链接：https://phys.org/news/2018-07-rethinking-homo-sapiens-story-dizzyingly.html
智人的进化发生在东非大裂谷，但后来发生了什么？（图片来源: 26781577@N07/flickr , CC BY-SA）
可以说，发现人类进化的时间、地点和原因是科学的终极大奖。
长久以来，一直都有充分的证据表明智人的进化发生在非洲，后来又从非洲大陆迁徙到地球上的其他地方定居。
但我们的起源故事真的这样简单吗？7月12日发表的一篇由英国和德国科学家撰写的评论表明，事实或许并非如此。
太简单了，但是……
几年前，一切都显得很简单。这个问题已经盖棺论定。大约20万年前，智人在东非大裂谷进化，又在6万年前离开这片祖辈的大陆，定居在地球上的其他地方。
然而，经过几十年的思考，我们在解决这一终极谜题上取得了重大进展，我们的起源故事正迎来一场姗姗迟来的全面修正。
近期的一系列发现将我们在非洲的起源推回到了31万5千年前，并且显示智人在接近19万4千年前第一次走出非洲，不久之后（可能在13万9千年前）到达中国南部。
更复杂的是，在最初的“走出非洲”之后，似乎还发生了一次或多次其他行动，这些行动源自不同的智人群体，他们大约于6万5千年前来到遥远的地方，如澳大利亚。
现在，我们还陆续发现了一些人类，例如在非洲和亚洲不同地区存活到很晚期的Iwo Eleru、Nazlet Khater以及马鹿洞穴人（Red Deer Cave），他们的身体特征与我们预想的智人概念并不相符。
他们有可能是我们猜想过的混血人种吗？因为经过几十年来的争论，遗传学家最终证明，我们的祖先曾与尼安德特人、丹尼索瓦人以及其他（甚至是非洲的）古人类通婚。
当然，时常更新是科学界的正常现象，因为新发现改变了我们认为自己知道的东西，有时甚至会把知识扩展到新的领域。
但作为一个对人类进化一直保持着密切关注和积极研究的人，我很清楚，一些事情正在发生改变。变化的趋势正在显现！
人类起源学里的转变是如此深刻，它见证了由23个研究人员组成的团队（由牛津大学的Eleanor Scerri领队）迈出的不寻常的一步，他们在7月12日发布了新的证据综论——《Did Our Species Evolve in Subdivided Populations across Africa, and Why Does It Matter?》——摒弃过去过于简单的想法，拥抱新的复杂性。这个团队当中有考古学家、人类学家、遗传学家和气候专家。
这篇文章读起来像一份宣言，概述了在新研究中考古学应该遵循的主要方向，以解决令人困惑的人类起源。一个关键信息是，在这些学科之中没有哪一个能够独立完成这个任务。那种方法只会让我们捕捉到复杂问题的一些简单答案。
这里有许多重大问题有待解决。其中最主要的是认可这样一个事实，即今天地球上区别各人类种群的生理差异似乎是在10万至4万年前才出现的。这使得识别我们的祖先并追踪他们的进化史变得非常困难。
来自埃塞俄比亚赫托（Herto）的头骨就是一个很好的例子。这是一个非常古老的人，无疑是我们这个物种的一员，然而他并不符合那些区别现今人类种群的任何一种生物特征。
为什么会出现这种情况？在人类历史的最初几十万年里，我们的非洲祖先似乎生活在非常小的、在地理上孤立的群体里，这些群体或多或少是彼此独立进化的。
在某个时候，他们中的一个或多个群体产生了更多群体，其中一些离开了非洲，然而没有明确的证据表明这一切是如何发生的。
这将我们引向了另外一个问题，或许也是最重要的问题。首先，我们如何定义智人？这种方法将如何提供有用的标准，让我们在20万、30万甚至40万年前的非洲化石记录中辨别人类种群？
史密森学会总结了我们用来定义人类物种的主要特征，但遗憾的是，许多特征很难在早期非洲祖先的头骨上得到印证。
这是否意味着他们不是我们的祖先？不一定，只是我们目前的方法非常有限，我们需要记住的是，我们最早的祖先可能看起来，嗯，很古老！
最后，人们对冰河期气候变化与非洲环境变化之间的复杂联系有了更好的理解，这些变化是自然选择和基因漂变（generic drift）的主要驱动力。这些力量塑造了我们的外貌和行为，在今天仍对我们的生理产生着影响。
尽管在过去十年，我们在梳理人类起源方面已经取得了各种进步，但Scerri及其团队的宣言包含的不只是“回到未来”的一种线索。
尽管这篇文章非常大胆且雄心勃勃，但它留给我的问题远比答案更多。让我挥之不去的一种感觉是，这些问题远比我们迄今为止准备接受的还要复杂得多。
原文
Rethinking Homo sapiens? The story of our origins gets dizzyingly complicated
Homo sapiens evolved in the East African rift valley…but then what happened? Credit: 26781577@N07/flickr , CC BY-SA
You might say it's the ultimate prize of science, to discover when, where and why humans evolved.
For a long time, the evidence has been overwhelming that Homo sapiens evolved in Africa and later spilled out of the continent to settle the rest of the planet.
But is the story of our origins really as simple as it seems? Published today, a new commentary written by UK and German scientists suggests not.
Too easy, but then…
A few years ago, it seemed all too easy. The matter was settled. Homo sapiens had evolved in the East African rift valley roughly 200,000 years ago and exited the mother continent to settle the remaining planet around 60,000 years ago.
But after decades of thinking we'd made major inroads into solving this ultimate riddle, the story of our origins is starting to get a long overdue overhaul.
A string of recent discoveries has pushed our origins in Africa back to more than 315,000 years ago – and suggested sapiens first exited Africa close to 194,000 years ago making it to southern China soon after, perhaps by 139,000 years ago.
And to add further complexity, this initial 'Out-of-Africa' seems to have been followed by one or more later movements, by different sapiens populations, that settled far flung places such as Australia by around 65,000 years ago.
We've also begun to find humans whose physical traits don't fit with our preconceived notions of what sapiens should look like, such as the Iwo Eleru, Nazlet Khater and Red Deer Cave people surviving quite late in various parts of Africa and in Asia.
Could they be hybrids we've wondered? Because after decades of scientists locking horns over the issue, geneticists have finally proven that our ancestors mated with the Neanderthals, Denisovans and other archaic humans even in Africa.
Change in the wind
Sure, it's normal for science to be regularly brought up to date, with new discoveries shifting what we think we know and sometimes even extending knowledge into new places.
But as someone who's kept a keen eye on developments in, and indeed actively researching, our evolution, it's clear to me that there's something's going on here. Change is in the wind!
So profound is the shift underway in human origins science that it's seen the unusual step of a team of 23 researchers (led by Eleanor Scerri of the University of Oxford) publish today's new synthesis of the evidence – and in doing so embrace the emerging picture of complexity and ditch the old simplistic ideas. Among their ranks are archaeologists, anthropologists, geneticists and climate specialists.
It reads like a manifesto, and outlines the major new research directions archaeology should follow to solve our puzzling origins. A key message is that none of these disciplines on their own is capable of doing it and going it alone. That approach only leads to us grasping for simple answers to complex questions.
Big issues on the line
There are number of big issues being wrestled with here. Among the major ones is acknowledgement of the fact that the physical variation that characterises our species around the planet today seems to have emerged only within the period 100,000-40,000 years ago. This has made identifying our ancestors and tracing their evolutionary history exceedingly difficult.
The cranium from Herto in Ethiopia is a great example of a very ancient person who was undoubtedly a member of our species, but who doesn't really fit neatly into the variation characterising any living group.
How would this occur? It seems that for the first couple of hundred thousand years of our history, our African forebears lived in very small and geographically isolated populations which evolved more or less independently of each other.
At some point one or more of them gave rise to one or more populations of living people, some of whom exited Africa, and with no clear evidence for how this all came about.
This leads us onto another, and perhaps the most important issue of all. How do we define Homo sapiens in the first place? How will this process provide useful criteria for recognising our kind in the African fossil record 200,000, 300,000 or even 400,000 years ago?
Defining Homo sapiens
The Smithsonian Institution has provided a summary of the major features we use to define our species but sadly many can't easily be found on the skulls of our early African ancestors.
Does this mean they aren't our forebears? Not necessarily, just that our current approach is pretty limited and we need to keep in mind that our earliest ancestors would have looked, well, ancestral!
Finally, the complicated links between Ice Age climate change and environmental shifts in Africa acting as key drivers through natural selection and genetic drift are becoming better understood. These forces have shaped how we look and behave and continue to exert their influence over our biology today.
Despite all the progress we've made over the last decade in teasing apart our origins, the manifesto of Scerri and her team has more than a hint of 'back to the future' about it.
As bold and ambitious as it is, it leaves me with far more questions than answers, and a lingering feeling that the issues are far more complicated than we've been prepared to admit until now.
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