Kaas J H
Department of Psychology, Vanderbilt University, Nashville, TN 37240-0009, USA.
Brain Behav Evol. 1995;46(4-5):187-96. doi: 10.1159/000113273.
There are several reasons why we lack detailed and comprehensive theories of how isocortex evolved in the various lines of mammalian evolution. Although current methods allow cortical areas to be defined with a high degree of assurance, few taxa have been studied in detail, and even the most-studied taxa are incompletely understood. In addition, concepts persist from early studies, based on limited data, that confound current theories, and some theories of isocortical evolution have been based on questionable premises. Nevertheless, some conclusions are clearly supportable. Early mammals had small brains with proportionately little isocortex. Mammals with larger brains and proportionately more isocortex evolved in several lines of descent. All mammals appear to have roughly 20 cortical areas, 'the organs of the brain', in common as retentions from an early ancestor, with primary and secondary sensory fields occupying much of cortex. Some of these cortical areas have been greatly modified in some taxa to become significantly expanded in size, highly laminated structurally, or both. Numbers of areas have increased independently in several branches of mammalian evolution, and the functioning of large brains may be enhanced by having more subdivisions. Finally, over many generations new areas may emerge from old by the formation of functionally distinct modules within areas, followed by the fusion of modules to ultimately form separate ones.
我们缺乏关于等皮质在各种哺乳动物进化谱系中如何演化的详细且全面理论，原因有以下几点。尽管当前方法能高度确定地定义皮质区域，但仅有少数分类群得到了详细研究，即便研究得最深入的分类群也未被完全理解。此外，基于有限数据的早期研究中的一些概念依然存在，这些概念混淆了当前的理论，而且一些等皮质进化理论是基于有问题的前提。然而，一些结论显然是有依据的。早期哺乳动物的大脑较小，等皮质所占比例相对较小。在多个进化谱系中出现了大脑较大且等皮质所占比例相对更大的哺乳动物。所有哺乳动物似乎都大致共有20个皮质区域，即“大脑器官”，这是从早期祖先那里保留下来的，其中初级和次级感觉区域占据了大部分皮质。在一些分类群中，这些皮质区域中的一些发生了很大变化，在大小上显著扩大，在结构上高度分层，或两者皆有。在哺乳动物进化的几个分支中，区域数量独立增加，大脑功能的增强可能得益于更多的细分。最后，经过许多代，新的区域可能会通过在区域内形成功能不同的模块，然后模块融合最终形成独立区域，从旧区域中产生。
