内容来源：https://www.quantamagazine.org/how-the-bird-eye-was-pushed-to-an-evolutionary-extreme-20260513/

内容总结：

鸟类视网膜“无氧生存”机制揭秘：颠覆传统认知的演化极端

长期以来，科学界一直困惑于一个悖论：鸟类拥有动物界最敏锐的视力，其视网膜却是体内代谢最活跃的组织之一，但令人费解的是，鸟类视网膜竟然几乎不含血管。这一结构意味着，这片高耗能的神经组织似乎缺乏直接的血液供氧。近日，一项发表于《自然》杂志的研究终于解开了这个百年谜题。

由奥胡斯大学进化生理学家克里斯蒂安·达姆斯高领导的研究团队发现，鸟类视网膜并非通过某种未知的奇特方式获取氧气，而是彻底放弃了有氧代谢。为了维持能量供应，其视网膜细胞采用了一种名为“厌氧糖酵解”的代谢方式。虽然这种无氧代谢的效率远低于依赖氧气的方式（相同葡萄糖分子产能仅为有氧代谢的十五分之一），但却足以支撑其功能。

研究团队利用微传感器对斑胸草雀、鸽子和鸡的视网膜进行检测，发现其内部视网膜层完全处于“慢性缺氧”状态，氧气浓度为零。通过空间转录组学技术，他们绘制了视网膜不同区域的基因活性图谱：在有血管的外层视网膜，与有氧呼吸相关的基因活跃；而在完全缺氧的内层视网膜，仅有与无氧呼吸相关的基因被激活。

那么，能量从何而来？研究指出，鸟类眼中一个名为“栉膜”（pecten oculi）的神秘结构扮演了关键角色。长期以来，科学家猜测栉膜是为视网膜供氧，但新研究证实，它的实际功能是像泵一样为视网膜输送大量葡萄糖，以维持无氧代谢。同时，栉膜还负责清除无氧代谢产生的有毒副产物——乳酸。

这种“无氧生存”策略在动物界极为罕见。此前已知仅有癌细胞或剧烈运动时的人类肌肉会短暂使用无氧代谢，而鸟类视网膜则终身处于这种状态。研究推测，这一特性可能演化自恐龙时代。当时，鸟类的祖先——兽脚类恐龙——为了在追踪猎物和识别伴侣时获得更锐利的视力，选择了消除视网膜血管以避免遮挡光线。这一结构在后来的鸟类中得以保留，并帮助它们在高空缺氧环境下飞行时依然保持卓越视力。

这项研究不仅解开了鸟类视觉进化的谜团，也为医学研究提供了新思路。人类大脑在完全缺氧状态下仅能耐受约一分钟，而中风导致的脑组织缺氧往往会造成永久性损伤。科学家希望通过研究鸟类组织这种极端耐受缺氧的机制，为治疗中风等缺血性疾病带来新的灵感。

中文翻译：

鸟类眼睛如何被推向进化极限
引言
当验光师将明亮的光线照入你的眼睛时，视野中会浮现出一棵巨大的、枝杈纵横的树影。这是血管的阴影。尽管我们通常无法感知它们，但这些血管总是遮挡着我们所见之物的一部分，而这背后有一个重要的原因：它们为视网膜提供能量。视网膜是位于眼球后部的一层薄薄的神经组织，负责将光信号传递给大脑。

视网膜是人体中能量消耗最高的组织之一。它由复杂的神经网络构成，有时包含超过100种不同类型的神经元，其组织消耗的能量是同等质量普通脑组织的两到三倍。这就是为什么大多数脊椎动物的视网膜（包括我们人类的）都布满密集、分支的血管网络：它们负责输送氧气以及其他产生能量所需的物质。

但这一规则有一个显著的例外。鸟类的视网膜基本上没有血管。考虑到鸟类拥有超凡的视力，这尤其令人费解。奥胡斯大学的进化生理学家克里斯蒂安·达姆斯加德表示，鸟类视网膜是“动物王国中代谢最活跃的组织之一，但它在没有明显血液灌注的情况下运作。这完全是一个悖论。”几个世纪以来，这一现象一直困扰着科学家，他们认为鸟类视网膜必然通过某种独特的、未被发现的途径获取氧气。

达姆斯加德是2026年1月发表在《自然》期刊上的一项研究的主要作者。该研究首次证明，鸟类视网膜并没有某种特殊的适应机制来获取氧气——它们完全在没有氧气的情况下存活。相反，为了为组织提供能量，它们使用一种称为“无氧糖酵解”的过程，这种过程的效率远低于依赖氧气的代谢，但仍能完成任务。

通过研究组织如何在无氧条件下存活，研究人员有可能开发治疗中风等缺氧状况的疗法。更根本的是，他们希望理解进化的极限。

“生命的极端状态是什么？”达姆斯加德问道，“我们能在多大程度上改变高代谢活性组织赖以生存的条件？”他发现，鸟类可以将这些条件推得很远。

有氧生命
大约34亿年前，蓝藻发明了光合作用。起初缓慢，随后迅速，它们新进化出的从阳光中获取能量的方法成功并传播开来。这些细胞向大气中释放了大量氧气（光合作用的副产物），从而改变了地球生命的进程。

氧气分子使细胞内的能量产生变得极其高效。为了提取能量，细胞将一分子葡萄糖分解成两分子丙酮酸。这一过程释放出两分子ATP（三磷酸腺苷），即生命通用的能量货币。缺乏氧气的细胞只能进行到这一步。然而，氧气能够促成进一步的生化反应，分解丙酮酸并额外产生30分子ATP。换句话说，氧气的存在使从单个葡萄糖分子中提取能量的效率提高了15倍，有时甚至更多。

通过有氧呼吸过程，氧气的能量优势是变革性的。一旦氧气充满大气，进化就选择了能够利用氧气的生物。“数百万年来，我们一直依赖20%的大气氧气，”柏林马克斯·德尔布吕克中心的分子生理学家加里·莱温说道。这一“大氧化事件”之后发生了大灭绝，因为利用氧气的生物胜过了几乎所有其他生物。尽管某些生命形式（如某些细菌）适应了无氧生活，但所有复杂的多细胞生物都需要这种能量优势来生存。

人类和大多数其他动物在几乎没有氧气的条件下最多只能存活几分钟。已知耐低氧能力最强的哺乳动物是裸鼹鼠，它可以在地下洞穴中呼吸无氧空气存活长达18分钟。一些冷血水生生物，包括淡水龟和金鱼，可以在冰冻湖底的低氧条件下坚持一到两年。但对于大多数动物而言，稳定的氧气供应是必不可少的。

没有氧气，许多过程会停止——尤其是在代谢需求高的组织，如大脑中。缺乏能量，我们的细胞会功能失常并死亡。

一个神秘的结构
正因为如此，当达姆斯加德在2019年听说鸟类视网膜没有血管时，他感到困惑。这种高能量组织在没有氧气的情况下如何存活，更不用说达到在视力敏锐的鸟类身上观察到的水平了？

他仔细研究了大量关于这一主题的文献，所有研究都指向鸟类眼睛中一个名为“栉膜”的神秘结构。在17世纪，解剖学家首次描述了这一不寻常的器官：它看起来像散热器，梳状，布满血管，表面积很大。在随后的几个世纪里，研究人员争论它是否有助于为鸟类眼睛的视网膜组织输送氧气。达姆斯加德阅读了大约30种仅基于解剖学提出的关于栉膜功能的理论。

“没有人真正对这一结构进行过直接的生理测量，”他说，“这正是我们介入的地方。”

在他的实验室里，达姆斯加德的团队使用微传感器测量了斑胸草雀、鸽子和鸡的视网膜中的氧气水平。果然，在完全缺乏血管的内部视网膜中，他们没有发现氧气。（他们在眼球后部有部分血管的外部视网膜中确实测到了氧气。）

这“令人震惊，”达姆斯加德说，“一半的视网膜处于慢性缺氧状态，那里完全没有氧气。”

通过使用空间转录组学（一种将细胞成像与RNA测序相结合的方法），研究人员绘制了视网膜组织不同部位活跃的基因图谱。与典型有氧呼吸相关的基因在存在血管的外部视网膜中表达。在缺氧的内部视网膜中，只有与无氧呼吸相关的基因活跃。

为了追踪营养物质的路径，达姆斯加德和他的团队与专注于无氧代谢的癌症科学家合作（肿瘤细胞常利用无氧糖酵解产生能量）。他们发现内部视网膜对葡萄糖的需求是鸟类大脑其他部分的2.5倍。

随后他们检查了栉膜。他们的空间转录组学数据显示，与葡萄糖相关的基因在此高度活跃。这表明，这个奇怪的结构并非将氧气带入鸟类视网膜，而是帮助泵入葡萄糖，从而支持效率较低的无氧过程。

作为副产物，无氧糖酵解产生乳酸，乳酸会积累并变得有毒。研究人员还发现，与乳酸转运蛋白（将乳酸移出组织的分子）相关的基因在栉膜中活跃。

他们的发现为栉膜在支持无氧糖酵解中的作用提供了令人信服的证据，这一作用“长期以来一直是个谜，”未参与该研究的苏塞克斯大学神经科学家托马斯·巴登说。“关于视网膜基本上是无氧的这一见解，至少在某些层面上令人惊讶……它确实降到了零。”

这种代谢途径被癌细胞使用，也被我们的肌肉在疲劳且无法获得足够氧气时暂时使用——比如我们在跑步时。但已知没有脊椎动物的组织能在完全无氧的条件下终身存活。

鹰一样的眼睛
鸟类的视网膜及其无氧供能系统如此不寻常，自然引发了关于它们如何进化的问题。

这“是一系列精彩的实验，”未参与这项研究的加州大学伯克利分校的卡蒂克·谢卡尔说。这是一个例子，说明动物如何将脊椎动物眼睛——一种高度保守的结构，其起源可追溯到约5.6亿年前原始生物身上的光敏斑块——进行改造以适应自身需求。“进化并不像真正的发明家；它更像一个修补匠，”他引用法国生物学家弗朗索瓦·雅各布1977年的文章《进化与修补》说道，“它采用早已存在的部件，并进行重组、再发明和重塑。”

研究人员试图通过比较鸟类视网膜与不太远的近亲——两种爬行动物（中华鳖和宽吻凯门鳄）——的氧气水平，来确定栉膜可能何时出现。爬行动物的视网膜具有正常的氧气水平，并且没有无氧糖酵解的迹象。这使达姆斯加德的团队得出结论：无氧组织很可能是在恐龙时代的某个时刻进化的，当时鸟类谱系已与鳄鱼分离，但尚未进化成现代鸟类。这与视网膜增厚的时间大致相同。

尽管如此，这个粗略的时间估计无法解释何种进化压力可能选择了这种不寻常的视网膜组织。研究人员只能推测。“我认为这个系统是在兽脚类恐龙中进化出来的，以应对对锐利视力的选择压力，用于追踪猎物和识别配偶，”达姆斯加德提出。然后，后来当鸟类飞向天空时，它“在低氧水平的高空飞行中，充当了维持视网膜功能的生理基础，”他推测道。

缺乏血管也可能为鸟类提供更好的视力优势。鸟类视网膜复杂且密集地排列着超过一百种细胞类型，这些细胞共同工作，以极高的分辨率呈现世界。鸟类利用其非凡的视觉进行狩猎和觅食——比如猫头鹰从空中追踪老鼠，信天翁观察海面寻找鱼类的迹象，或蜂鸟每天定位数百朵花——以及迁徙时跟随地面上的地标。没有血管阻碍视线，鸟类的视网膜细胞可能能够接收更多的视觉信息。

这究竟是适应还是进化历史的巧合？无法确切知道鸟类惊人的视力是如何进化的。这个谜团“一直萦绕在我们身边，”巴登说，“鸟类的眼睛到底有什么特别之处？”它们的视网膜供能系统似乎可以解释是什么使它们如此独特。然而，生理学家莱温对于将结果和解释过度扩展到每一种鸟类持谨慎态度，因为研究人员尚未研究任何迁徙物种。

其影响远远超出鸟类适应性的范畴，延伸到了生物医学领域。许多医学状况的一个共同线索是组织供氧下降，这根据发生部位的不同，可能导致疤痕或脑损伤。莱温说，人类大脑可能只能承受大约一分钟的完全缺氧。这就是为什么中风（切断大脑部分区域的血液和氧气供应）如此具有破坏性。通过研究裸鼹鼠和鸟类等生物的缺氧状况，科学家可以深入了解组织如何耐受低氧条件。

“也许我们可以从大自然如何通过数百万年的自然选择解决这些问题中获得灵感，”达姆斯加德说，“从这些能够做到我们无法做到之事的动物身上，有太多东西可以学习。”

英文来源：

How the Bird Eye Was Pushed to an Evolutionary Extreme
Introduction
When an optometrist shines a bright light into your eyes, a vast, branching tree sprouts in your field of vision. This is the shadow of blood vessels. Though we normally can’t perceive them, these vessels always occlude a portion of what we see, and for an important reason. They power the retina, a thin layer of nerve tissue in the back of the eye that communicates light signals to the brain.
The retina is one of the body’s most energetically expensive tissues. Built from complex networks of sometimes more than 100 different types of neurons, retinal tissue consumes two to three times more energy than the same mass of typical brain tissue. That’s why most vertebrate retinas, including our own, are furrowed with dense, branching networks of blood vessels: to deliver oxygen and other ingredients for producing energy.
But there’s a significant exception to this rule. Birds have retinas that mostly lack blood vessels. This may seem especially strange given birds’ exceptional vision. The bird retina is “one of the most metabolically active tissues in the animal kingdom, yet it worked with no apparent blood perfusion,” said Christian Damsgaard, an evolutionary physiologist at Aarhus University. “It was a complete paradox.” For centuries this has puzzled scientists, who figured that the bird retina must obtain oxygen through a unique, undiscovered process.
Damsgaard is the lead author of a study, published in the journal Nature in January 2026, that showed for the first time that bird retinas don’t have some unusual adaptation for acquiring oxygen — they survive without it entirely. Instead, to bring energy to the tissue, they use a process called anaerobic glycolysis that is significantly less efficient than oxygen-powered metabolism but gets the job done.
By studying how tissues can survive without oxygen, researchers can potentially develop therapeutics to treat conditions of oxygen deprivation, such as strokes. More fundamentally, they want to understand the limits of evolution.
“What are the extremes of life?” Damsgaard said. “How far can we bend the conditions under which highly metabolically active tissues can actually survive?”
A bird, he learned, can bend them pretty far.
Oxygenated Life
Around 3.4 billion years ago, cyanobacteria invented photosynthesis. Slowly at first, then quickly, their newly evolved method of making energy from sunlight succeeded and spread. The cells pumped so much oxygen, a by-product of photosynthesis, into the atmosphere that it changed the course of life on Earth.
Oxygen molecules make energy production in cells extremely efficient. To extract energy, cells break down a glucose molecule into two pyruvate molecules. This process releases two molecules of ATP (adenosine triphosphate), life’s universal energy currency. A cell lacking oxygen can go only this far. Oxygen, however, enables further biochemical reactions that break down pyruvate and produce another 30 molecules of ATP. In other words, the presence of oxygen makes energy extraction from a single glucose molecule 15 times as efficient, and sometimes more.
The energetic advantage of oxygen, through the process of aerobic respiration, was transformative. Once oxygen imbued the atmosphere, evolution selected for organisms that could use it. “We’ve been hooked on 20% [atmospheric] oxygen for millions of years,” said Gary Lewin, a molecular physiologist at the Max Delbrück Center in Berlin. This Great Oxidation Event was followed by mass extinction, as organisms using oxygen outcompeted just about everybody else. While some life forms, such as certain bacteria, are adapted to life without oxygen, all complex, multicellular organisms need that energy advantage to survive.
Humans and most other animals can survive with little or no oxygen for several minutes at most. The mammal with the highest known tolerance for low-oxygen conditions is the naked mole rat, which can survive for up to 18 minutes breathing anoxic air in underground burrows. A few cold-blooded aquatic creatures, including freshwater turtles and goldfish, can persist in low-oxygen conditions at the bottom of a frozen lake for a year or two. But for most animals, a steady supply of oxygen is a must-have.
Without oxygen, a variety of processes shut down — especially in metabolically demanding tissues such as the brain. Without that energy, our cells malfunction and die.
A Mysterious Structure
All this is why, in 2019, when Damsgaard learned that bird retinas lack blood vessels, he was confused. How could this high-energy tissue survive, let alone perform at the level observed in sharp-sighted bird species, without oxygen?
He pored over the voluminous research on the subject, all of which pointed at a mysterious structure in the bird eye known as the pecten oculi. In the 17th century, anatomists first described the unusual organ: It looked like a radiator, comblike, riveted with blood vessels, and with a large surface area. In the centuries that followed, researchers debated whether it helps deliver oxygen to retinal tissue in bird eyes. Damsgaard read about 30 different theories about the pecten oculi’s function based on anatomy alone.
“Nobody had really done direct physiological measurements on this structure,” he said. “That’s where we came in.”
In his lab, which studies the exchange of gases such as oxygen and carbon dioxide between vertebrates and their environments, Damsgaard’s team used microsensors to measure oxygen levels in the retinas of zebra finches, pigeons, and chickens. Indeed, in the inner retina, which completely lacks blood vessels, they found no oxygen. (They did measure oxygen in the outer retina, at the back of the eye, which has some blood vessels.)
That was “striking,” Damsgaard said. “Half of the retina lives in a chronic state of anoxia, where there’s no oxygen present at all.”
Using spatial transcriptomics, a method that combines cell imaging with RNA sequencing, the researchers mapped which genes were active in different parts of the retinal tissue. Genes associated with typical aerobic respiration were expressed in the outer retina, where there are blood vessels. In the oxygen-depleted inner retina, only genes associated with anaerobic respiration were active.
To trace the paths of nutrients, Damsgaard and his team worked with cancer scientists who are experts on oxygen-free metabolism (tumor cells often use anaerobic glycolysis to make energy). They found that the inner retina demanded 2.5 times more glucose than other parts of the bird brain.
Then they examined the pecten oculi. Their spatial transcriptomics data showed that the genes for glucose were highly active there. This suggested that the strange structure wasn’t bringing oxygen into the bird’s retina; rather, it was helping to pump glucose in, thereby enabling the less efficient anaerobic process.
As a by-product, anaerobic glycolysis creates lactic acid, which can accumulate and become toxic. The researchers also saw that genes for lactic acid transporters — the molecules that move lactic acid out of tissues — were active in the pecten oculi.
Their findings provide compelling evidence for the role of the pecten oculi in supporting anaerobic glycolysis, which “has been a mystery for a long time,” said Thomas Baden, a neuroscientist at the University of Sussex who was not involved in the study. “The insight that the retina basically goes oxygen-free, at least in some layers, is surprising. … It really gets properly down to zero.”
This pathway is used by cancer cells and temporarily by our muscles when they’re strained and can’t get enough oxygen — such as when we’re running. But no known vertebrate tissue was known to survive in completely anoxic conditions for a lifetime.
Eyes Like a Hawk
The bird’s retina and its no-oxygen power system are so unusual that they naturally raise questions about how they could have evolved.
This is “a series of beautiful experiments,” said Karthik Shekhar of the University of California, Berkeley, who was not involved in the research. It’s an example of how an animal took the vertebrate eye — a highly conserved structure whose origins go back some 560 million years to a light-sensitive patch on a primitive creature — and tinkered with it to fit its own needs. “Evolution is not really like an inventor; it acts more like a tinkerer,” he said, citing a 1977 essay, “Evolution and Tinkering,” by the French biologist François Jacob. “It takes parts that have existed long before, and it recombines, reinvents, and reshapes.”
The researchers tried to pinpoint when the pecten oculi might have arisen by comparing oxygen levels in the bird retina to those in not-so-distant relatives: two reptile species, Chinese pond turtles and broad-snouted caimans. The reptile retinas had normal oxygen levels and no indication of anaerobic glycolysis. This led Damsgaard’s team to conclude that the oxygen-free tissue likely evolved sometime during the dinosaur era, after the avian lineage had split from crocodiles but hadn’t yet evolved into modern birds. This was around the same time that the retina thickened.
Still, that rough time estimate can’t explain what evolutionary pressure might have selected for the unusual retinal tissue. Researchers can only speculate. “I think the system evolved in theropod dinosaurs in response to selection for sharp vision for tracking prey and identifying mates,” Damsgaard suggested. Then, later, when birds took to the skies, it “served as the physiological basis for maintaining retinal function” during high-altitude flights, when oxygen levels are low, he speculated.
The lack of blood vessels could also offer birds the advantage of better vision. The bird retina is complex and densely packed with more than a hundred cell types that work to render the world in great resolution. Birds use their exceptional visual sense for hunting and foraging — consider an owl tracking a mouse from the sky, an albatross watching for signs of fish on the ocean’s surface, or a hummingbird locating hundreds of flowers every day — as well as for following landmarks across the landscape during migration. Without blood vessels obstructing their view, birds’ retinal cells might be able to take in more visual information.
Could this be an adaptation, or is it a coincidence of evolutionary history? There’s no way to know for sure how birds’ incredible vision evolved. There’s this mystery “that has lingered around us,” Baden said. “What is it about birds that makes their eyes so special?” Their retinal power system seems as if it could explain what makes them so unique. However, Lewin, the physiologist, is cautious about overextending the results and interpretations to every bird, given that the researchers haven’t looked at any migratory species.
The implications stretch well beyond bird adaptations to biomedicine. A common thread in many medical conditions is a drop in oxygen delivery to tissues, which, depending on where it occurs, can lead to scars or brain damage. Human brains can tolerate maybe a minute of total anoxia, Lewin said. That’s what makes strokes, which cut off blood and oxygen supply to parts of the brain, so devastating. By studying low-oxygen conditions in creatures such as naked mole rats and birds, scientists can gain insight into how tissues can tolerate low-oxygen conditions.
“Maybe we can get inspiration for how nature solved these problems by millions of years of natural selection,” Damsgaard said. “There’s so much to be learned from these animals that are able to do something that we cannot do.”