地球最早的有脊动物是什么

地球最早的有脊动物是什么
地球最早的有脊动物是什么

地球最早的有脊动物是什么
1999年昆明鱼和海口鱼的发现被英国《自然》杂志评论为“逮住第一鱼”,为难题的破解投进了一缕曙光.2003年初,舒德干等人再度在《自然》杂志著文,他们通过对数百枚海口鱼标本的深入研究,揭示出它们一方面已经开始演化出原始脊椎骨和眼睛等重要头部感官,另一方面却仍保留着无头类的原始性器官,从而证实了它们不仅是已知最古老的脊椎动物,而且还属于地球上一类最原始的脊椎动物.早期后口动物的系列性发现,不仅与现代动物学关于脊椎动物起源分“四步走”假说相一致,更重要的是添加了比这“四步走”更为原始的“第一步”,从而首次提出了脊椎动物起源至少分“五步走”的新假说.这些始见于澄江化石库地层最底部的“第一步”动物群古虫类和云南虫类,是一些创生出咽腔型鳃系统的原始分节后口动物,极可能代表着学术界期盼已久的原口动物和后口动物分节的共同祖先与由于躯体特化而丧失分节性的后口动物（包括棘皮动物和半索动物）之间的过渡类型.十分有趣的是,尽管它们由于咽鳃的出现而引发了动物体在取食、呼吸等新陈代谢方式的重大革新而成为真正的后口动物,但其躯体却仍保留着其祖先的分节性特征.舒德干解释说：“实际上,既出现创新特征又继承祖先某些原始性状的镶嵌演化是生物界一种十分常见的现象.”
在这分“五步走”的演化系列中,“第一步”的动物类群十分奇特：对1400多枚海口虫标本进行比较解剖学研究表明,它们不仅缺少脊索构造,而且在皮肤、肌肉、呼吸、循环、神经等器官系统上与脊索动物存在着根本区别；其中最为独特的是其由6对外鳃组成的呼吸系统,这与较为高等的后口动物的内鳃迥然有别.海口虫与同处“第一步”的古虫动物门在躯体构型上却相当一致：两者皆明显分节,而且躯体也都呈独特的“双重二分型”,即身体沿纵轴分为前体和后体两大部分,而前体又被一个能自由扩张的“中带”构造分为背、腹两个单元.所不同的是,海口虫兼具背神经索和腹神经索,这显示出它比古虫动物门稍略进步些,从而更靠近“第二步”中的半索动物.
舒德干指出,尽管我们提出了脊椎动物起源分“五步走”的新假说,但这仍只给出了一个演化轮廓,在其相邻演化步骤之间仍缺乏中间环节的证据.

1999 kunming fish and haikou fish are found to be British journal nature to "catch the first fish," for the problem of crack into a light. In early 2003, ShuDeGan etc. Again in the journal nature, the...

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1999 kunming fish and haikou fish are found to be British journal nature to "catch the first fish," for the problem of crack into a light. In early 2003, ShuDeGan etc. Again in the journal nature, they through whose hundreds of fish samples coin haikou research reveals that they had started on the evolution of the original backbones and eyes and other important head senses, on the other hand was still retains the original organ of the headless, confirming them not only is the oldest vertebrates known on earth, but also belongs to the category of the most primitive vertebrates. After a series of early mouth animals, not only with modern zoology found on vertebrates origin points "four steps" consistent hypothesis, more important than it is added to the "four step" more primitive "step", which offers the first vertebrate origin "at least five walk" new hypothesis. These beginning in the bottom stratum chengjiang fossil library "step" GuChong fauna of yunnan, and created some swallow cavity is the system of the gills after animals, mouth may represent the original academic long-expected mouth after the animals and animal stanza mouth with due to the common ancestor of body specialized lose points after the day JiPi mouth animals (including the animal and half cable animals) transition between type. Very interesting is, although they are due to swallow the gills appear sparked animal feeding, breathing in the way of metabolism and become major innovations of animals, but its mouth after the body still retains its ancestor's stanza sexual characteristics. ShuDeGan explained: "actually, innovative features and inheriting appear some primitive traits of ancestors phyla evolution is a very common phenomenon".
In this "five-step", "the evolution of the first step in the series of groups of animals very strange: 14 more haikou insect specimens comparative anatomy studies show that, they not only, and the lack of chordomas in skin, muscle, breathing, circulation, neural organs such as the system with the fundamental difference between the existing chordates, One of the most distinctive is the gills of six foreign composed of respiratory system, and this is the mouth of higher animals in the gills. Haikou insect with "first" GuChong animal body configuration on the door was quite consistent: both obvious stanza, but also has a unique body of "double half body type", namely the longitudinal axis is divided into two parts and the body precursor, a precursor to free expansion "with" structure is divided into two units, abdomen. Different is, haikou bug back and nerve concurrently ventral nerve, this shows that it is a GuChong animals gate, which is closer to the progress of the second step "and" cable animals.
ShuDeGan points out, although we proposed vertebrate origin points "five-step" new hypothesis, but this is still only a contour evolution in the adjacent evolutionary step, lack of evidence intermediate links.

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昆明鱼呀！简洁明了

应该就是鱼了....