Comparative neurobiologists have long wondered when and how the dorsal pallium (e.g., mammalian neocortex) evolved. For the last 50 years, the most widely accepted answer has been that this structure was already present in the earliest vertebrates and, therefore, homologous between the major vertebrate lineages. One challenge for this hypothesis is that the olfactory bulbs project throughout most of the pallium in the most basal vertebrate lineages (notably lampreys, hagfishes, and lungfishes) but do not project to the putative dorsal pallia in teleosts, cartilaginous fishes, and amniotes (i.e., reptiles, birds, and mammals). To make sense of these data, one may hypothesize that a dorsal pallium existed in the earliest vertebrates and received extensive olfactory input, which was subsequently lost in several lineages. However, the dorsal pallium is notoriously difficult to delineate in many vertebrates, and its homology between the various lineages is often based on little more than its topology. Therefore, we suspect that dorsal pallia evolved independently in teleosts, cartilaginous fishes, and amniotes. We further hypothesize that the emergence of these dorsal pallia was accompanied by the phylogenetic restriction of olfactory projections to the pallium and the expansion of inputs from other sensory modalities. We do not deny that the earliest vertebrates may have possessed nonolfactory sensory inputs to some parts of the pallium, but such projections alone do not define a dorsal pallium.
Brain Behav Evol

中文翻译：

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多脊椎动物谱系中背侧苍白的独立进化

比较神经生物学家长期以来一直想知道背侧大脑皮层（例如哺乳动物新皮质）何时以及如何进化。在过去的 50 年中，最广泛接受的答案是这种结构已经存在于最早的脊椎动物中，因此在主要脊椎动物谱系之间是同源的。这一假设面临的一个挑战是，嗅球在最基础的脊椎动物谱系（尤其是七鳃鳗、盲鳗和肺鱼）的大部分大脑皮层中投射，但不会投射到硬骨鱼、软骨鱼和羊膜动物中推定的背侧大脑皮层（即、爬行动物、鸟类和哺乳动物）。为了理解这些数据，人们可能会假设背侧大脑皮层存在于最早的脊椎动物中并接受了广泛的嗅觉输入，随后在几个谱系中丢失了。然而，众所周知，背侧大脑皮层在许多脊椎动物中难以描绘，它在各个谱系之间的同源性通常仅基于其拓扑结构。因此，我们怀疑硬骨鱼、软骨鱼和羊膜动物的背侧苍白是独立进化的。我们进一步假设，这些背侧大脑皮层的出现伴随着对大脑大脑皮层的嗅觉投射的系统发育限制以及来自其他感觉方式的输入的扩展。我们不否认最早的脊椎动物可能对大脑皮层的某些部分拥有非嗅觉的感觉输入，但仅这些投射并不能定义背侧大脑皮层。我们怀疑硬骨鱼、软骨鱼类和羊膜动物的背侧苍白是独立进化的。我们进一步假设，这些背侧大脑皮层的出现伴随着对大脑大脑皮层的嗅觉投射的系统发育限制以及来自其他感觉方式的输入的扩展。我们不否认最早的脊椎动物可能对大脑皮层的某些部分拥有非嗅觉的感觉输入，但仅这些投射并不能定义背侧大脑皮层。我们怀疑硬骨鱼、软骨鱼类和羊膜动物的背侧苍白是独立进化的。我们进一步假设，这些背侧大脑皮层的出现伴随着对大脑大脑皮层的嗅觉投射的系统发育限制以及来自其他感觉方式的输入的扩展。我们不否认最早的脊椎动物可能对大脑皮层的某些部分拥有非嗅觉的感觉输入，但仅这些投射并不能定义背侧大脑皮层。
大脑行为进化