Northcutt R G
Neurobiology Unit, Scripps Institution of Oceanography, La Jolla, Calif., USA.
Brain Behav Evol. 1996;48(5):237-47. doi: 10.1159/000113203.
An outgroup analysis of head organization of tunicates, lancelets and craniates indicates that the origin of craniates involved the development of a new muscular pharyngeal pump, which was a key innovation underlying the evolution of a more metabolically active group of chordates. This innovation involved the genesis of new embryonic tissues, neural crest and neurogenic placodes, whose derivatives formed skeletal elements of the new pump as well as new receptors advantageous to the more mobile craniates. These changes also involved the evolution of a more complex brain by elaboration and reorganization of a lancelet-like cephalic neural tube. All of these events appear to be linked to a number of homeobox genes that were duplicated in the ancestral chordates leading to craniates. An outgroup analysis of the life histories of lancelets, hagfishes and lampreys suggests that hagfishes and lampreys have life histories that are substantially derived in comparison to that of ancestral craniates and that the highly derived features of the brain of hagfishes evolved in conjunction with their invasion of a bathyal benthic environment.
对被囊动物、文昌鱼和有头类动物头部结构的外类群分析表明，有头类动物的起源涉及一种新的肌肉咽泵的发育，这是一群代谢更活跃的脊索动物进化的关键创新。这一创新涉及新的胚胎组织、神经嵴和神经源性基板的产生，其衍生物形成了新泵的骨骼成分以及对更具移动性的有头类动物有利的新受体。这些变化还涉及通过对类似文昌鱼的头部神经管进行细化和重组，使大脑变得更加复杂。所有这些事件似乎都与一些在导致有头类动物的祖先脊索动物中发生复制的同源框基因有关。对文昌鱼、盲鳗和七鳃鳗生活史的外类群分析表明，与祖先有头类动物相比，盲鳗和七鳃鳗的生活史有很大的衍生性，并且盲鳗高度衍生的大脑特征是在它们侵入深海海底环境的过程中进化而来的。
