromosoom 21 van die Altai (Siberië), El Sidrón (Spanje) en Vindija (Kroasië) Neanderdalmense te ontleed, word vasgestel dat—van hierdie drie afstammelinge—net die El Sidrón- en Vindija-neanderdalmense toon beduidende tempo van geenvloei (0,3–2,6) %) in moderne mense, wat daarop dui dat die El Sidrón en Vindija Neanderdallers nader verwant is as die Altai Neanderdallers aan die Neanderdallers wat ongeveer 47 000–65 000 jaar gelede met moderne mense gekruis het. [28] Omgekeerd word daar ook vasgestel dat beduidende tempo's van moderne menslike geenvloei na Neanderdalmense voorgekom het—van die drie ondersoekde afstammelinge—vir slegs die Altai Neanderdallers (0.1–2.1%), wat daarop dui dat moderne menslike genevloei in Neanderdalmense hoofsaaklik plaasgevind het. na die skeiding van die Altai Neanderdallers van die El Sidrón en Vindija Neanderdalers wat ongeveer 110 000 jaar gelede plaasgevind het. [28] Die bevindinge toon dat die bron van moderne menslike genevloei na Neanderdalmense ontstaan ​​het uit 'n bevolking van vroeë moderne mense van ongeveer 100 000 jaar gelede, wat die uit-Afrika-migrasie van die moderne menslike voorouers van hedendaagse nie-Afrikane voorafgegaan het. . [28]

Mitochondriale DNA en Y-chromosoom Edit

Geen bewyse van Neanderdal-mitochondriale DNA is by moderne mense gevind nie. [29] [30] [31] Dit dui daarop dat suksesvolle Neanderdal-vermenging plaasgevind het in pare met Neanderdal-mannetjies en moderne menslike wyfies. [32] [33] Possible hypotheses are that Neanderthal mitochondrial DNA had detrimental mutations that led to the extinction of carriers, that the hybrid offspring of Neanderthal mothers were raised in Neanderthal groups and became extinct with them, or that female Neanderthals and male Sapiens did not produce fertile offspring. [32] However this is contested by recent findings that suggest that the Neanderthal's Y chromosome was replaced by Sapiens' Y chromosomes after the human Y chromosome entered the Neanderthal gene pool, meaning that male Sapiens must have mated with female Neanderthals at some point. [2]

As shown in an interbreeding model produced by Neves and Serva (2012), the Neanderthal admixture in modern humans may have been caused by a very low rate of interbreeding between modern humans and Neanderthals, with the exchange of one pair of individuals between the two populations in about every 77 generations. [34] This low rate of interbreeding would account for the absence of Neanderthal mitochondrial DNA from the modern human gene pool as found in earlier studies, as the model estimates a probability of only 7% for a Neanderthal origin of both mitochondrial DNA and Y chromosome in modern humans. [34]

Reduced contribution Edit

There is a presence of large genomic regions with strongly reduced Neanderthal contribution in modern humans due to negative selection, [14] [18] partly caused by hybrid male infertility. [18] These large regions of low Neanderthal contribution were most-pronounced on the X chromosome—with fivefold lower Neanderthal ancestry compared to autosomes. [4] [18] They also contained relatively high numbers of genes specific to testes. [18] This means that modern humans have relatively few Neanderthal genes that are located on the X chromosome or expressed in the testes, suggesting male infertility as a probable cause. [18] It may be partly affected by hemizygosity of X chromosome genes in males. [4]

Deserts of Neanderthal sequences may also be caused by genetic drift involving intense bottlenecks in the modern human population and background selection as a result of strong selection against deleterious Neanderthal alleles. [4] The overlap of many deserts of Neanderthal and Denisovan sequences suggests that repeated loss of archaic DNA occur at specific loci. [4]

It has also been shown that Neanderthal ancestry has been selected against in conserved biological pathways, such as RNA processing. [18]

Consistent with the hypothesis that purifying selection has reduced Neanderthal contribution in present-day modern human genomes, Upper Paleolithic Eurasian modern humans (such as the Tianyuan modern human) carry more Neanderthal DNA (about 4–5%) than present-day Eurasian modern humans (about 1–2%). [35]

Rates of selection against Neanderthal sequences varied for European and Asian populations. [4]

Changes in modern humans Edit

In Eurasia, modern humans inherited adaptive introgression from archaic humans, which provided a source of advantageous genetic variants that are adapted to local environments and a reservoir for additional genetic variation. [4] Adaptive introgression from Neanderthals has targeted genes involved with keratin filaments, sugar metabolism, muscle contraction, body fat distribution, enamel thickness, and oocyte meiosis, as well as brain size and functioning. [36] There are signals of positive selection, as the result of adaptation to diverse habitats, in genes involved with variation in skin pigmentation and hair morphology. [36] In the immune system, introgressed variants have heavily contributed to the diversity of immune genes, of which there's an enrichment of introgressed alleles that suggest a strong positive selection. [36]

Genes affecting keratin were found to have been introgressed from Neanderthals into modern humans (shown in East Asians and Europeans), suggesting that these genes gave a morphological adaptation in skin and hair to modern humans to cope with non-African environments. [14] [18] This is likewise for several genes involved in medical-relevant phenotypes, such as those affecting systemic lupus erythematosus, primary biliary cirrhosis, Crohn's disease, optic disk size, smoking behavior, interleukin 18 levels, and diabetes mellitus type 2. [18]

Researchers found Neanderthal introgression of 18 genes—several of which are related to UV-light adaptation—within the chromosome 3p21.31 region (HYAL region) of East Asians. [37] The introgressive haplotypes were positively selected in only East Asian populations, rising steadily from 45,000 years BP until a sudden increase of growth rate around 5,000 to 3,500 years BP. [37] They occur at very high frequencies among East Asian populations in contrast to other Eurasian populations (e.g. European and South Asian populations). [37] The findings also suggests that this Neanderthal introgression occurred within the ancestral population shared by East Asians and Native Americans. [37]

Evans et al. (2006) had previously suggested that a group of alleles collectively known as haplogroup D of microcephalin, a critical regulatory gene for brain volume, originated from an archaic human population. [38] The results show that haplogroup D introgressed 37,000 years ago (based on the coalescence age of derived D alleles) into modern humans from an archaic human population that separated 1.1 million years ago (based on the separation time between D and non-D alleles), consistent with the period when Neanderthals and modern humans co-existed and diverged respectively. [38] The high frequency of the D haplogroup (70%) suggest that it was positively selected for in modern humans. [38] The distribution of the D allele of microcephalin is high outside Africa but low in sub-Saharan Africa, which further suggest that the admixture event happened in archaic Eurasian populations. [38] This distribution difference between Africa and Eurasia suggests that the D allele originated from Neanderthals according to Lari et al. (2010), but they found that a Neanderthal individual from the Mezzena Rockshelter (Monti Lessini, Italy) was homozygous for an ancestral allele of microcephalin, thus providing no support that Neanderthals contributed the D allele to modern humans and also not excluding the possibility of a Neanderthal origin of the D allele. [39] Green et al. (2010), having analyzed the Vindija Neanderthals, also could not confirm a Neanderthal origin of haplogroup D of the microcephalin gene. [8]

It has been found that HLA-A*02, A*26/*66, B*07, B*51, C*07:02, and C*16:02 of the immune system were contributed from Neanderthals to modern humans. [40] After migrating out of Africa, modern humans encountered and interbred with archaic humans, which was advantageous for modern humans in rapidly restoring HLA diversity and acquiring new HLA variants that are better adapted to local pathogens. [40]

It is found that introgressed Neanderthal genes exhibit cis-regulatory effects in modern humans, contributing to the genomic complexity and phenotype variation of modern humans. [41] Looking at heterozygous individuals (carrying both Neanderthal and modern human versions of a gene), the allele-specific expression of introgressed Neanderthal alleles was found to be significantly lower in the brain and testes relative to other tissues. [4] [41] In the brain, this was most pronounced at the cerebellum and basal ganglia. [41] This downregulation suggests that modern humans and Neanderthals possibly experienced a relative higher rate of divergence in these specific tissues. [41]

Furthermore, correlating the genotypes of introgressed Neanderthal alleles with the expression of nearby genes, it is found that archaic alleles contribute proportionally more to variation in expression than nonarchaic alleles. [4] Neanderthal alleles affect expression of the immunologically genes OAS1/2/3 and TLR1/6/10, which can be specific to cell-type and is influenced by environmental stimuli. [4]

Studying the high-coverage female Vindija Neanderthal genome, Prüfer et al. (2017) identified several Neanderthal-derived gene variants, including those that affect levels of LDL cholesterol and vitamin D, and has influence on eating disorders, visceral fat accumulation, rheumatoid arthritis, schizophrenia, as well as the response to antipsychotic drugs. [11]

Examining European modern humans in regards to the Altai Neanderthal genome in high-coverage, results show that Neanderthal admixture is associated with several changes in cranium and underlying brain morphology, suggesting changes in neurological function through Neanderthal-derived genetic variation. [42] Neanderthal admixture is associated with an expansion of the posterolateral area of the modern human skull, extending from the occipital and inferior parietal bones to bilateral temporal locales. [42] In regards to modern human brain morphology, Neanderthal admixture is positively correlated with an increase in sulcal depth for the right intraparietal sulcus and an increase in cortical complexity for the early visual cortex of the left hemisphere. [42] Neanderthal admixture is also positively correlated with an increase in white and gray matter volume localized to the right parietal region adjacent to the right intraparietal sulcus. [42] In the area overlapping the primary visual cortex gyrification in the left hemisphere, Neanderthal admixture is positively correlated with gray matter volume. [42] The results also show evidence for a negative correlation between Neanderthal admixture and white matter volume in the orbitofrontal cortex. [42]

In Papuans, assimilated Neanderthal inheritance is found in highest frequency in genes expressed in the brain, whereas Denisovan DNA has the highest frequency in genes expressed in bones and other tissues. [43]

Population substructure theory Edit

Although less parsimonious than recent gene flow, the observation may have been due to ancient population sub-structure in Africa, causing incomplete genetic homogenization within modern humans when Neanderthals diverged while early ancestors of Eurasians were still more closely related to Neanderthals than those of Africans to Neanderthals. [8] On the basis of allele frequency spectrum, it was shown that the recent admixture model had the best fit to the results while the ancient population sub-structure model had no fit–demonstrating that the best model was a recent admixture event that was preceded by a bottleneck event among modern humans—thus confirming recent admixture as the most parsimonious and plausible explanation for the observed excess of genetic similarities between modern non-African humans and Neanderthals. [44] On the basis of linkage disequilibrium patterns, a recent admixture event is likewise confirmed by the data. [45] From the extent of linkage disequilibrium, it was estimated that the last Neanderthal gene flow into early ancestors of Europeans occurred 47,000–65,000 years BP. [45] In conjunction with archaeological and fossil evidence, the gene flow is thought likely to have occurred somewhere in Western Eurasia, possibly the Middle East. [45] Through another approach—using one genome each of a Neanderthal, Eurasian, African, and chimpanzee (outgroup), and dividing it into non-recombining short sequence blocks—to estimate genome-wide maximum-likelihood under different models, an ancient population sub-structure in Africa was ruled out and a Neanderthal admixture event was confirmed. [10]

Morphology Edit

The early Upper Paleolithic burial remains of a modern human child from Abrigo do Lagar Velho (Portugal) features traits that indicate Neanderthal interbreeding with modern humans dispersing into Iberia. [46] Considering the dating of the burial remains (24,500 years BP) and the persistence of Neanderthal traits long after the transitional period from a Neanderthal to a modern human population in Iberia (28,000–30,000 years BP), the child may have been a descendant of an already heavily admixed population. [46]

The remains of an early Upper Paleolithic modern human from Peștera Muierilor (Romania) of 35,000 years BP shows a morphological pattern of European early modern humans, but possesses archaic or Neanderthal features, suggesting European early modern humans interbreeding with Neanderthals. [47] These features include a large interorbital breadth, a relatively flat superciliary arches, a prominent occipital bun, an asymmetrical and shallow mandibular notch shape, a high mandibular coronoid processus, the relative perpendicular mandibular condyle to notch crest position, and a narrow scapular glenoid fossa. [47]

The early modern human Oase 1 mandible from Peștera cu Oase (Romania) of 34,000–36,000 14 C years BP presents a mosaic of modern, archaic, and possible Neanderthal features. [49] It displays a lingual bridging of the mandibular foramen, not present in earlier humans except Neanderthals of the late Middle and Late Pleistocene, thus suggesting affinity with Neanderthals. [49] Concluding from the Oase 1 mandible, there was apparently a significant craniofacial change of early modern humans from at least Europe, possibly due to some degree of admixture with Neanderthals. [49]

The earliest (before about 33 ka BP) European modern humans and the subsequent (Middle Upper Paleolithic) Gravettians, falling anatomically largely inline with the earliest (Middle Paleolithic) African modern humans, also show traits that are distinctively Neanderthal, suggesting that a solely Middle Paleolithic modern human ancestry was unlikely for European early modern humans. [50]

A late-Neanderthal jaw (more specifically, a corpus mandibulae remnant) from the Mezzena rockshelter (Monti Lessini, Italy) shows indications of a possible interbreeding in late Italian Neanderthals. [51] The jaw falls within the morphological range of modern humans, but also displayed strong similarities with some of the other Neanderthal specimens, indicating a change in late Neanderthal morphology due to possible interbreeding with modern humans. [51] However, a more recent aDNA analysis of this jaw has shown that it does not belong to a Neanderthal, but to a fully modern human of the Holocene. Previous reports of a Mezzena "Neanderthal hybrid" were based on a faulty DNA analysis. [52]

Manot 1, a partial calvarium of a modern human that was recently discovered at the Manot Cave (Western Galilee, Israel) and dated to 54.7±5.5 kyr BP, represents the first fossil evidence from the period when modern humans successfully migrated out of Africa and colonized Eurasia. [53] It also provides the first fossil evidence that modern humans inhabited the southern Levant during the Middle to Upper Palaeolithic interface, contemporaneously with the Neanderthals and close to the probable interbreeding event. [53] The morphological features suggest that the Manot population may be closely related to or given rise to the first modern humans who later successfully colonized Europe to establish early Upper Palaeolithic populations. [53]

Geskiedenis Redigeer

The interbreeding has been discussed ever since the discovery of Neanderthal remains in the 19th century, though earlier writers believed that Neanderthals were a direct ancestor of modern humans. Thomas Huxley suggested that many Europeans bore traces of Neanderthal ancestry, but associated Neanderthal characteristics with primitivism, writing that since they "belong to a stage in the development of the human species, antecedent to the differentiation of any of the existing races, we may expect to find them in the lowest of these races, all over the world, and in the early stages of all races". [54]

Until the early 1950s, most scholars thought Neanderthals were not in the ancestry of living humans. [55] : 232–34 [56] Nevertheless, Hans Peder Steensby proposed interbreeding in 1907 in the article Race studies in Denmark. He strongly emphasised that all living humans are of mixed origins. [57] He held that this would best fit observations, and challenged the widespread idea that Neanderthals were ape-like or inferior. Basing his argument primarily on cranial data, he noted that the Danes, like the Frisians and the Dutch, exhibit some Neanderthaloid characteristics, and felt it was reasonable to "assume something was inherited" and that Neanderthals "are among our ancestors."

Carleton Stevens Coon in 1962 found it likely, based upon evidence from cranial data and material culture, that Neanderthal and Upper Paleolithic peoples either interbred or that the newcomers reworked Neanderthal implements "into their own kind of tools." [58]

By the early 2000s, the majority of scholars supported the Out of Africa hypothesis, [59] [60] according to which anatomically modern humans left Africa about 50,000 years ago and replaced Neanderthals with little or no interbreeding. Yet some scholars still argued for hybridisation with Neanderthals. The most vocal proponent of the hybridisation hypothesis was Erik Trinkaus of Washington University. [61] Trinkaus claimed various fossils as products of hybridised populations, including the skeleton of a child found at Lagar Velho in Portugal [62] [63] [64] and the Peștera Muierii skeletons from Romania. [47]

Genetics Edit

Proportion of admixture Edit

It has been shown that Melanesians (e.g. Papua New Guinean and Bougainville Islander) share relatively more alleles with Denisovans when compared to other Eurasians and Africans. [65] It is estimated that 4% to 6% of the genome in Melanesians derives from Denisovans, while no other Eurasians or Africans displayed contributions of the Denisovan genes. [65] It has been observed that Denisovans contributed genes to Melanesians but not to East Asians, indicating that there was interaction between the early ancestors of Melanesians with Denisovans but that this interaction did not take place in the regions near southern Siberia, where as-of-yet the only Denisovan remains have been found. [65] In addition, Aboriginal Australians also show a relative increased allele sharing with Denisovans, compared to other Eurasians and African populations, consistent with the hypothesis of increased admixture between Denisovans and Melanesians. [66]

Reich et al. (2011) produced evidence that the highest presence of Denisovan admixture is in Oceanian populations, followed by many Southeast Asian populations, and none in East Asian populations. [67] There is significant Denisovan genetic material in eastern Southeast Asian and Oceanian populations (e.g. Aboriginal Australians, Near Oceanians, Polynesians, Fijians, eastern Indonesians, Philippine Mamanwa and Manobo), but not in certain western and continental Southeast Asian populations (e.g. western Indonesians, Malaysian Jehai, Andaman Onge, and mainland Asians), indicating that the Denisovan admixture event happened in Southeast Asia itself rather than mainland Eurasia. [67] The observation of high Denisovan admixture in Oceania and the lack thereof in mainland Asia suggests that early modern humans and Denisovans had interbred east of the Wallace Line that divides Southeast Asia according to Cooper and Stringer (2013). [68]

Skoglund and Jakobsson (2011) observed that particularly Oceanians, followed by Southeast Asians populations, have a high Denisovans admixture relative to other populations. [69] Furthermore, they found possible low traces of Denisovan admixture in East Asians and no Denisovan admixture in Native Americans. [69] In contrast, Prüfer et al. (2013) found that mainland Asian and Native American populations may have a 0.2% Denisovan contribution, which is about twenty-five times lower than Oceanian populations. [9] The manner of gene flow to these populations remains unknown. [9] However, Wall et al. (2013) stated that they found no evidence for Denisovan admixture in East Asians. [17]

Findings indicate that the Denisovan gene flow event happened to the common ancestors of Aboriginal Filipinos, Aboriginal Australians, and New Guineans. [67] [70] New Guineans and Australians have similar rates of Denisovan admixture, indicating that interbreeding took place prior to their common ancestors' entry into Sahul (Pleistocene New Guinea and Australia), at least 44,000 years ago. [67] It has also been observed that the fraction of Near Oceanian ancestry in Southeast Asians is proportional to the Denisovan admixture, except in the Philippines where there is a higher proportional Denisovan admixture to Near Oceanian ancestry. [67] Reich et al. (2011) suggested a possible model of an early eastward migration wave of modern humans, some who were Philippine/New Guinean/Australian common ancestors that interbred with Denisovans, respectively followed by divergence of the Philippine early ancestors, interbreeding between the New Guinean and Australian early ancestors with a part of the same early-migration population that did not experience Denisovan gene flow, and interbreeding between the Philippine early ancestors with a part of the population from a much-later eastward migration wave (the other part of the migrating population would become East Asians). [67]

Finding components of Denisovan introgression with differing relatedness to the sequenced Denisovan, Browning et al. (2018) suggested that at least two separate episodes of Denisovan admixture has occurred. [71] Specifically, introgression from two distinct Denisovan populations is observed in East Asians (e.g. Japanese and Han Chinese), whereas South Asians (e.g. Telugu and Punjabi) and Oceanians (e.g. Papuans) display introgression from one Denisovan population. [71]

Exploring derived alleles from Denisovans, Sankararaman et al. (2016) estimated that the date of Denisovan admixture was 44,000–54,000 years ago. [5] They also determined that the Denisovan admixture was the greatest in Oceanian populations compared to other populations with observed Denisovan ancestry (i.e. America, Central Asia, East Asia, and South Asia). [5] The researchers also made the surprising finding that South Asian populations display an elevated Denisovan admixture (when compared to other non-Oceanian populations with Denisovan ancestry), albeit the highest estimate (which are found in Sherpas) is still ten times lower than in Papuans. [5] They suggest two possible explanations: There was a single Denisovan introgression event that was followed by dilution to different extents or at least three distinct pulses of Denisovan introgressions must have occurred. [5]

It has been shown that Eurasians have some but significantly lesser archaic-derived genetic material that overlaps with Denisovans, stemming from the fact that Denisovans are related to Neanderthals—who contributed to the Eurasian gene pool—rather than from interbreeding of Denisovans with the early ancestors of those Eurasians. [16] [65]

The skeletal remains of an early modern human from the Tianyuan cave (near Zhoukoudian, China) of 40,000 years BP showed a Neanderthal contribution within the range of today's Eurasian modern humans, but it had no discernible Denisovan contribution. [72] It is a distant relative to the ancestors of many Asian and Native American populations, but post-dated the divergence between Asians and Europeans. [72] The lack of a Denisovan component in the Tianyuan individual suggests that the genetic contribution had been always scarce in the mainland. [9]

Reduced contribution Edit

There are large genomic regions devoid of Denisovan-derived ancestry, partly explained by infertility of male hybrids, as suggested by the lower proportion of Denisovan-derived ancestry on X chromosomes and in genes that are expressed in the testes of modern humans. [5]

Changes in modern humans Edit

Exploring the immune system's HLA alleles, it has been suggested that HLA-B*73 introgressed from Denisovans into modern humans in western Asia due to the distribution pattern and divergence of HLA-B*73 from other HLA alleles. [40] Even though HLA-B*73 is not present in the sequenced Denisovan genome, HLA-B*73 was shown to be closely associated to the Denisovan-derived HLA-C*15:05 from the linkage disequilibrium. [40] From phylogenetic analysis, however, it has been concluded that it is highly likely that HLA-B*73 was ancestral. [36]

The Denisovan's two HLA-A (A*02 and A*11) and two HLA-C (C*15 and C*12:02) allotypes correspond to common alleles in modern humans, whereas one of the Denisovan's HLA-B allotype corresponds to a rare recombinant allele and the other is absent in modern humans. [40] It is thought that these must have been contributed from Denisovans to modern humans, because it is unlikely to have been preserved independently in both for so long due to HLA alleles' high mutation rate. [40]

Tibetan people received an advantageous EGLN1 and EPAS1 gene variant, associated with hemoglobin concentration and response to hypoxia, for life at high altitudes from the Denisovans. [36] The ancestral variant of EPAS1 upregulates hemoglobin levels to compensate for low oxygen levels—such as at high altitudes—but this also has the maladaption of increasing blood viscosity. [73] The Denisovan-derived variant on the other hand limits this increase of hemoglobin levels, thus resulting in a better altitude adaption. [73] The Denisovan-derived EPAS1 gene variant is common in Tibetans and was positively selected in their ancestors after they colonized the Tibetan plateau. [73]

Rapid decay of fossils in Sub-Saharan African environments makes it currently unfeasible to compare modern human admixture with reference samples of archaic Sub-Saharan African hominins. [4] [74]

From three candidate regions with introgression found by searching for unusual patterns of variations (showing deep haplotype divergence, unusual patterns of linkage disequilibrium, and small basal clade size) in 61 non-coding regions from two hunter-gatherer groups (Biaka Pygmies and San who have significant admixture) and one West African agricultural group (Mandinka, who don't have significant admixture), it is concluded that roughly 2% of the genetic material found in the Biaka Pygmies and San was inserted into the human genome approximately 35,000 years ago from archaic hominins that separated from the ancestors of the modern human lineage around 700,000 years ago. [75] A survey for the introgressive haplotypes across many Sub-Saharan populations suggest that this admixture event happened with archaic hominins who once inhabited Central Africa. [75]

Researching high-coverage whole-genome sequences of fifteen Sub-Saharan hunter-gatherer males from three groups—five Pygmies (three Baka, a Bedzan, and a Bakola) from Cameroon, five Hadza from Tanzania, and five Sandawe from Tanzania—there are signs that the ancestors of the hunter-gatherers interbred with one or more archaic human populations, [74] probably over 40,000 years ago. [76] Analysis of putative introgressive haplotypes in the fifteen hunter-gatherer samples suggests that the archaic African population and modern humans diverged around 1.2 to 1.3 million years ago. [74]

According to a study published in 2020, there are indications that 2% to 19% (or about ≃6.6 and ≃7.0%) of the DNA of four West African populations may have come from an unknown archaic hominin which split from the ancestor of humans and Neanderthals between 360 kya to 1.02 mya. However, the study also finds that at least part of this proposed archaic admixture is also present in Eurasians/non-Africans, and that the admixture event or events range from 0 to 124 ka B.P, which includes the period before the Out-of-Africa migration and prior to the African/Eurasian split (thus affecting in part the common ancestors of both Africans and Eurasians/non-Africans). [77] [78] [79] Another recent study, which discovered substantial amounts of previously undescribed human genetic variation, also found ancestral genetic variation in Africans that predates modern humans and was lost in most non-Africans. [80]

In 2019, scientists discovered evidence, based on genetics studies using artificial intelligence (AI), that suggests the existence of an unknown human ancestor species, not Neanderthal or Denisovan, in the genome of modern humans. [81] [82]


Ancient mixers

The new research illustrates the complexity of humanity's deep history. Evidence has long been accumulating that humans and Neanderthals mated while their populations overlapped in Europe, before Neanderthals went extinct around 30,000 years ago. In 2010, researchers reported that between 1% and 4% of modern human genes in people in Asia, Europe and Oceania came from Neanderthal ancestors. When you add up all the snippets of Neanderthal DNA present in all modern humans today, some 20% of the Neanderthal genome may be preserved, according to 2014 research.

As scientists have been able to sequence more fragile fragments of DNA from fossils of ancient human ancestors, they've discovered a complex web of interbreeding stretching back millennia. Some Pacific Islanders, for example, carry pieces of the DNA of a mysterious ancient species of humans known as Denisovans.

The researchers of the new study used a computational method of comparing the genomes of two Neanderthals, a Denisovan and two modern African individuals. (Africans were chosen because modern people in Africa don't carry Neanderthal genes from the well-known human-Neanderthal interbreeding that occurred in Europe starting 50,000 years ago.) This method allowed the researchers to capture recombination events, in which segments of chromosome — which are made up of DNA — from one individual get incorporated into the chromosomes of another.

"We are trying to build a complete model for the evolutionary history of every segment of the genome, jointly across all of the analyzed individuals," Siepel said. "The ancestral recombination graph, as it is known, includes a tree that captures the relationships among all individuals at every position along the genome, and the recombination events that cause those trees to change from one position to the next."

One advantage of the method, Siepel said, is that it allows researchers to find recombination events inside of recombination events. For example, if a bit of ancient hominin DNA from an unknown ancestor were incorporated in the Neanderthal genome, and then a later mating event between Neanderthals and humans inserted that mystery DNA into the human genome, the method allows for the identification of this "nested" DNA.


Splitting apart

This kind of speciation, known as sympatry, was once thought to be extremely unlikely, says Chris Bird of Texas A&M University Corpus Christi, who studies how organisms are evolving by analysing their genomes. The conventional view is that speciation almost always requires two populations to be physically separated to prevent interbreeding, for example, living on different sides of a mountain, or on different islands in an archipelago.

This is because when animals mate, a process called recombination mixes up gene variants, meaning the genes of a mother and a father will be shuffled together in future generations. As long as interbreeding continues, it’s unlikely that two groups with distinctly different genetic traits will arise.

But Marques’ team found that the genetic differences between the two fish types are concentrated on the parts of chromosomes that are less likely to undergo recombination. As a result, the sets of gene variants that give the two types their distinct characteristics are less likely to get split up.


Genetic exchange

Professor Michael Kohn from Rice University in Houston, Texas, led the team of researchers who carried out the work.

"Our study is so special because it involves hybridisation between two species of mouse that are 1.5-3 million years removed from each other.

"Most of the offspring. do not reproduce, they are sterile - but there is a small window, which remains open for genes to be moved from one species to the other, and that's through a few fertile females - so there is a chance to leak genes from one species to another."

Thanks to these few fertile females, the vast majority of mice in Spain and a growing number in Germany have acquired resistance over a very short period of time, although scientists aren't exactly sure when the first genetic exchanges took place.

And while they may not look any different from normal household mice, in their genetic code they now have the ability to survive the strongest chemicals in the pest control armoury.

"There are a lot of genetic barriers between these species of mice, to see them hybridise and transfer genetic material is quite spectacular, to be frank," said Professor Kohn.

The researchers say that increased human travel and population growth are responsible for bringing these mice species together and putting them under evolutionary stress by trying to poison them.

They are concerned that similar human pressures could afford rats both the necessity and the opportunity to breed across species, resulting in rodents that are almost impossible to control.


Aanpasbare bestraling

The processes described in this page can occur over and over. In the case of Darwin's finches, they must have been repeated a number of times forming new species that gradually divided the available habitats between them. From the first arrival have come a variety of ground-feeding and tree-feeding finches as well as the warblerlike finch and the tool-using woodpeckerlike finch. The formation of a number of diverse species from a single ancestral one is called an aanpasbare bestraling.

Speciateion in theHouse mice on the island of Madeira

A report in the 13 January 2000 issue of Natuur describes a study of house mouse populations on the island of Madeira off the Northwest coast of Africa. These workers (Janice Britton-Davidian et al) examined the karyotypes of 143 house mice (Mus musculus domesticus) from various locations along the coast of this mountainous island.

  • There are 6 distinct populations (shown by different colors)
  • Each of these has a distinct karyotype, with a diploid number less than the "normal" (2n = 40).
  • The reduction in chromosome number has occurred through Robertsonian fusions. Mouse chromosomes tend to be acrocentric that is, the centromere connects one long and one very short arm. Acrocentric chromosomes are at risk of translocations that fuse the long arms of two different chromosomes with the loss of the short arms.
  • The different populations are allopatric isolated in different valleys leading down to the sea.
  • The distinct and uniform karyotype found in each population probably arose from genetic drift rather than natural selection.
  • The 6 different populations are technically described as races because there is no opportunity for them to attempt interbreeding.
  • However, they surely meet the definition of true species. While hybrids would form easily (no prezygotic isolating mechanisms), these would probably be infertile as proper synapsis and segregation of such different chromosomes would be difficult when the hybrids attempted to form gametes by meiosis.

Pizzly Bears

Scientists confirmed last week that a bear shot by an Inuvialuit hunter in the Northwest Territories is a second-generation grizzly-polar bear hybrid—a “pizzly” or “grolar” bear. Why can some interbreeding species produce fertile offspring, while others—like horses and donkeys—cannot?

Because they have more recent common ancestry. When geographical barriers—such as rising sea levels or retracting ice floes—separate populations, they may develop genetic, physiological, or behavioral differences changes in chromosome structure or number differently shaped genitalia or incompatible mating times and rituals—any of which can prevent successful reproduction. Take horses and donkeys, which probably diverged about 2.4 million years ago. Horses have 64 chromosomes, while donkeys have 62, and when they mate, their chromosomes don’t pair up properly, inhibiting meiosis in their offspring. As a result, mules are sterile. Brown bears and polar bears, by contrast, evolved from the same ancestor only about 150,000 years ago—a relatively brief period—and have not developed significant genetic differences.

The prevailing theory holds that polar bears diverged from brown bears at the end of the last ice age (the Pleistocene), when a population followed retreating ice northward. As they adapted to their new arctic home, the separated population lost the brown bear’s hump and developed the polar bear’s characteristic hair (which is actually clear), narrower shoulders, longer neck, smaller head, and partially webbed toes. Despite appearances, polar bears and grizzlies are still genetically quite similar. In fact, there are multiple instances of the two species successfully interbreeding in zoos.

The reason grizzlies and polar bears rarely interbreed in the wild is that, generally speaking, they don’t cross paths during mating season. Barren-ground grizzlies live primarily on land, where they feast on caribou and berries, and mate from May to July meanwhile, polar bears mate from April to June while hunting for seals along the sea ice. But four years ago, a sports hunter shot a male grizzly-polar bear hybrid near Banks Island (just west of Victoria Island), proving that at least a couple of wild bears bridged their differences. The hybrid shot last month was the offspring of a female hybrid and male grizzly, bringing the total known wild hybrid count to three (counting the two dead bears and hybrid mother). It’s possible there could be more out there. Some scientists are re-evaluating past sightings of bears that they assumed, at the time, were blonde grizzlies.

Some scientists believe that global warming could cause the hybridization of many arctic animals, particularly marine mammals. The thinking goes that as Arctic sea ice melts, closely related species in the North Pacific and North Atlantic will come into contact and interbreed. In the case of polar bears, loss of habitat could drive them to land, where they may come into contact with grizzlies. Other scientists, however, aren’t convinced that climate change is the trigger for the hybrid bears. Changes in sea ice have been less drastic in the Beaufort Sea than in other parts of the arctic, and it seems quite possible that the pizzlies resulted from breeding pairs that met on ice, rather than on land.

Got a question about today’s news? Ask the Explainer.

Explainer thanks Brendan P. Kelly of the International Arctic Research Center, Karyn D. Rode of U.S. Fish and Wildlife Service, Lily Peacock of the USGS Alaska Science Center, Sandra Talbot of the USGS Alaska Science Center, and Marsha Branigan of the Department of Environment and Natural Resources in the Northwest Territories.

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