最近，瓮安生物群动物胚胎化石的研究又有了新进展。以中科院南京地质古生物研究所陈均远研究员为首的国际科学小组利用同步辐射相衬显微CT技术研究了两颗保存精美且有极性分化的胚胎化石。相关成果不久前发表在《美国科学院院刊》（PNAS）上（Complex embryos displaying bilaterian characters from Precambrian Doushantuo phosphate deposits, Weng’an, Guizhou, China. PNAS, 2009, 45(106): 19056–19060）。这是该科学小组继2004年和2006年在Science上分别报道最古老的“两侧对称动物小春虫化石”和“具极叶的动物胚胎化石”以来，为论证5.8亿年前的瓮安生物群中已经存在两侧对称动物添加的又一力证。
包括人类在内的复杂多细胞动物的始祖究竟是什么时候开始出现在地球上的呢？这是目前科学界最引人入胜同样也是人们知之甚少的重大科学问题之一。虽然分子生物学家使用分子钟对此进行过许多推测，但不同的分子钟测算的结果差异较大，且因为缺乏相应的化石记录来佐证而未能给出令人满意的答案。瓮安生物群中成吨产出的动物胚胎化石为解决这一难题带来了新的机遇。被磷酸盐化后以三维立体的形态保存在贵州瓮安埃迪卡拉纪陡山沱组的动物胚胎化石是迄今为止地球上最古老的后生动物化石记录，给科学家实证研究后生动物尤其是两侧对称动物的起源和早期演化提供了独一无二的材料。然而，瓮安生物群中绝大部分动物化石处在囊胚期之前的卵裂阶段，囊胚期之后的化石稀少，成体化石更难觅踪影，故曾有学者认为，瓮安生物群中动物胚胎在细胞增长到成百上千的数量时仍然没有分化，缺失原肠胚期及之后的阶段。并由此断定它们代表了早期后生动物的基干类群，真正的海绵动物以及更高等的后生动物在瓮安生物群中可能并不存在。虽然该观点与分子钟的预测相左，但瓮安陡山沱组动物化石记录的偏差使得该观点长期存在。而陈均远等人的研究成果使得该观点受到了严峻挑战。
陈均远等人使用目前最为先进的同步辐射X射线相衬显微CT技术，对新发现的两颗胚胎化石进行了三维结构重建。三维重建结果显示这两颗胚胎化石已经开始了细胞的迁移和分化，并在此基础上产生了极性，即出现了前后轴、背腹轴和左右轴。这些显著生物学特征表明它们与两侧对称动物有着十分密切的亲缘关系。值得指出的是，这两颗胚胎化石在细胞迁移和重排的过程中采用了完全不同的机制，暗示两者可能来自不同的分类群，说明两侧对称动物不仅仅在新元古代就已经崛起，而且有了相当程度的分化，这为了解寒武纪大爆发之前后生动物的演化历程提供了重要线索。（生物谷Bioon.com）
生物谷推荐原始出处：
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.
