3月4日,，刊登在《生物學(xué)評論》（Biological Reviews）上的一項研究成果稱，劍橋大學(xué)的研究人員證實了發(fā)現(xiàn)地球上出現(xiàn)最早的脊索動物,，該生物類似蠕狀，并生活于海洋中，體形大約有兩英寸長,，其被稱為纖細(xì)皮凱亞蟲(Pikaia gracilens）。該生物生存于5億年前,，被認(rèn)為是現(xiàn)代脊椎動物的最早祖先,，當(dāng)然也包括人類。本項最新的研究解決了一個具有爭議性的議題,，即該生物是否是脊索動物門大家庭中的最早成員,，而脊椎動物僅僅是脊索動物門中的一個亞門。
根據(jù)該研究的結(jié)論,，人類,、魚類、兩棲類,、鳥類,、爬行動物以及哺乳動物都是這個兩英寸長的海洋生物的后代。更確切地說,，我們的祖先可追溯至5.05億年前的海洋中,。脊索動物有一個特點，即背側(cè)有一條脊索,，對中空的背神經(jīng)管起到靈活的支撐作用,，在經(jīng)歷數(shù)百萬年的演化之后，逐漸形成了脊柱,，尤其是更高級的后代中,，脊柱是一個明顯的特征。
這只凱亞蟲化石顯示其有足足兩英寸長
研究人員在纖細(xì)皮凱亞蟲的脊索兩側(cè)發(fā)現(xiàn)了整齊的鋸齒狀結(jié)構(gòu),，它們是大約100個微小的肌肉組織塊,，也可稱其為肌分節(jié)。這些肌肉組織塊的作用便是可使該生物通過左右搖擺彎曲身體,，推動身體前進(jìn),。目前發(fā)現(xiàn)的距今5.05億年的化石保存較為完好,，可以看出纖細(xì)皮凱亞蟲沒有眼睛或者牙齒的部位，但是已經(jīng)有了一個清楚的頭部特征,，還有鰓,，用于呼吸氧氣，此外還生長著兩個小小的觸角,，科學(xué)家推測觸角的功能可能包含著感官能力,，可以尋找、覓食水中的微量成分,。
根據(jù)領(lǐng)導(dǎo)這項研究的劍橋大學(xué)教授西蒙·康韋（Simon Conway）介紹：在纖細(xì)皮凱亞蟲脊索兩側(cè)發(fā)現(xiàn)的肌分節(jié)是一個我們此前一直尋找的證據(jù),。我們現(xiàn)在已經(jīng)掌握了神經(jīng)傳導(dǎo)、脊索,、血液循環(huán)系統(tǒng)還有現(xiàn)在發(fā)現(xiàn)的肌分節(jié),，這些結(jié)構(gòu)都為我們搭建出皮凱亞蟲的面貌，清晰地表明它是這個星球上最原始的脊索動物,。如果我們在爐壁上擺上脊索生物的全家福,，那么在最后面的將會是皮凱亞蟲。
在整個發(fā)現(xiàn)過程中,，科學(xué)家研究了加拿大落基山脈布爾吉斯頁巖采集到的114個生物標(biāo)本,，其中一個最值得慶祝的標(biāo)本便是可追溯至5.05億年前的脊索生物。研究人員通過一系列新的圖像處理技術(shù)（如掃描電子顯微鏡）揭示了皮凱亞蟲化石中精細(xì)的生理結(jié)構(gòu),。但是目前尚不清楚皮凱亞蟲為什么會演化出脊椎結(jié)構(gòu),，也可能是為了更快地擺動身體以逃避獵食者。
加拿大多倫多大學(xué)吉恩伯納德·卡倫（Jean-Bernard Caron）博士也參與了這項研究,，他認(rèn)為在原始脊索動物的化石是極為罕見的,，皮凱亞蟲的化石在正常情況是無法得到保存，正是由于發(fā)現(xiàn)地點的特殊性,，即加拿大布爾吉斯頁巖的特殊環(huán)境有利于這些數(shù)億年前的化石得以保存,。我們目前希望在相關(guān)研究領(lǐng)域繼續(xù)探索，其他物種也將被發(fā)現(xiàn),，使我們對人類自己早期演化歷史有更完善的理解,。（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.
