大約在30億年前，地球上只有一個(gè)名為“最后普遍共同祖先(LUCA)”的生物,。它形態(tài)龐大,，是一種史無(wú)前例的巨大生物，填充了地球上的所有海洋,。隨后它分裂為三個(gè)部分,，最終誕生了今天地球上所有生命的祖先。
科學(xué)家最新的研究成果顯示,，LUCA是早期生命努力生存的結(jié)果。
在數(shù)百萬(wàn)年的時(shí)間內(nèi),，LUCA試圖將海洋轉(zhuǎn)變?yōu)槿蚧蚪粨Q工廠,。試圖努力存活下來(lái)的細(xì)胞在無(wú)競(jìng)爭(zhēng)環(huán)境下相互交換有用物質(zhì)，有效地創(chuàng)造出覆蓋全球的巨大生物,。
約29億年前,，LUCA分裂成三種不同的生命形態(tài)：?jiǎn)渭?xì)胞細(xì)菌,、原始細(xì)菌以及能夠演變?yōu)閯?dòng)物和植物的更為復(fù)雜的真核細(xì)胞。要想知道在分裂之前發(fā)生了什么很困難,。那個(gè)時(shí)代幾乎沒有留下任何化石遺跡,，任何能追溯到那個(gè)年代的基因可能均已發(fā)生無(wú)法辨認(rèn)的變異。
伊利諾伊大學(xué)厄巴納-尚佩恩分校的古斯塔沃·卡埃塔諾-阿諾勒說,，要描繪LUCA的樣貌并不存在什么無(wú)法克服的困難,。盡管基因序列的變化十分迅速，但基因編碼形成蛋白質(zhì)的三維結(jié)構(gòu)卻更能經(jīng)受得住時(shí)間的考驗(yàn),。他說,，如果現(xiàn)存所有生物都產(chǎn)生一種大致結(jié)構(gòu)相同的蛋白質(zhì)，那么在LUCA中存在這種蛋白質(zhì)結(jié)構(gòu)的可能性就很大,。他將這種結(jié)構(gòu)稱作活化石,，并且指出由于蛋白質(zhì)功能非常依賴其結(jié)構(gòu)，這些活化石能夠告訴我們LUCA能做什么,。
為重建LUCA能夠產(chǎn)生的蛋白質(zhì)集合,，卡埃塔諾-阿諾勒在一個(gè)包含420種現(xiàn)代生物蛋白質(zhì)的數(shù)據(jù)庫(kù)中搜索，尋找全部類別蛋白質(zhì)共同具有的結(jié)構(gòu),。根據(jù)他尋找到的結(jié)構(gòu),，只有5%至11%的結(jié)構(gòu)具有普遍性，這意味著它們保存了足夠多源于LUCA的結(jié)構(gòu),。
德國(guó)奧斯納布呂克大學(xué)的阿芒·米爾基賈尼安說：“有充分論述支持(這種巨大生物)共享基因,、酶和代謝物。”在只能生存在混合族群中的微生物族群里還能看到這種基因交換系統(tǒng)的痕跡,。（生物谷Bioon.com）
doi:
PMC:
PMID:
Life began with a planetary mega-organism
Hiroyasu Yamamoto, Evan G. Williams, Laurent Mouchiroud, Carles Cantó, Weiwei Fan, Michael Downes, Christophe Héligon, Grant D. Barish, Béatrice Desvergne, Ronald M. Evans et al.
ONCE upon a time, 3 billion years ago, there lived a single organism called LUCA. It was enormous: a mega-organism like none seen since, it filled the planet's oceans before splitting into three and giving birth to the ancestors of all living things on Earth today.This strange picture is emerging from efforts to pin down the last universal common ancestor - not the first life that emerged on Earth but the life form that gave rise to all others.The latest results suggest LUCA was the result of early life's fight to survive, attempts at which turned the ocean into a global genetic swap shop for hundreds of millions of years. Cells struggling to survive on their own exchanged useful parts with each other without competition - effectively creating a global megaorganism.It was around 2.9 billion years ago that LUCA split into the three domains of life: the single-celled bacteria and archaea, and the more complex eukaryotes that gave rise to animals and plants (see timeline). It's hard to know what happened before the split. Hardly any fossil evidence remains from this time, and any genes that date that far back are likely to have mutated beyond recognition.
