3月4日,刊登在《生物学评论》(Biological Reviews)上的一项研究成果称,剑桥大学的研究人员证实了发现地球上出现最早的脊索动物,该生物类似蠕状,并生活于海洋中,体形大约有两英寸长,其被称为纤细皮凯亚虫(Pikaia gracilens)。该生物生存于5亿年前,被认为是现代脊椎动物的最早祖先,当然也包括人类。本项最新的研究解决了一个具有争议性的议题,即该生物是否是脊索动物门大家庭中的最早成员,而脊椎动物仅仅是脊索动物门中的一个亚门。
根据该研究的结论,人类、鱼类、两栖类、鸟类、爬行动物以及哺乳动物都是这个两英寸长的海洋生物的后代。更确切地说,我们的祖先可追溯至5.05亿年前的海洋中。脊索动物有一个特点,即背侧有一条脊索,对中空的背神经管起到灵活的支撑作用,在经历数百万年的演化之后,逐渐形成了脊柱,尤其是更高级的后代中,脊柱是一个明显的特征。
这只凯亚虫化石显示其有足足两英寸长
研究人员在纤细皮凯亚虫的脊索两侧发现了整齐的锯齿状结构,它们是大约100个微小的肌肉组织块,也可称其为肌分节。这些肌肉组织块的作用便是可使该生物通过左右摇摆弯曲身体,推动身体前进。目前发现的距今5.05亿年的化石保存较为完好,可以看出纤细皮凯亚虫没有眼睛或者牙齿的部位,但是已经有了一个清楚的头部特征,还有鳃,用于呼吸氧气,此外还生长着两个小小的触角,科学家推测触角的功能可能包含着感官能力,可以寻找、觅食水中的微量成分。
根据领导这项研究的剑桥大学教授西蒙·康韦(Simon Conway)介绍:在纤细皮凯亚虫脊索两侧发现的肌分节是一个我们此前一直寻找的证据。我们现在已经掌握了神经传导、脊索、血液循环系统还有现在发现的肌分节,这些结构都为我们搭建出皮凯亚虫的面貌,清晰地表明它是这个星球上最原始的脊索动物。如果我们在炉壁上摆上脊索生物的全家福,那么在最后面的将会是皮凯亚虫。
在整个发现过程中,科学家研究了加拿大落基山脉布尔吉斯页岩采集到的114个生物标本,其中一个最值得庆祝的标本便是可追溯至5.05亿年前的脊索生物。研究人员通过一系列新的图像处理技术(如扫描电子显微镜)揭示了皮凯亚虫化石中精细的生理结构。但是目前尚不清楚皮凯亚虫为什么会演化出脊椎结构,也可能是为了更快地摆动身体以逃避猎食者。
加拿大多伦多大学吉恩伯纳德·卡伦(Jean-Bernard Caron)博士也参与了这项研究,他认为在原始脊索动物的化石是极为罕见的,皮凯亚虫的化石在正常情况是无法得到保存,正是由于发现地点的特殊性,即加拿大布尔吉斯页岩的特殊环境有利于这些数亿年前的化石得以保存。我们目前希望在相关研究领域继续探索,其他物种也将被发现,使我们对人类自己早期演化历史有更完善的理解。(生物谷 Bioon.com)
doi:10.1111/j.1469-185X.2012.00220.x
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Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia
Simon Conway Morris, Jean-Bernard Caron
The Middle Cambrian Pikaia gracilens (Walcott) has an iconic position as a Cambrian chordate, but until now no detailed description has been available. Here on the basis of the 114 available specimens we review its anatomy, confirm its place in the chordates and explore with varying degrees of confidence its relationships to both extant and extinct chordates and other deuterostomes. The body of Pikaia is fusiform, laterally compressed and possesses about 100 myomeres. The head is small, bilobed and bears two narrow tentacles. There is no evidence for eyes. Apart from a thin dorsal fin (without finrays) and a series of at least nine bilaterally arranged appendages with possible pharyngeal pores at the anterior end, there are no other external features. In addition to the musculature the internal anatomy includes an alimentary canal, the anterior of which forms a prominent lenticular unit that is almost invariably preserved in positive relief. The cavity is interpreted as pharyngeal, implying that the mouth itself was almost terminal. The posterior extension of the gut is unclear although the anus appears to have been terminal. The most prominent internal structure is a reflectively preserved unit, possibly hollow, termed here the dorsal organ. Although formerly interpreted as a notochord its position and size make this less likely. Its original function remains uncertain, but it could have formed a storage organ. Ventral to the dorsal organ a narrower strand of tissue is interpreted as representing the nerve chord and notochord. In addition to these structures, there is also evidence for a vascular system, including a ventral blood vessel.
The position of Pikaia in the chordates is largely based on the presence of sigmoidal myomeres, and the more tentative identification of a notochord. In many other respects, Pikaia differs from the expected nature of primitive chordates, especially as revealed in amphioxus and the Cambrian record (including Cathaymyrus, Haikouichthys, Metaspriggina, Myllokunmingia, and Zhongxiniscus). Whilst the possibility that Pikaia is simply convergent on the chordates cannot be dismissed, we prefer to build a scenario that regards Pikaia as the most stem-ward of the chordates with links to the phylogenetically controversial yunnanozoans. This hypothesis has implications for the evolution of the myomeres, notochord and gills. Finally, the wealth of material of Pikaia indicates that, although by definition there must be some sort of taphonomic imprint, the consistency of preservational details allows a reliable reconstruction of the anatomy and does not significantly erode phylogenetically relevant characters.