A popular hypothesis about animal diversification is that unique changes occurred in the Precambrian or Cambrian (ca. 700-500 millions of years [Myr] ago) to produce the distinctive features of all animal "Bauplane" ("body plans") and that such changes have not occurred since (Gould 1989:47). In contrast, we suggest that changes similar to the key innovations initiating the appearance of these distinctive features occur repeatedly during evolution. A major example is the "inversion" of the dorsoventral axis in the evolution of chordates (Arendt and Nubler-Jung 1994), initiated by a switch in mouth position from the neural to the abneural side. Here we note that similar changes in mouth position evolved <50 Myr ago at least twice in a group of nematodes related to Caenorhabditis elegans. Because this means that such changes were not unique to the Cambrian, they can be studied by experimental approaches in closely related related extant organisms. A direct consequence of this focus on studying elemental key changes is that "Bauplan" becomes a less useful concept for understanding how animal diversity evolved.
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机译：关于动物多样化的一个普遍假设是，前寒武纪或寒武纪（大约在700-500亿年以前）发生了独特的变化，以产生所有动物“ Bauplane”（“人体计划”）的独特特征，并且这种变化从那以后就没有发生过任何变化（Gould 1989：47）。相反，我们建议，与启动这些独特功能的外观的关键创新相似的变化在进化过程中会反复发生。一个主要的例子是在脊索进化过程中背腹轴的“倒置”（Arendt and Nubler-Jung 1994），这是由嘴侧位置从神经侧向腹侧转变引起的。在这里，我们注意到在与秀丽隐杆线虫有关的线虫组中，至少两次口嘴位置的类似变化在<50 Myr之前演化。因为这意味着这种变化不是寒武纪所特有的，所以可以通过实验方法在紧密相关的现存生物中进行研究。专注于研究基本关键变化的直接结果是，“ Bauplan”成为了解动物多样性如何演变的不太有用的概念。
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