我们在宇宙中是孤独的吗？归根结底，智力是自然选择的可能结果，还是不太可能的侥幸。根据定义，可能的事件经常发生，不太可能的事件很少发生——或者只发生一次。我们的进化史表明，许多关键的适应——不仅是智力，还有复杂的动物、复杂的细胞、光合作用和生命本身——都是独特的一次性事件，因此极不可能发生。我们的进化可能就像中了彩票……只是可能性小得多。

宇宙大得惊人。银河系有超过1000亿星星，还有超过一万亿个星系在可见的宇宙中，我们能看到的宇宙的一小部分。即使可居住的世界很少，但它们的数量——有像星星一样多的行星，也许更多——表明外面有很多生命。那么每个人都在哪里？这就是费米悖论。宇宙又大又旧，有时间和空间让智力进化，但没有证据表明这一点。

智力不可能进化吗？不幸的是，我们不能研究外星生命来回答这个问题。但是我们可以研究大约45亿年的地球历史，看看进化在哪里重复，或者不重复。

进化有时会重复，不同的物种会独立地汇聚到相似的结果上。如果进化经常自我重复，那么我们的进化可能是可能的，甚至是不可避免的。

趋同进化的惊人例子确实存在。澳大利亚已经灭绝的有袋类袋狼有一个像袋鼠一样的袋，但在其他方面看起来像狼，尽管是从不同的哺乳动物谱系进化而来的。还有有袋鼹鼠、有袋食蚁兽和有袋飞鼠。值得注意的是，澳大利亚的整个进化史与其他大陆相似，恐龙灭绝后哺乳动物多样化。

其他引人注目的趋同案例包括海豚和灭绝的鱼龙，它们进化出类似的形状在水中滑行，以及鸟类、蝙蝠和翼龙，它们趋同进化飞行。

我们也看到个体器官的趋同。眼睛不仅在脊椎动物中进化，也在节肢动物、章鱼、蠕虫和水母中进化。脊椎动物、节肢动物、章鱼和蠕虫独立发明了颌骨。腿在节肢动物、章鱼和四种鱼(四足动物、蛙鱼、鳐、弹涂鱼)中逐渐进化。)

关键是。所有这些融合都发生在一个谱系中，欧洲后生动物。真后生动物是具有对称性、嘴、内脏、肌肉和神经系统的复杂动物。不同的真后生动物进化出相似的解决方案来解决相似的问题，但是使这一切成为可能的复杂身体计划是独一无二的。复杂的动物进化了一次在生命的历史中，暗示它们是不可能的。

令人惊讶的是，我们进化史中的许多关键事件都是独一无二的，而且可能是不太可能的。一个是脊椎动物的骨骼，它让大型动物移动到陆地上。所有动物和植物都是由复杂的真核细胞构成的，包含细胞核和线粒体，只进化一次。性只进化了一次。光合作用增加了生命可用的能量并产生氧气一次性的。就此而言，人类水平的智力也是如此。有袋动物有狼和鼹鼠，但没有有袋动物。

有进化重复的地方，也有进化不重复的地方。如果我们只寻求趋同，就会产生确认偏差。趋同似乎是规则，我们的进化看起来是可能的。但是当你寻找不收敛时，它无处不在，关键的、复杂的适应似乎是最不可重复的，因此是不可能的。

2019年5月10日晚些时候，在安德烈伊托村附近的苏里亚诺省乌拉圭农村，可以看到银河系。

此外，这些事件相互依赖。直到鱼进化出骨骼让它们爬上陆地，人类才能进化。直到复杂的动物出现，骨头才能进化。复杂的动物需要复杂的细胞，复杂的细胞需要光合作用产生的氧气。没有生命的进化，这一切都不会发生，生命是单一事件中的单一事件。所有生物都来自一个祖先；据我们所知，生活只发生过一次。

奇怪的是，所有这些花费了令人惊讶的长时间。光合作用进化了十五亿年后地球形成后，复杂的细胞27亿年，复杂的动物之后40亿年和人类智慧45亿年地球形成后。这些创新是如此有用，但花了这么长时间才进化出来，这意味着它们是极其不可能的。

一系列不太可能的事件

这些一次性的创新，关键的失误，可能会造成一连串的进化瓶颈过滤。如果是这样，我们的进化就不像中了彩票。就像一次又一次中了彩票。在其他世界，这些关键的适应可能进化得太晚了，以至于智力无法在它们的太阳成为新星之前出现，或者根本没有出现。

想象一下，智力依赖于一系列七种不太可能的创新——生命起源、光合作用、复杂细胞、性别、复杂动物、骨骼和智力本身——每一种都有10%的进化机会。进化出智力的几率是千分之一。

但是复杂的适应可能更不可能。光合作用需要蛋白质、色素和细胞膜的一系列适应。真后生动物需要多种解剖学创新(神经、肌肉、嘴巴等)。)所以也许这七项关键创新中的每一项都只有1%的时间在进化。如果是这样，智能将在100万亿分之一的可居住世界中进化。如果可居住的世界很少，那么我们可能是银河系乃至可见宇宙中唯一的智慧生命。

然而，我们在这里。那一定很重要，对吧？如果进化在100万亿次中幸运一次，我们在发生进化的星球上的几率有多大？事实上，在那个不太可能的世界上的可能性是100%，因为我们不能在光合作用、复杂细胞或动物没有进化的世界上进行这样的对话。那是人择原理:地球的历史一定允许智慧生命进化，否则我们不会在这里思考它。

尼克·朗里奇是美国科学院古生物学和进化生物学的高级讲师巴斯大学英国。

这篇文章中表达的观点是作者自己的。

HUMANS MAY BE ONLY INTELLIGENT LIFE IN THE UNIVERSE | OPINION

​Are we alone in the universe? It comes down to whether intelligence is a probable outcome of natural selection, or an improbable fluke. By definition, probable events occur frequently, improbable events occur rarely—or once. Our evolutionary history shows that many key adaptations—not just intelligence, but complex animals, complex cells, photosynthesis, and life itself—were unique, one-off events, and therefore highly improbable. Our evolution may have been like winning the lottery … only far less likely.

The universe is astonishingly vast. The Milky Way has more than 100 billion stars, and there are over a trillion galaxies in the visible universe, the tiny fraction of the universe we can see. Even if habitable worlds are rare, their sheer number—there are as many planets as stars, maybe more—suggests lots of life is out there. So where is everyone? This is the Fermi paradox. The universe is large, and old, with time and room for intelligence to evolve, but there's no evidence of it.

Could intelligence simply be unlikely to evolve? Unfortunately, we can't study extraterrestrial life to answer this question. But we can study some 4.5 billion years of Earth's history, looking at where evolution repeats itself, or doesn't.

Evolution sometimes repeats, with different species independently converging on similar outcomes. If evolution frequently repeats itself, then our evolution might be probable, even inevitable.

And striking examples of convergent evolution do exist. Australia's extinct, marsupial thylacine had a kangaroo-like pouch but otherwise looked like a wolf, despite evolving from a different mammal lineage. There are also marsupial moles, marsupial anteaters and marsupial flying squirrels. Remarkably, Australia's entire evolutionary history, with mammals diversifying after the dinosaur extinction, parallels other continents.

Other striking cases of convergence include dolphins and extinct ichthyosaurs, which evolved similar shapes to glide through the water, and birds, bats and pterosaurs, which convergently evolved flight.

We also see convergence in individual organs. Eyes evolved not just in vertebrates, but in arthropods, octopi, worms and jellyfish. Vertebrates, arthropods, octopi and worms independently invented jaws. Legs evolved convergently in the arthropods, octopi and four kinds of fish (tetrapods, frogfish, skates, mudskippers.)

Here's the catch. All this convergence happened within one lineage, the Eumetazoa. Eumetazoans are complex animals with symmetry, mouths, guts, muscles, a nervous system. Different eumetazoans evolved similar solutions to similar problems, but the complex body plan that made it all possible is unique. Complex animals evolved once in life's history, suggesting they're improbable.

Surprisingly, many critical events in our evolutionary history are unique and, probably, improbable. One is the bony skeleton of vertebrates, which let large animals move onto land. The complex, eukaryotic cells that all animals and plants are built from, containing nuclei and mitochondria, evolved only once. Sex evolved just once. Photosynthesis, which increased the energy available to life and produced oxygen, is a one-off. For that matter, so is human-level intelligence. There are marsupial wolves and moles, but no marsupial humans.

There are places where evolution repeats, and places where it doesn't. If we only look for convergence, it creates confirmation bias. Convergence seems to be the rule, and our evolution looks probable. But when you look for non-convergence, it's everywhere, and critical, complex adaptations seem to be the least repeatable, and therefore improbable.

The Milky Way's seen late on May 10, 2019 from the Uruguayan countryside in the department of Soriano, near the village of Andresito.

What's more, these events depended on one another. Humans couldn't evolve until fish evolved bones that let them crawl onto land. Bones couldn't evolve until complex animals appeared. Complex animals needed complex cells, and complex cells needed oxygen, made by photosynthesis. None of this happens without the evolution of life, a singular event among singular events. All organisms come from a single ancestor; as far as we can tell, life only happened once.

Curiously, all this takes a surprisingly long time. Photosynthesis evolved 1.5 billion years after the Earth's formation, complex cells after 2.7 billion years, complex animals after 4 billion years, and human intelligence 4.5 billion years after the Earth formed. That these innovations are so useful but took so long to evolve implies that they're exceedingly improbable.

An unlikely series of events

These one-off innovations, critical flukes, may create a chain of evolutionary bottlenecks or filters. If so, our evolution wasn't like winning the lottery. It was like winning the lottery again, and again, and again. On other worlds, these critical adaptations might have evolved too late for intelligence to emerge before their suns went nova, or not at all.

Imagine that intelligence depends on a chain of seven unlikely innovations—the origin of life, photosynthesis, complex cells, sex, complex animals, skeletons and intelligence itself—each with a 10 percent chance of evolving. The odds of evolving intelligence become one in 10 million.

But complex adaptations might be even less likely. Photosynthesis required a series of adaptations in proteins, pigments and membranes. Eumetazoan animals required multiple anatomical innovations (nerves, muscles, mouths and so on.) So maybe each of these seven key innovations evolves just 1 percent of the time. If so, intelligence will evolve on just 1 in 100 trillion habitable worlds. If habitable worlds are rare, then we might be the only intelligent life in the galaxy, or even the visible universe.

And yet, we're here. That must count for something, right? If evolution gets lucky one in 100 trillion times, what are the odds we happen to be on a planet where it happened? Actually, the odds of being on that improbable world are 100 percent, because we couldn't have this conversation on a world where photosynthesis, complex cells, or animals didn't evolve. That's the anthropic principle: Earth's history must have allowed intelligent life to evolve, or we wouldn't be here to ponder it.

Nick Longrich is a Senior lecturer of paleontology and evolutionary biology at the University of Bath, United Kingdom.

 

Views expressed in this article are the author's own.