Abstract：
A crucial evolutionary change in vertebrate history was the Palaeozoic (Devonian 419-359 million years ago) water-to-land transition, allowed by key morphological and physiological modifications including the acquisition of lungs. Nonetheless, the origin and early evolution of vertebrate lungs remain highly controversial, particularly whether the ancestral state was paired or unpaired. Due to the rarity of fossil soft tissue preservation, lung evolution can only be traced based on the extant phylogenetic bracket. Here we investigate, for the first time, lung morphology in extensive developmental series of key living lunged osteichthyans using synchrotron x-ray microtomography and histology. Our results shed light on the primitive state of vertebrate lungs as unpaired, evolving to be truly paired in the lineage towards the tetrapods. The water-to-land transition confronted profound physiological challenges and paired lungs were decisive for increasing the surface area and the pulmonary compliance and volume, especially during the air-breathing on land.
All life on Earth started out under water. However, around 400 million years ago some vertebrates, such as fish, started developing limbs and other characteristics that allowed them to explore life on land. One of the most pivotal features to evolve was the lungs, which gave vertebrates the ability to breathe above water. Most land-living vertebrates, including humans, have two lungs which sit on either side of their chest. The lungs extract oxygen from the atmosphere and transfer it to the bloodstream in exchange for carbon dioxide which then gets exhaled out in to the atmosphere. How this important organ first evolved is a hotly debated topic. This is largely because lung tissue does not preserve well in fossils, making it difficult to trace how the lungs of vertebrates changed over the course of evolution. To overcome this barrier, Cupello et al. compared the lungs of living species which are crucial to understand the early stages of the water-to-land transition. This included four species of lunged bony fish which breathe air at the water surface, and a four-legged salamander that lives on land. Cupello et al. used a range of techniques to examine how the lungs of the bony fish and salamander changed shape during development. The results suggested that the lungs of vertebrates started out as a single organ, which became truly paired later in evolution once vertebrates started developing limbs. This anatomical shift increased the surface area available for exchanging oxygen and carbon dioxide so that vertebrates could breathe more easily on land. These findings provide new insights in to how the lung evolved into the paired structure found in most vertebrates alive today. It likely that this transition allowed vertebrates to fully adapt to breathing above water, which may explain why this event only happened once over the course of evolution.
All life on Earth started out under water. However, around 400 million years ago some vertebrates, such as fish, started developing limbs and other characteristics that allowed them to explore life on land. One of the most pivotal features to evolve was the lungs, which gave vertebrates the ability to breathe above water. Most land-living vertebrates, including humans, have two lungs which sit on either side of their chest. The lungs extract oxygen from the atmosphere and transfer it to the bloodstream in exchange for carbon dioxide which then gets exhaled out in to the atmosphere. How this important organ first evolved is a hotly debated topic. This is largely because lung tissue does not preserve well in fossils, making it difficult to trace how the lungs of vertebrates changed over the course of evolution. To overcome this barrier, Cupello et al. compared the lungs of living species which are crucial to understand the early stages of the water-to-land transition. This included four species of lunged bony fish which breathe air at the water surface, and a four-legged salamander that lives on land. Cupello et al. used a range of techniques to examine how the lungs of the bony fish and salamander changed shape during development. The results suggested that the lungs of vertebrates started out as a single organ, which became truly paired later in evolution once vertebrates started developing limbs. This anatomical shift increased the surface area available for exchanging oxygen and carbon dioxide so that vertebrates could breathe more easily on land. These findings provide new insights in to how the lung evolved into the paired structure found in most vertebrates alive today. It likely that this transition allowed vertebrates to fully adapt to breathing above water, which may explain why this event only happened once over the course of evolution.
摘要：
脊椎动物历史上一个至关重要的进化变化是古生代（泥盆纪419-3.59亿年前）水陆过渡，通过关键的形态学和生理学改变，包括肺部的采集。尽管如此，脊椎动物肺的起源和早期进化仍然存在很大争议，特别是祖先状态是成对还是不成对。由于软组织化石保存的稀有性,肺进化只能根据现存的系统发育括号来追踪。在这里我们调查，第一次,使用同步加速器X射线显微断层摄影术和组织学检查，在关键的活的骨科医生的广泛发育系列中的肺形态。我们的结果揭示了脊椎动物肺部未配对的原始状态，进化为在四足动物的谱系中真正配对。水陆过渡面临深刻的生理挑战，成对的肺对增加表面积，肺顺应性和体积具有决定性作用。尤其是在陆地上呼吸空气的时候.
地球上所有的生命都是从水下开始的。然而,大约4亿年前一些脊椎动物，比如鱼,开始发展四肢和其他特征，使他们能够探索陆地上的生命。进化的最关键的特征之一是肺，这使脊椎动物能够在水面上呼吸。大多数陆地生活的脊椎动物，包括人类,有两个位于胸部两侧的肺。肺部从大气中提取氧气，并将其转移到血液中，以换取二氧化碳，然后将其呼出到大气中。这个重要的器官如何首先进化是一个激烈争论的话题。这主要是因为肺组织在化石中保存不好，这使得很难追踪脊椎动物的肺在进化过程中是如何变化的。为了克服这个障碍，Cupelloetal.比较了生物物种的肺，这些肺对于了解水陆过渡的早期阶段至关重要。其中包括四种在水面呼吸空气的龙骨鱼，和一只生活在陆地上的四条腿的sal。Cupelloetal.使用一系列技术来检查骨鱼和sal的肺在发育过程中如何改变形状。结果表明，脊椎动物的肺最初是一个单一的器官，一旦脊椎动物开始发育四肢，它就在进化的后期变得真正配对。这种解剖变化增加了可用于交换氧气和二氧化碳的表面积，因此脊椎动物可以更容易地在陆地上呼吸。这些发现提供了新的见解，以了解肺如何演变成当今大多数脊椎动物中发现的配对结构。这种转变可能使脊椎动物完全适应在水面上呼吸，这可以解释为什么这个事件在进化过程中只发生过一次。
地球上所有的生命都是从水下开始的。然而,大约4亿年前一些脊椎动物，比如鱼,开始发展四肢和其他特征，使他们能够探索陆地上的生命。进化的最关键的特征之一是肺，这使脊椎动物能够在水面上呼吸。大多数陆地生活的脊椎动物，包括人类,有两个位于胸部两侧的肺。肺部从大气中提取氧气，并将其转移到血液中，以换取二氧化碳，然后将其呼出到大气中。这个重要的器官如何首先进化是一个激烈争论的话题。这主要是因为肺组织在化石中保存不好，这使得很难追踪脊椎动物的肺在进化过程中是如何变化的。为了克服这个障碍，Cupelloetal.比较了生物物种的肺，这些肺对于了解水陆过渡的早期阶段至关重要。其中包括四种在水面呼吸空气的龙骨鱼，和一只生活在陆地上的四条腿的sal。Cupelloetal.使用一系列技术来检查骨鱼和sal的肺在发育过程中如何改变形状。结果表明，脊椎动物的肺最初是一个单一的器官，一旦脊椎动物开始发育四肢，它就在进化的后期变得真正配对。这种解剖变化增加了可用于交换氧气和二氧化碳的表面积，因此脊椎动物可以更容易地在陆地上呼吸。这些发现提供了新的见解，以了解肺如何演变成当今大多数脊椎动物中发现的配对结构。这种转变可能使脊椎动物完全适应在水面上呼吸，这可以解释为什么这个事件在进化过程中只发生过一次。
