Charles Darwin stated, “community in embryonic structure reveals community of descent”. Thus, to understand how the neocortex emerged during mammalian evolution we need to understand the evolution of the development of the pallium, the source of the neocortex. In this article, we review the variations in the development of the pallium that enabled the production of the six-layered neocortex. We propose that an accumulation of subtle modifications from very early brain development accounted for the diversification of vertebrate pallia and the origin of the neocortex. Initially, faint differences of expression of secretable morphogens promote a wide variety in the proportions and organization of sectors of the early pallium in different vertebrates. It prompted different sectors to host varied progenitors and distinct germinative zones. These cells and germinative compartments generate diverse neuronal populations that migrate and mix with each other through radial and tangential migrations in a taxon-specific fashion. Together, these early variations had a profound influence on neurogenetic gradients, lamination, positioning, and connectivity. Gene expression, hodology, and physiological properties of pallial neurons are important features to suggest homologies, but the origin of cells and their developmental trajectory are fundamental to understand evolutionary changes. Our review compares the development of the homologous pallial sectors in sauropsids and mammals, with a particular focus on cell lineage, in search of the key changes that led to the appearance of the mammalian neocortex.

查尔斯·达尔文说过，“群体的胚胎结构揭示了血统的群体”。因此，为了了解新皮质在哺乳动物进化过程中是如何出现的，我们需要了解大脑皮层（新皮质的来源）发育的演变。在本文中，我们回顾了能够产生六层新皮质的大脑皮层发育的变化。我们认为，早期大脑发育过程中微妙变化的积累解释了脊椎动物大脑皮层的多样化和新皮质的起源。最初，分泌型成形素表达的微弱差异促进了不同脊椎动物早期大脑皮层部分的比例和组织的广泛变化。它促使不同的部门容纳不同的祖细胞和不同的发芽区。这些细胞和萌发区室产生不同的神经元群体，这些神经元群体通过径向和切向迁移以分类单元特异性的方式相互迁移和混合。总之，这些早期变异对神经发生梯度、分层、定位和连接产生了深远的影响。大脑皮层神经元的基因表达、同源性和生理特性是表明同源性的重要特征，但细胞的起源及其发育轨迹是理解进化变化的基础。我们的综述比较了蜥脚类动物和哺乳动物同源大脑皮层部分的发育，特别关注细胞谱系，以寻找导致哺乳动物新皮质出现的关键变化。