鳃的气体交换和离子调节在脊椎动物1,2,3,4的进化中具有关键作用。据推测，鳃首先从干脊椎动物的皮肤中获得了这些重要的稳态功能，从而促进了更大、更活跃的生活方式2,3,5的进化。然而，这一假设缺乏相关分类群的功能支持。在这里，我们描述了脊椎动物的鳃和皮肤的功能（七鳃鳗；Entosphenus tridentatus），一种头孢动物（文昌鱼；Branchiostoma floridae）和一种半脊索动物（橡子蠕虫；Saccoglossus kowalevskii），具有脊椎动物祖先的假定挖洞、滤食性特征6,7,8,9. 随着体型和活动的增加，我们为脊椎动物在鳃处气体交换的起源提供功能支持，因为体内直接测量表明，鳃是仅在 ammocoetes 中气体交换的主要部位，并且仅在体型增加或对氧气的挑战时才会发生供需。相反，所有三个分类群的鳃都与离子调节有关。Ammocoete 鳃负责所有体型的所有离子通量，而离子调节的分子标记在鳃中高于文昌鱼和橡子虫的皮肤。这表明鳃的离子调节比气体交换更早起源，这与脊椎动物的大小和活动无关 - 可能是在茎氘核中的咽孔开始时。
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Ion regulation at gills precedes gas exchange and the origin of vertebrates
Gas exchange and ion regulation at gills have key roles in the evolution of vertebrates1,2,3,4. Gills are hypothesized to have first acquired these important homeostatic functions from the skin in stem vertebrates, facilitating the evolution of larger, more-active modes of life2,3,5. However, this hypothesis lacks functional support in relevant taxa. Here we characterize the function of gills and skin in a vertebrate (lamprey ammocoete; Entosphenus tridentatus), a cephalochordate (amphioxus; Branchiostoma floridae) and a hemichordate (acorn worm; Saccoglossus kowalevskii) with the presumed burrowing, filter-feeding traits of vertebrate ancestors6,7,8,9. We provide functional support for a vertebrate origin of gas exchange at the gills with increasing body size and activity, as direct measurements in vivo reveal that gills are the dominant site of gas exchange only in ammocoetes, and only with increasing body size or challenges to oxygen supply and demand. Conversely, gills of all three taxa are implicated in ion regulation. Ammocoete gills are responsible for all ion flux at all body sizes, whereas molecular markers for ion regulation are higher in the gills than in the skin of amphioxus and acorn worms. This suggests that ion regulation at gills has an earlier origin than gas exchange that is unrelated to vertebrate size and activity—perhaps at the very inception of pharyngeal pores in stem deuterostomes.