Would humans evolve again if we rewound time?
如果时间倒流，人类会再次进化吗？

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.

如果让时光退回到人类进化史上的任意一个时间点，重新开始会怎样？美国古生物学家古尔德（Stephen Jay Gould）在上世纪80年代末提出了这一著名的假设思考，至今仍牵动着进化生物学家的想象力。

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.

古尔德认为，如果时间倒流，进化会促使生命走上另一条与人类生命截然不同的道路，人类永远不会再进化成现在的样子。实际上，他认为人类的进化非常罕见，如果把生命诞生的过程重复一百万遍，也不会再次看到类似智人的物种出现。

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，偶然事件在进化过程中发挥了巨大的作用。包括大规模的灭绝事件，如灾难性的小行星撞击和火山爆发。但偶然事件也在微观上发挥着作用。作为进化适应的基础的基因突变，就有赖于偶然事件。

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.

简单地说，进化是随机突变的产物。能够提高某种有机体，在特定环境中相对于其他有机体的生存机会的突变极少，但随着时间的推移，这种突变会变得普遍。从一个物种分化成两个物种就始于这种罕见的突变。然而，进一步的随机过程仍能产生干扰，可能会导致有益突变减少，并增加有害突变。这种内在的随机性表明，如果回放生命诞生的过程，会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.

当然，在现实中，是不可能让时光倒流的。我们永远无法确切地知道，人类进化到目前程度的可能性有多大。然而，幸运的是，实验进化生物学家的确有办法在细菌的微观尺度上检验古尔德的部分理论。

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.

微生物的分裂和进化非常迅速。因此，我们可以在时间上冷冻数十亿个相同的细胞，并将它们无限期存储。这样我们便可以提取其中一部分细胞，看它们会不会在新的环境中生长，并实时观察它们的适应性变化。我们可以从"现在"去到"未来"然后再回来，想重复多少次就重复多少次——本质上就是在试管里回放生命诞生的过程。

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.

令人惊讶的是，很多细菌进化研究发现，在短期内的进化往往遵循非常容易预测到的路径，经常出现相同的特征和遗传解决方案。比如，设想一个长期的实验。在这个实验中，由一次克隆产生的12份独立的大肠杆菌种群，自1988年开始一直在不断进化。这其中涉及逾65000代——自现代智人出现以来，人类只繁衍了7500至10000代。该实验中所有的进化菌群都表现出比它们的祖先更强的适应性、更快的生长速度和更大的细胞体积。这表明，有机体在如何进化上存在一些限制。

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.

有一种进化力量让不断进化的有机体保持中规中矩。自然选择是进化的“引导者”，控制着随机突变引发的混乱，并鼓励有益突变。这意味着很多基因变异会随着时间的推移而逐渐消失，只有最有利的变异才会持续下去。这可能导致同样的生存解决方案在完全不相关的物种中实现。

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.

我们在进化史上发现了这方面的证据，亲缘关系不密切，但生存环境相似的物种，会进化出相似的特征。比如，已经灭绝的翼龙和鸟类都进化出了翅膀和独特的喙，但它们的近代祖先并不相同。所以翅膀和喙进化了两次，并且是同时进行的，原因在于进化压力。

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.

但基因结构也很重要。并非所有基因都生来平等：相比于其他基因，有些基因有非常重要的功能。基因常被组织成网络，像电路一样，有冗余开关和“主开关”。“主开关”的突变自然会引起更大的变异，因为在其控制下的所有基因都能感受到连锁反应。这意味着基因组中的某些位置会比其他位置更频繁地为进化做出贡献，或者说产生更大的影响，从而使进化结果发生偏转。

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.

那么基本的物理规律呢？它们是否支持可以预测的进化？在非常大的尺度上看起来是这样的。我们知道很多关于宇宙规律的理论是确定的。比如，重力是一种可以预测的力量。因为有重力，才有海洋、厚厚的大气层和为我们提供能量的太阳核聚变。牛顿（Isaac Newton）理论基于大范畴的确定性，也可以用来描述大规模的系统。它们将宇宙描述为完全可预测的。

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.

如果牛顿的观点完全正确，那么人类的进化便是一种必然。然而，这种令人欣慰的可预测性，被20世纪发现的量子力学理论打破了。在最小的原子和粒子尺度上，真正的随机性在发挥作用——这意味着我们这个世界在最基本的层面上是不可预测的。

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.

这意味着，无论将这个过程重复多少次，广泛的进化“规律”是一样的。接收太阳能的有机体永远具有进化上的优势。对于那些利用大气中丰富气体的人来说，总是有机会的。从这些适应中，我们可以预见到熟悉的生态系统的出现。但最终，很多进化过程固有的随机性，会让我们失去完全确定地“预见未来”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.

用天文学中的一个问题可以恰当的类比。在18世纪，一个数学研究机构设立奖项，奖励解答"三体问题"的人。解答这个问题需要精确描述太阳、地球和月球之间的引力关系以及由此形成的轨道。

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.

获奖者从本质上证明了这个问题根本无解。就像随机突变带来的混乱，起初的细微错误会不可避免地进一步发展，这意味着你无法轻易断定这三者未来最终会走向何方。但作为其中占据支配地位的一方，太阳在一定程度上决定着了3颗行球的轨道——让我们能够将它们可能的位置缩小在一个范围内。

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.

这很像进化的指引者，它们将不断适应的生物体固定在熟悉的路线上。如果时间倒流，我们不能确定，人类最终会到达哪里，但进化生物体可选择的道路也不是无限的。所以，人类或许不会再出现了，但无论取代人类世界的是一个什么样的外星世界，它都会是一个我们熟悉的地方。