Vertebrates develop an olfactory system that detects odorants and pheromones through their interaction with specialized cell surface receptors on olfactory sensory neurons. During development, the olfactory system forms from the olfactory placodes, specialized areas of the anterior ectoderm that share cellular and molecular properties with placodes involved in the development of other cranial senses. The early-diverging chordate lineages amphioxus, tunicates, lampreys and hagfishes give insight into how this system evolved. Here, we review olfactory system development and cell types in these lineages alongside chemosensory receptor gene evolution, integrating these data into a description of how the vertebrate olfactory system evolved. Some olfactory system cell types predate the vertebrates, as do some of the mechanisms specifying placodes, and it is likely these two were already connected in the common ancestor of vertebrates and tunicates. In stem vertebrates, this evolved into an organ system integrating additional tissues and morphogenetic processes defining distinct olfactory and adenohypophyseal components, followed by splitting of the ancestral placode to produce the characteristic paired olfactory organs of most modern vertebrates.

脊椎动物开发出一种嗅觉系统，通过与嗅觉感觉神经元上的专门细胞表面受体的相互作用来检测气味和信息素。在发育过程中，嗅觉系统由嗅觉基板形成，嗅觉基板是前外胚层的专门区域，与参与其他颅感觉发育的基板共享细胞和分子特性。早期分化的脊索动物谱系 amphioxus、被囊动物、七鳃鳗和盲鳗提供了对该系统如何进化的深入了解。在这里，我们回顾了这些谱系中的嗅觉系统发育和细胞类型以及化学感应受体基因的进化，将这些数据整合到对脊椎动物嗅觉系统如何进化的描述中。一些嗅觉系统细胞类型早于脊椎动物，一些指定基板的机制也是如此，很可能这两者在脊椎动物和被囊动物的共同祖先中已经联系在一起。在干脊椎动物中，这演变成一个器官系统，整合了额外的组织和形态发生过程，定义了不同的嗅觉和腺垂体成分，然后分裂了祖先基板，产生了大多数现代脊椎动物的特征配对嗅觉器官。