最近,，甕安生物群動物胚胎化石的研究又有了新進(jìn)展,。以中科院南京地質(zhì)古生物研究所陳均遠(yuǎn)研究員為首的國際科學(xué)小組利用同步輻射相襯顯微CT技術(shù)研究了兩顆保存精美且有極性分化的胚胎化石。相關(guān)成果不久前發(fā)表在《美國科學(xué)院院刊》（PNAS）上（Complex embryos displaying bilaterian characters from Precambrian Doushantuo phosphate deposits, Weng’an, Guizhou, China. PNAS, 2009, 45(106): 19056–19060）,。這是該科學(xué)小組繼2004年和2006年在Science上分別報道最古老的“兩側(cè)對稱動物小春蟲化石”和“具極葉的動物胚胎化石”以來,，為論證5.8億年前的甕安生物群中已經(jīng)存在兩側(cè)對稱動物添加的又一力證。
包括人類在內(nèi)的復(fù)雜多細(xì)胞動物的始祖究竟是什么時候開始出現(xiàn)在地球上的呢,？這是目前科學(xué)界最引人入勝同樣也是人們知之甚少的重大科學(xué)問題之一,。雖然分子生物學(xué)家使用分子鐘對此進(jìn)行過許多推測,，但不同的分子鐘測算的結(jié)果差異較大,，且因為缺乏相應(yīng)的化石記錄來佐證而未能給出令人滿意的答案。甕安生物群中成噸產(chǎn)出的動物胚胎化石為解決這一難題帶來了新的機(jī)遇,。被磷酸鹽化后以三維立體的形態(tài)保存在貴州甕安埃迪卡拉紀(jì)陡山沱組的動物胚胎化石是迄今為止地球上最古老的后生動物化石記錄,，給科學(xué)家實證研究后生動物尤其是兩側(cè)對稱動物的起源和早期演化提供了獨(dú)一無二的材料。然而,，甕安生物群中絕大部分動物化石處在囊胚期之前的卵裂階段,，囊胚期之后的化石稀少，成體化石更難覓蹤影,，故曾有學(xué)者認(rèn)為,，甕安生物群中動物胚胎在細(xì)胞增長到成百上千的數(shù)量時仍然沒有分化，缺失原腸胚期及之后的階段,。并由此斷定它們代表了早期后生動物的基干類群,，真正的海綿動物以及更高等的后生動物在甕安生物群中可能并不存在。雖然該觀點(diǎn)與分子鐘的預(yù)測相左,，但甕安陡山沱組動物化石記錄的偏差使得該觀點(diǎn)長期存在,。而陳均遠(yuǎn)等人的研究成果使得該觀點(diǎn)受到了嚴(yán)峻挑戰(zhàn)。
陳均遠(yuǎn)等人使用目前最為先進(jìn)的同步輻射X射線相襯顯微CT技術(shù),，對新發(fā)現(xiàn)的兩顆胚胎化石進(jìn)行了三維結(jié)構(gòu)重建,。三維重建結(jié)果顯示這兩顆胚胎化石已經(jīng)開始了細(xì)胞的遷移和分化,，并在此基礎(chǔ)上產(chǎn)生了極性，即出現(xiàn)了前后軸,、背腹軸和左右軸,。這些顯著生物學(xué)特征表明它們與兩側(cè)對稱動物有著十分密切的親緣關(guān)系。值得指出的是,，這兩顆胚胎化石在細(xì)胞遷移和重排的過程中采用了完全不同的機(jī)制,，暗示兩者可能來自不同的分類群，說明兩側(cè)對稱動物不僅僅在新元古代就已經(jīng)崛起,，而且有了相當(dāng)程度的分化,，這為了解寒武紀(jì)大爆發(fā)之前后生動物的演化歷程提供了重要線索。（生物谷Bioon.com）
生物谷推薦原始出處：
PNAS October 26, 2009, doi: 10.1073/pnas.0904805106
Complex embryos displaying bilaterian characters from Precambrian Doushantuo phosphate deposits, Weng'an, Guizhou, China
Jun-Yuan Chena,1, David J. Bottjerb, Gang Lic, Michael G. Hadfieldd, Feng Gaoe, Andrew R. Camerone, Chen-Yu Zhanga, Ding-Chang Xianc, Paul Tafforeauf, Xin Liaoa and Zong-Jun Yina
aLPS of Nanjing Institute of Geology and Paleontology, Institute of Evo/Developmental Biology, and State Key Laboratory of Pharmaceutical Biotechnology, Nanjing University, Nanjing 210093, China;
bDepartment of Earth Sciences, University of Southern California, Los Angeles, CA 90089;
cInstitute of High Energy Physics, Beijing 100049, China;
dKewalo Marine Laboratory, University of Hawaii at Manoa, Honolulu, HI 96813;
eDivision of Biology, 156-29, California Institute of Technology, Pasadena, CA 91125; and
fEuropean Synchrotron Radiation Facility, F-38043 Grenoble, France
Three-dimensionally preserved embryos from the Precambrian Ediacaran Doushantuo Formation, Weng'an, Guizhou, southern China, have attracted great attention as the oldest fossil evidence yet found for multicellular animal life on Earth. Many embryos are early cleavage embryos and most of them yield a limited phylogenetic signal. Here we report the discovery of two Doushantuo embryos that are three-dimensionally preserved and complex. Imaging techniques using propagation phase-contrast based synchrotron radiation microtomography (PPC-SR-μCT) reveal that the organization of cells demonstrates several bilaterian features, including the formation of anterior-posterior, dorso-ventral, and right-left polarities, and cell differentiation. Unexpectedly, our observations show a noticeable difference in organization patterns between the embryos, suggesting that they represent two distinct taxa. These embryos provide further evidence for the presence of bilaterian animals in the Doushantuo biota. Furthermore, these bilaterians had already diverged into distantly related groups at least 40 million years before the Cambrian radiation, indicating that the last common ancestor of the bilaterians lived much earlier than is usually thought.
