Cummings Frederick W
University of California Riverside, California, USA.
Int J Dev Biol. 2006;50(2-3):193-208. doi: 10.1387/ijdb.052058fc.
The 'Cambrian explosion', about 540 million years ago, may have occurred within 10 to 50 million years. Almost all of the modern phyla, a very restricted group and many groups that may represent extinct phyla, suddenly appear near that time in the fossil record. Numerous extensive periods of mass extinction since that time led to no new phyla. This is taken as an impetus to examine a possible source, beyond Darwinian adaptation, of the apparently restricted number of phyla. Such a postulated constraint or restriction beyond adaptation is proposed to be based on a mutation or mutations allowing single celled or colonial precursors to begin forming into epithelial sheets and gene activation patterns of a particular kind, those giving rise to the very earliest metazoans. The interaction of signaling pathways in pairs, with different pairs acting sequentially are proposed as key to this earliest patterning, such patterning being extensively elaborated over the last approximately 550 million years. Restrictions on the very large set of possible forms and patterns on which adaptation acts are discussed.
约5.4亿年前的“寒武纪大爆发”可能发生在1000万至5000万年的时间内。几乎所有现代门类，这是一个非常有限的类群，以及许多可能代表已灭绝门类的类群，在化石记录中几乎在同一时期突然出现。自那时以来，无数次大规模灭绝的漫长时期并未导致新门类的产生。这促使人们去研究除达尔文式适应之外，门类数量明显有限的可能来源。这种超出适应范畴的假定限制被认为是基于一个或多个突变，这些突变使得单细胞或群体前体开始形成上皮层以及特定类型的基因激活模式，正是这些模式产生了最早的后生动物。成对信号通路的相互作用，不同的成对通路依次发挥作用，被认为是这种最早模式形成的关键，这种模式在过去约5.5亿年里得到了广泛的细化。文中还讨论了对适应作用的大量可能形式和模式的限制。
