Why Do These Fossil Shells Flip Their Spirals Every Few Millennia?
For thousands and sometimes millions of years, marine plankton all over the world built their spiral shells in one direction.
数千年甚至数百万年以来，世界各地的海洋浮游生物都朝一个方向建造螺旋壳。
Then, suddenly, the spirals switched direction at the same time, everywhere, only to switch back again later.
然后，突然间，螺旋在各处同时改变了方向，但稍后又又变回来了。
These microorganisms are types of foraminifera, or forams.
这些微生物是有孔虫或有孔虫的类型。
Found in all oceans, from the tropics to high latitudes, they’re among the most abundant eukaryotic organisms on Earth.
它们存在于从热带到高纬度地区的所有海洋中，是地球上最丰富的真核生物之一。
The single-celled protists secrete a hard shell perforated with many small holes; most species live on the seafloor, while some are planktonic, at the mercy of currents.
单细胞原生生物分泌出坚硬的外壳，外壳上有许多小孔；大多数物种生活在海底，而有些则是浮游生物，受洋流的支配。
When they die, their shells blanket the seafloor across the world, forming a natural archive of Earth’s history that goes back some 560 million years.
当它们死亡时，它们的壳会覆盖世界各地的海底，形成一个可追溯到约 5.6 亿年前的地球历史的自然档案。
By studying the composition of species, or measuring isotopes or trace elements, scientists use accumulated foram shells to reconstruct past climates and ocean conditions.
通过研究物种的组成，或测量同位素或微量元素，科学家利用积累的有孔虫壳来重建过去的气候和海洋条件。
Over the years, they have noticed a strange phenomenon.
多年来，他们注意到一个奇怪的现象。
Among the many planktonic forams that have snail-like coiled shells, some species strongly prefer one spiral direction — left or right — over the other, with as many as 97% of the individuals in a species coiling in the same direction.
在许多拥有蜗牛状盘绕壳的浮游有孔虫中，一些物种强烈喜欢一个螺旋方向（左或右）而不是另一个，一个物种中多达 97% 的个体沿着同一方向盘绕。
And sometimes, a new coiling direction dominates seemingly everywhere across the oceans all at once in the fossil record.
有时，在化石记录中，一个新的卷绕方向似乎同时在整个海洋中占据主导地位。
Ever since the 1950s, when this curiosity was observed, scientists have been trying to figure out what might cause shell direction to change in quadrillions of microorganisms in unison.
自 20 世纪 50 年代观察到这种好奇心以来，科学家们一直试图找出可能导致数万亿微生物的壳方向一致改变的原因。
Recently, micropaleontologists integrated and synthesized data on the shell-coiling direction, or chirality , of several species from case studies of forams dating back as far as 56 million years.
最近，微型古生物学家从可追溯至 5600 万年前的有孔虫案例研究中整合并综合了几个物种的贝壳卷曲方向或手性数据。
Some researchers speculated that the shell flips were connected to changes in temperature or other climate factors.
一些研究人员推测，贝壳翻转与温度或其他气候因素的变化有关。
The new study , however, hypothesizes that shell direction is not itself an adaptive response, but rather is an accidental marker of a significant evolutionary event that starts in a small subpopulation and then sweeps across vast ocean basins.
然而，这项新研究假设，贝壳方向本身并不是一种适应性反应，而是一个重大进化事件的偶然标记，该事件始于一个小亚群，然后席卷广阔的海洋盆地。
“It’s probably one of the first times that people who usually do more biostratigraphy — basically, deep-time research — on foraminifera are approaching such a question,” said Julie Meilland , a researcher at the Cerege, a research institute in France, who was not involved in the study.
“这可能是那些通常对有孔虫进行更多生物地层学研究（基本上是深度研究）的人第一次探讨这样的问题，”法国塞雷吉研究所的研究员朱莉·梅兰德（Julie Meilland）说，她没有参与这项研究。
In addition to data on coiling direction from multiple species going back millions of years, the study incorporates insights from modern foram genetics and biology.
除了数百万年前多个物种的盘绕方向数据外，该研究还结合了现代有孔虫遗传学和生物学的见解。
“It was very refreshing to see these worlds connect because very often people doing more modern research don’t necessarily connect to people doing deep-time research,” Meilland added.
梅兰补充道：“看到这些世界相互联系真是令人耳目一新，因为通常进行更现代研究的人不一定与进行深度研究的人有联系。”
The work offers a new perspective on a decades-old mystery and a rare glimpse into an obscure process that enables new traits to sweep across large populations.
这项工作为一个数十年之久的谜团提供了一个新的视角，也让我们难得一睹一个使新性状席卷大量人群的晦涩过程。
The phenomenon of the flipping forams was first described in the early 1950s, when advances in seafloor coring techniques allowed researchers to analyze accumulated layers of shells.
翻转有孔虫现象首次在 20 世纪 50 年代初被描述，当时海底取芯技术的进步使研究人员能够分析堆积的贝壳层。
The Swiss micropaleontologist Hans Bolli first noted that among forams with coiled shells, several species had a directional preference, and that sometimes this preference changed through time.
瑞士微型古生物学家汉斯·博利（Hans Bolli）首先指出，在具有盘绕壳的有孔虫中，有几个物种具有方向性偏好，并且有时这种偏好会随着时间而改变。
An explanation for this curious occurrence came from a seminal 1959 study , for which the marine geologist David Ericson, a core specialist at Columbia University’s Lamont Geological Observatory (now the Lamont-Doherty Earth Observatory), sifted through hundreds of coiled shells from the species Neogloboquadrina pachyderma collected from the North Atlantic.
对这一奇怪现象的解释来自 1959 年的一项开创性研究，在这项研究中，哥伦比亚大学拉蒙特地质观测站（现为拉蒙特-多尔蒂地球观测站）的核心专家、海洋地质学家大卫·埃里克森（David Ericson）筛选了数百个从北大西洋收集的 Neogloboquadrina pachderma 物种的盘绕贝壳。
Ericson observed that in cold climates during the ice ages, the shells tended to coil left, while during warmer periods they turned right.
埃里克森观察到，在冰河时期的寒冷气候中，贝壳倾向于向左盘绕，而在温暖时期则向右盘绕。
He wasn’t sure why coiling direction would relate to climate, but he speculated that temperature was this species’ determining factor.
他不确定为什么盘绕方向与气候有关，但他推测温度是该物种的决定因素。
However, as more cores were retrieved from around the globe, and with advances in genetics, the temperature hypothesis didn’t hold up.
然而，随着从全球各地取回更多的核心，并且随着遗传学的进步，温度假说不再成立。
Kate Darling, an honorary professor at the University of Stirling, has studied the genetics and evolution of forams for decades.
斯特灵大学名誉教授凯特·达林（Kate Darling）几十年来一直在研究有孔虫的遗传学和进化。
“Flipping, in my opinion, means they’ve speciated,” she said.
“在我看来，翻转意味着它们已经形成，”她说。
In 2006, Kate Darling , now an honorary professor at the University of Stirling, published genetic work showing that the variants of N.
2006 年，现为斯特灵大学名誉教授的凯特·达林 (Kate Darling) 发表了基因研究成果，表明 N.
pachyderma are, in fact, two distinct species , each with its own coiling direction.
事实上，厚皮虫是两个不同的物种，每个物种都有自己的卷绕方向。
Then, in 2013 , the evolutionary paleobiologist Yurika Ujiié , now a professor at Kochi University in Japan, found that shell chirality in different foram species, collected from multiple oceans, did not correspond to temperature.
然后，在 2013 年，进化古生物学家、现任日本高知大学教授 Yurika Ujiié 发现，从多个海洋收集的不同有孔虫物种的贝壳手性与温度并不对应。
Each study countered Ericson’s hypothesis in a different way.
每项研究都以不同的方式反驳埃里克森的假设。
Plus, it was hard for many researchers to imagine what advantage coiling direction would offer a single-celled organism with no obvious handedness.
另外，许多研究人员很难想象卷曲方向会给没有明显左手习惯的单细胞生物带来什么优势。
Half a century after Ericson’s initial observation, the driving force behind the flips once again became a mystery.
在埃里克森最初观察半个世纪后，翻转背后的驱动力再次成为一个谜。
It wouldn’t take long to pique another researcher’s interest.
很快就会激起另一位研究人员的兴趣。
Bridget Wade , a micropaleontologist at University College London, had been studying sediment cores for decades when her team noticed a curious pattern.
伦敦大学学院的微型古生物学家布里奇特·韦德（Bridget Wade）几十年来一直在研究沉积物核心，当时她的团队注意到了一种奇怪的模式。
Several foram species seemed to flip their shell direction around the same time at different latitudes in the Atlantic, Indian, and Pacific oceans.
在大西洋、印度洋和太平洋的不同纬度，有几种有孔虫似乎在同一时间翻转它们的壳方向。
In one species, the flips seemed almost instantaneous in the tropics as well as in higher latitudes.
在一个物种中，无论是在热带地区还是在高纬度地区，这种翻转似乎几乎是瞬间发生的。
This evidence that the phenomenon extended far beyond a single ocean basin suggested a global process with more than temperature at work.
这一证据表明，这种现象远远超出了单一海洋盆地的范围，这表明全球过程的作用不仅仅是温度。
To sate their curiosity, Wade’s team synthesized data from five decades of studies and analyzed changes in coiling patterns in several planktonic foraminifera species from the past 56 million years.
为了满足他们的好奇心，Wade 的团队综合了 50 年研究的数据，并分析了过去 5600 万年来几种浮游有孔虫物种的盘绕模式的变化。
For each species, they found evidence of flipping across multiple ocean basins and climate belts.
对于每个物种，他们都发现了跨越多个海洋盆地和气候带的证据。
Paragloborotalia siakensis changed from mixed to left-handed coiling 15 million years ago.
1500万年前，Paragloborotalia siakensis从混合盘绕变为左旋盘绕。
Globorotalia scitula flipped twice: from mixed to left-handed 15 million years ago, and then to right-handed 10 million years ago.
Globorotalia scitula 翻转了两次：1500 万年前从混合性变成左撇子，然后在 1000 万年前变成右撇子。
“It seems truly puzzling that a species could exist for millions of years coiling one way, and then suddenly reverse, for no apparent reason,” the authors wrote.
作者写道：“一个物种可以以一种方式盘绕数百万年，然后突然逆转，没有明显的原因，这似乎确实令人费解。”
Bridget Wade, a micropaleontologist at University College London, points to a species of foram she described at the Smithsonian National Museum of Natural History.
伦敦大学学院的微型古生物学家布里奇特·韦德（Bridget Wade）指出了她在史密森国家自然历史博物馆描述的一种有孔虫。
Pulleniatina obliquiloculata was an especially useful example, Wade said: It has an exceptionally detailed fossil record, is still living today, and occurs throughout tropical oceans worldwide.
韦德说，Plleniatina obliquiloculata 是一个特别有用的例子：它有非常详细的化石记录，至今仍然存在，并且分布在全世界的热带海洋中。
For the past 860,000 years, its shell has coiled almost exclusively to the right.
在过去的 86 万年里，它的壳几乎完全向右卷曲。
But before that, it went through a sequence of rapid shell-coiling flips that occurred globally every few thousand years.
但在此之前，它经历了一系列快速的卷壳翻转，每隔几千年在全球范围内发生一次。
The shifts were far too sudden and widespread to be explained by gradual evolution .
这些转变太过突然和广泛，无法用渐进进化来解释。
“That was quite a surprise because if it was a local event, it would be easier to think about a local, changing environment,” Wade said.
韦德说：“这真是令人惊讶，因为如果这是一场当地活动，就会更容易考虑当地不断变化的环境。”
The fact that it was happening everywhere suggested a different process at work.
事实上，这种情况随处可见，这表明工作过程有所不同。
What could explain the worldwide sweep of a chirality switch?
什么可以解释全球范围内的手性转变？
Wade knew that despite their apparent uniformity, oceans hide many distinct habitats that differ in temperature, currents, ultraviolet light, chemistry, and oxygen.
韦德知道，尽管海洋表面上很均匀，但它们隐藏着许多不同的栖息地，这些栖息地在温度、洋流、紫外线、化学和氧气方面都有所不同。
Likewise, an apparently global population of a foram species can hide cryptic species.
同样，看似全球性的有孔虫物种种群也可能隐藏着神秘物种。
Genetic studies have revealed that often what was considered a single species, based on shell shapes (including coiling direction), was in fact more than one.
遗传学研究表明，根据贝壳形状（包括卷绕方向）被认为是单一物种的物种实际上不止一个。
What if, her team hypothesized, one of these cryptic species developed some broad adaptive advantage.
她的团队假设，如果这些神秘物种中的一种发展出了一些广泛的适应性优势，结果会怎样呢？
This cryptic species might spread across the globe, carried by ocean currents and its own success, in a gigantic population sweep — and bring a single coiling direction to dominance along the way, thereby preserving the event in the fossil record.
这种神秘的物种可能会在洋流和自身成功的推动下，在巨大的种群迁移中传播到全球各地，并沿途以单一盘绕方向占据主导地位，从而将这一事件保留在化石记录中。
Unlike shelled species most familiar to us, forams do not live fully inside their shell.
与我们最熟悉的带壳物种不同，有孔虫并不完全生活在壳内。
The perforated shell sits beneath the cell membrane and is engulfed by jelly-like protoplasm.
穿孔的外壳位于细胞膜下方，并被果冻状的原生质吞没。
All the specimens here are smaller than 1 millimeter.
这里所有的标本都小于1毫米。
Biogeosciences 16, 3377-3396 (2019)
生物地球科学 16, 3377-3396 (2019)
“Flipping, in my opinion, means they’ve speciated,” said Darling, who was not involved in Wade’s study.
“在我看来，翻转意味着它们已经形成了，”达林说，他没有参与韦德的研究。
Whether the flipped forams are a new species or a genetic variant, they would need to have a significant advantage over other forams to sweep all around the world.
无论翻转有孔虫是新物种还是基因变异，它们都需要比其他有孔虫具有显着优势才能席卷全世界。
“It’s a bit like how Covid-19 variants became really abundant in one place,” Wade said, “and then had a slight fitness advantage over another genetic variant” and rapidly spread to become the dominant strain in virtually every country on Earth.
韦德说：“这有点像 Covid-19 变体在一个地方变得非常丰富，然后比另一种基因变体具有轻微的适应性优势”，并迅速传播成为地球上几乎每个国家的主导菌株。
However, Ujiié cautioned against assuming any simple relationship between a species’ genetic identity and its shell chirality.
然而，乌杰警告不要假设物种的遗传特性与其贝壳手性之间有任何简单的关系。
“Coiling direction appears to have a genetic basis,” she said, “but it does not necessarily mean that dextral [right-coiling] and sinistral [left-coiling] individuals represent separate genetic populations.”
“卷绕方向似乎有遗传基础，”她说，“但这并不一定意味着右旋[右卷绕]和左旋[左卷绕]个体代表不同的遗传群体。”
It’s still unclear why an entire population would coil the same way in the first place; it happens only in a subset of planktonic foram species with coiled shells.
目前还不清楚为什么整个种群一开始会以同样的方式盘绕。它只发生在具有卷曲壳的浮游有孔虫物种的子集中。
In other species, shells coil in both directions in a roughly 50-50 split.
在其他物种中，贝壳以大约 50-50 的裂口向两个方向卷曲。
So why do some have a strong chiral bias?
那么为什么有些人有很强的手性偏见呢？
The only thing the scientists seem to agree on is that they don’t know yet.
科学家们似乎唯一同意的是他们还不知道。
Meilland, who studies reproduction in live cultures of forams, has observed that ratios of coiling directions in her cultures don’t necessarily match what one would expect to find in the wild, based on fossil studies.
梅兰德研究有孔虫活体培养物的繁殖，她观察到，根据化石研究，她培养的培养物中卷绕方向的比率不一定与人们期望在野外发现的比率相符。
She thinks that the drivers of coiling direction could be rather complex.
她认为卷绕方向的驱动因素可能相当复杂。
“I think there could be something with genes, with the recombination, with them trying to evolve, with the environment, and also with luck and just life,” she said.
“我认为可能有一些东西与基因有关，与重组有关，与它们试图进化有关，与环境有关，也与运气和生活有关，”她说。
While the near synchronicity of the worldwide foram flips might seem puzzling, Darling and Ujiié pointed out that what appears instantaneous in the fossil record might unfold over 1,000 years or more in real time.
尽管全球范围内有孔虫翻转的近乎同步性似乎令人费解，但 Darling 和 Ujiié 指出，化石记录中看似瞬时发生的事情可能会在 1000 年或更长时间内实时展开。
Over that period, ocean waters circulate around the entire globe.
在此期间，海水在全球范围内循环。
Water masses also change and move, which could speed up the spread of a new variant.
水团也会发生变化和移动，这可能会加速新变种的传播。
It’s possible that traits or species of other marine organisms can also sweep the globe this way.
其他海洋生物的特征或物种也可能以这种方式席卷全球。
But it would be difficult to observe without a clear fossil marker, such as the forams’ shell chirality.
但如果没有明确的化石标记，例如有孔虫壳的手性，就很难观察到。
These coiling flips are therefore a rare window into how evolutionary processes can play out on a global scale.
因此，这些卷曲翻转是了解进化过程如何在全球范围内发挥作用的难得窗口。
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