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如果時間倒流,人類會再次進化嗎?

Would humans evolve again if we rewound time?
如果時間倒流,人類會再次進化嗎?

What would happen if the hands of time were turned back to an arbitrary point in our evolutionary history and we restarted the clock? American palaeontologist Stephen Jay Gould proposed this famous thought experiment in the late 1980s – and it still grips the imagination of evolutionary biologists today.

如果讓時光退回到人類進化史上的任意一個時間點,重新開始會怎樣?美國古生物學家古爾德(Stephen Jay Gould)在上世紀80年代末提出了這一著名的假設思考,至今仍牽動著進化生物學家的想象力。

Gould reckoned that if time was rewound, then evolution would drive life down a completely different path and humans would never re-evolve. In fact, he felt humanity’s evolution was so rare that we could replay the tape of life a million times and we wouldn’t see anything like Homo sapiens arise again.

古爾德認為,如果時間倒流,進化會促使生命走上另一條與人類生命截然不同的道路,人類永遠不會再進化成現在的樣子。實際上,他認為人類的進化非常罕見,如果把生命誕生的過程重復一百萬遍,也不會再次看到類似智人的物種出現。

His reasoning was that chance events play a huge role in evolution. These include enormous mass extinction events – such as cataclysmic asteroid impacts and volcanic eruptions. But chance events also operate at the molecular scale. Genetic mutation, which forms the basis of evolutionary adaptation, is reliant on chance events.

他的理由是,偶然事件在進化過程中發揮了巨大的作用。包括大規模的滅絕事件,如災難性的小行星撞擊和火山爆發。但偶然事件也在微觀上發揮著作用。作為進化適應的基礎的基因突變,就有賴于偶然事件。

Put simply, evolution is the product of random mutation. A rare few mutations can improve an organism’s chance of survival in certain environments over others. The split from one species into two starts from such rare mutations that become common over time. But further random processes can still interfere, potentially leading to a loss of beneficial mutations and increasing harmful mutations over time. This inbuilt randomness ought to suggest you’d get different life forms if you replayed the tape of life.

簡單地說,進化是隨機突變的產物。能夠提高某種有機體,在特定環境中相對于其他有機體的生存機會的突變極少,但隨著時間的推移,這種突變會變得普遍。從一個物種分化成兩個物種就始于這種罕見的突變。然而,進一步的隨機過程仍能產生干擾,可能會導致有益突變減少,并增加有害突變。這種內在的隨機性表明,如果回放生命誕生的過程,會得到不同的生命形式。

Of course, in reality, it’s impossible to turn back the clock in this way. We’ll never know for sure just how likely it was to have arrived at this moment as we are. Fortunately, however, experimental evolutionary biologists do have the means to test some of Gould’s theories on a microscale with bacteria.

當然,在現實中,是不可能讓時光倒流的。我們永遠無法確切地知道,人類進化到目前程度的可能性有多大。然而,幸運的是,實驗進化生物學家的確有辦法在細菌的微觀尺度上檢驗古爾德的部分理論。

Microorganisms divide and evolve very quickly. We can therefore freeze billions of identical cells in time and store them indefinitely. This allows us to take a subset of these cells, challenge them to grow in new environments and monitor their adaptive changes in real time. We can go from the “present” to the “future” and back again as many times as we like – essentially replaying the tape of life in a test tube.

微生物的分裂和進化非常迅速。因此,我們可以在時間上冷凍數十億個相同的細胞,并將它們無限期存儲。這樣我們便可以提取其中一部分細胞,看它們會不會在新的環境中生長,并實時觀察它們的適應性變化。我們可以從"現在"去到"未來"然后再回來,想重復多少次就重復多少次——本質上就是在試管里回放生命誕生的過程。

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Many bacterial evolution studies have found, perhaps surprisingly, that evolution often follows very predictable paths over the short term, with the same traits and genetic solutions frequently cropping up. Consider, for example, a long-term experiment, in which 12 independent populations of Escherichia coli founded by a single clone, have been continuously evolving since 1988. That’s over 65,000 generations – there have only been 7,500-10,000 generations since modern Homo sapiens appeared. All the evolving populations in this experiment show higher fitness, faster growth and larger cells than their ancestor. This suggests that organisms have some constraints on how they can evolve.

令人驚訝的是,很多細菌進化研究發現,在短期內的進化往往遵循非常容易預測到的路徑,經常出現相同的特征和遺傳解決方案。比如,設想一個長期的實驗。在這個實驗中,由一次克隆產生的12份獨立的大腸桿菌種群,自1988年開始一直在不斷進化。這其中涉及逾65000代——自現代智人出現以來,人類只繁衍了7500至10000代。該實驗中所有的進化菌群都表現出比它們的祖先更強的適應性、更快的生長速度和更大的細胞體積。這表明,有機體在如何進化上存在一些限制。

There are evolutionary forces that keep evolving organisms on the straight and narrow. Natural selection is the “guiding hand” of evolution, reigning in the chaos of random mutations and abetting beneficial mutations. This means many genetic changes will fade from existence over time, with only the best enduring. This can also lead to the same solutions of survival being realised in completely unrelated species.

有一種進化力量讓不斷進化的有機體保持中規中矩。自然選擇是進化的“引導者”,控制著隨機突變引發的混亂,并鼓勵有益突變。這意味著很多基因變異會隨著時間的推移而逐漸消失,只有最有利的變異才會持續下去。這可能導致同樣的生存解決方案在完全不相關的物種中實現。

We find evidence for this in evolutionary history where species that are not closely related, but share similar environments, develop a similar trait. For example, extinct pterosaurs and birds both evolved wings as well as a distinct beak, but not from a recent common ancestor. So essentially wings and beaks evolved twice, in parallel, because of evolutionary pressures.

我們在進化史上發現了這方面的證據,親緣關系不密切,但生存環境相似的物種,會進化出相似的特征。比如,已經滅絕的翼龍和鳥類都進化出了翅膀和獨特的喙,但它們的近代祖先并不相同。所以翅膀和喙進化了兩次,并且是同時進行的,原因在于進化壓力。

But genetic architecture is also important. Not all genes are created equal: some have very important jobs compared to others. Genes are frequently organised into networks, that are comparable to circuits, complete with redundant switches and “master switches”. Mutations in “master switches” naturally result in much bigger changes, because of the knock-on effect felt by all genes under its control. This means that certain locations in the genome will contribute to evolution more frequently, or with a larger effect, than others – biasing evolutionary outcomes.

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但基因結構也很重要。并非所有基因都生來平等:相比于其他基因,有些基因有非常重要的功能。基因常被組織成網絡,像電路一樣,有冗余開關和“主開關”。“主開關”的突變自然會引起更大的變異,因為在其控制下的所有基因都能感受到連鎖反應。這意味著基因組中的某些位置會比其他位置更頻繁地為進化做出貢獻,或者說產生更大的影響,從而使進化結果發生偏轉。

But what about the underlying physical laws – do they favour predictable evolution? At very large scales, it appears so. We know of many laws governing our universe that are certain. Gravity, for example – for which we owe our oceans, thick atmosphere and the nuclear fusion in the sun that showers us with energy – is a predictable force. Isaac Newton’s theories, based on large scale deterministic forces, can also be used to describe many systems on large scales. These describe the universe as perfectly predictable.

那么基本的物理規律呢?它們是否支持可以預測的進化?在非常大的尺度上看起來是這樣的。我們知道很多關于宇宙規律的理論是確定的。比如,重力是一種可以預測的力量。因為有重力,才有海洋、厚厚的大氣層和為我們提供能量的太陽核聚變。牛頓(Isaac Newton)理論基于大范疇的確定性,也可以用來描述大規模的系統。它們將宇宙描述為完全可預測的。

If Newton’s view was to remain perfectly true, the evolution of humans was inevitable. However, this comforting predictability was shattered by the discovery of the contradictory but fantastical world of quantum mechanics in the 20th Century. At the smallest scales of atoms and particles, true randomness is at play – meaning our world is unpredictable at the most fundamental level.

如果牛頓的觀點完全正確,那么人類的進化便是一種必然。然而,這種令人欣慰的可預測性,被20世紀發現的量子力學理論打破了。在最小的原子和粒子尺度上,真正的隨機性在發揮作用——這意味著我們這個世界在最基本的層面上是不可預測的。

This means that the broad “rules” for evolution would remain the same no matter how many times we replayed the tape. There would always be an evolutionary advantage for organisms that harvest solar power. There would always be opportunity for those that make use of the abundant gases in the atmosphere. And from these adaptations, we may predictably see the emergence of familiar ecosystems. But ultimately, randomness, which is built into many evolutionary processes, will remove our ability to “see into the future” with complete certainty.

這意味著,無論將這個過程重復多少次,廣泛的進化“規律”是一樣的。接收太陽能的有機體永遠具有進化上的優勢。對于那些利用大氣中豐富氣體的人來說,總是有機會的。從這些適應中,我們可以預見到熟悉的生態系統的出現。但最終,很多進化過程固有的隨機性,會讓我們失去完全確定地“預見未來”的能力。

There is a problem in astronomy that acts as a fitting analogy. In the 1700s, a mathematical institute offered a prize for solving the “three-body problem”, involving accurately describing the gravitational relationship and resultant orbits of the sun, Earth and moon.

用天文學中的一個問題可以恰當的類比。在18世紀,一個數學研究機構設立獎項,獎勵解答"三體問題"的人。解答這個問題需要精確描述太陽、地球和月球之間的引力關系以及由此形成的軌道。

The winner essentially proved that the problem couldn’t be solved exactly. Much like the chaos introduced by random mutations, a little bit of starting error would inevitably grow, meaning that you couldn’t easily determine where the three bodies would end up in the future. But as the dominant partner, the sun dictates the orbits of all three to an extent – allowing us to narrow the possible positions of the bodies to within a range.

獲獎者從本質上證明了這個問題根本無解。就像隨機突變帶來的混亂,起初的細微錯誤會不可避免地進一步發展,這意味著你無法輕易斷定這三者未來最終會走向何方。但作為其中占據支配地位的一方,太陽在一定程度上決定著了3顆行球的軌道——讓我們能夠將它們可能的位置縮小在一個范圍內。

This is much like the guiding hands of evolution, which tether adapting organisms to familiar routes. We may not be entirely sure where we’d end up if we rewound time, but the paths available to evolving organisms are far from limitless. And so maybe humans would never appear again, but it’s likely that whatever alien world replaced ours, it would be a familiar place.

這很像進化的指引者,它們將不斷適應的生物體固定在熟悉的路線上。如果時間倒流,我們不能確定,人類最終會到達哪里,但進化生物體可選擇的道路也不是無限的。所以,人類或許不會再出現了,但無論取代人類世界的是一個什么樣的外星世界,它都會是一個我們熟悉的地方。

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