内容来源：https://www.quantamagazine.org/why-do-these-fossil-shells-flip-their-spirals-every-few-millennia-20260911/

内容总结：

研究人员近日综合分析了多物种浮游有孔虫的壳体旋向数据，提出新假说：壳体旋向的全球同步翻转，可能并非对温度等气候因素的直接适应，而是某一具有演化优势的隐秘物种从一小群体出发、借助洋流迅速扩散至全球海洋后留下的“偶然印记”。相关研究为困扰科学界数十年的有孔虫旋向之谜提供了新视角。

有孔虫是生活在全球海洋中的单细胞原生生物，其中不少种类会分泌带孔硬壳，死后壳体沉降至海底，形成可追溯至约5.6亿年前的自然档案。科学家长期利用这些壳体研究古气候和古海洋环境。早在20世纪50年代，研究者就注意到，一些浮游有孔虫的蜗状壳体会呈现明显的左旋或右旋偏好，某些物种中同一方向的比例甚至高达97%；更奇特的是，在化石记录中，这种旋向有时会在全球范围内近乎同时发生翻转。

1959年，海洋地质学家戴维·埃里克森曾根据北大西洋沉积岩芯提出，旋向变化可能与冰期和间冰期的冷暖气候有关。但后续研究逐渐动摇了这一解释。2006年的遗传学研究显示，原本被视为同一物种的壳体旋向差异，实际上可能对应两个不同物种；2013年的研究也发现，多海域有孔虫的壳体旋向与温度并不对应。

英国伦敦大学学院微体古生物学家布里奇特·韦德团队近日对过去5600万年间多个浮游有孔虫物种的旋向数据进行整合分析，发现多个物种在多个海域和气候带中都出现过旋向翻转。其中，一种名为Pulleniatina obliquiloculata的有孔虫在过去86万年中几乎全部右旋，但在此之前，其壳体旋向曾每隔数千年在全球范围内快速翻转。

研究人员认为，这种变化过于突然且分布广泛，难以用渐进演化或单一环境因素解释。他们提出，海洋中看似单一的有孔虫物种，可能隐藏着多个形态相似但遗传上不同的“隐秘物种”。当其中一个隐秘物种获得某种广泛适应优势后，便可能随洋流扩散至全球，并在此过程中使其壳体旋向占据主导，从而在化石记录中留下全球同步翻转的印象。

未参与该研究的法国科研人员朱莉·梅扬认为，这项研究首次将深时化石数据与现代有孔虫遗传学和生物学认识结合起来，令人耳目一新。也有专家提醒，壳体旋向与遗传种群之间的关系仍不宜简单化，旋向是否具有明确适应优势、为何部分物种会形成强烈旋向偏好，目前仍无定论。

研究人员表示，化石记录中看似“瞬间”完成的全球翻转，实际可能历经上千年甚至更久；洋流循环和水团运动可能加速新变种传播。类似的大规模演化扩散过程或许也存在于其他海洋生物中，但由于缺乏像有孔虫壳体旋向这样清晰的化石标记，往往难以观察。

中文翻译：

为什么这些化石贝壳每隔几千年就会翻转一次螺旋方向？

引言

在数千年乃至数百万年的时间里，全球各地的海洋浮游生物都朝着同一个方向构建它们的螺旋形壳体。然后，突然间，螺旋方向在全球范围内同时发生了翻转，之后又在某个时候翻了回去。

这些微生物属于有孔虫类。从热带到高纬度地区，在所有海洋中都能找到它们，它们是地球上数量最丰富的真核生物之一。这些单细胞原生生物分泌出布满小孔的坚硬壳体；大多数物种生活在海底，也有一些营浮游生活，随洋流漂泊。它们死后，壳体覆盖全球海底，形成了可追溯至约5.6亿年前的地球历史天然档案。

通过研究物种组成，或测量同位素和微量元素，科学家利用积累的有孔虫壳体来重建过去的气候和海洋条件。多年来，他们注意到一个奇特的现象。在许多具有蜗牛般螺旋壳体的浮游有孔虫中，一些物种强烈偏好某一螺旋方向——左旋或右旋——偏好比例可高达97%的个体朝同一方向卷曲。而且有时候，在化石记录中，一个新的卷曲方向似乎同时在整个大洋范围内占据主导地位。自20世纪50年代首次观察到这一奇异现象以来，科学家一直试图弄清楚，究竟是什么原因能让数以千万亿计的微生物同时改变壳体方向。

近年来，微体古生物学家整合并综合了多个物种壳体卷曲方向（即手性）的数据，这些数据来自可追溯至5600万年前的有孔虫案例研究。一些研究人员曾推测，壳体翻转与温度变化或其他气候因素有关。然而，这项新研究提出一个假说：壳体方向本身并非一种适应性响应，而是一个重大演化事件的偶然标记——该事件始于一个小型亚种群，随后席卷广阔的洋盆。

“这可能是第一次有通常从事有孔虫生物地层学——基本上是深时研究——的人来探讨这类问题，”法国CEREGE研究所的研究员朱莉·梅扬说，她未参与这项研究。除了来自多个物种跨越数百万年的卷曲方向数据外，该研究还融入了现代有孔虫遗传学和生物学的见解。“看到这些领域相互连接，令人耳目一新，因为很多时候做现代研究的人不一定与做深时研究的人有联系，”梅扬补充道。

这项工作为一个数十年之久的谜团提供了新视角，也让我们得以罕见地一窥一个晦涩的过程——它使新性状得以在庞大种群中迅速扩散。

一个被扭曲的假说

有孔虫翻转现象最早在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累积的壳体层。瑞士微体古生物学家汉斯·博利首先注意到，在具有螺旋壳体的有孔虫中，有几个物种存在方向偏好，而且这种偏好有时会随时间改变。

对这一奇特现象的解释来自1959年的一项开创性研究。海洋地质学家、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）的岩芯专家大卫·埃里克森，从北大西洋采集的厚壁新球房虫物种中，筛选了数百枚螺旋壳体。埃里克森观察到，在冰期的寒冷气候中，壳体倾向于左旋，而在温暖时期则转为右旋。

他不确定卷曲方向为何与气候相关，但推测温度是该物种的决定性因素。然而，随着全球范围内更多岩芯被提取，以及遗传学的进步，温度假说未能成立。

2006年，现为斯特灵大学荣誉教授的凯特·达林发表了遗传学研究，表明厚壁新球房虫的变种实际上是两个不同的物种，各自具有自己的卷曲方向。随后，在2013年，现为日本高知大学教授的演化古生物学家氏家由利香发现，从多个大洋采集的不同有孔虫物种的壳体手性并不与温度对应。

每项研究都以不同方式反驳了埃里克森的假说。此外，许多研究人员很难想象卷曲方向能给一个没有明显左右偏向的单细胞生物带来什么优势。在埃里克森最初观察半个世纪之后，翻转背后的驱动力再次成为一个谜。

没过多久，另一位研究者的兴趣就被激发了。伦敦大学学院的微体古生物学家布里奇特·韦德研究沉积物岩芯已有数十年，她的团队注意到一个奇特的模式。几个有孔虫物种似乎在大西洋、印度洋和太平洋的不同纬度几乎同时翻转了壳体方向。有一个物种，在热带和高纬度地区，翻转似乎几乎是瞬间发生的。这一现象远远超出单个洋盆的证据表明，这是一个全球性过程，起作用的不只是温度。

为满足好奇心，韦德的团队综合了五十年研究的数据，分析了过去5600万年间几种浮游有孔虫物种的卷曲模式变化。对于每个物种，他们都发现了跨越多个洋盆和气候带发生翻转的证据。西阿克副球房虫在1500万年前从混合卷曲变为左旋。细小球房虫翻转了两次：1500万年前从混合变为左旋，然后在1000万年前变为右旋。“一个物种以某种方式卷曲存在了数百万年，然后突然毫无明显原因地反转，这似乎确实令人费解，”作者们写道。

韦德说，斜室普林虫是一个特别有用的例子：它拥有异常详细的化石记录，至今仍然存活，并且遍布全球热带海洋。在过去86万年中，它的壳体几乎全部右旋。但在那之前，它经历了一系列快速的壳体卷曲翻转，每隔几千年就在全球范围内发生一次。

这些转变太过突然和广泛，无法用渐进演化来解释。“这相当令人惊讶，因为如果是局部事件，就更容易想象是局部环境变化所致，”韦德说。它 everywhere 都在发生这一事实表明，有另一种过程在起作用。

隐秘的含义

什么能解释手性转换的全球性席卷？韦德知道，尽管海洋表面看似均一，但实际上隐藏着许多不同的栖息地，它们在温度、洋流、紫外线、化学性质和氧气方面各不相同。同样，一个有孔虫物种看似全球统一的种群，也可能隐藏着隐存种。遗传学研究表明，许多仅凭壳体形状（包括卷曲方向）被认定为单一物种的，实际上是多个物种。

她的团队提出假说：如果这些隐存种之一演化出了某种广泛的适应性优势呢？这个隐存种可能借助洋流和自身的成功，以庞大的种群规模席卷全球——并在此过程中使某一卷曲方向占据主导地位，从而将这一事件保存在化石记录中。

“在我看来，翻转意味着它们已经物种化了，”未参与韦德研究的达林说。

无论翻转的有孔虫是新物种还是遗传变种，它们都需要比其他有孔虫具有显著优势，才能席卷全球。“这有点像新冠病毒变种在一个地方变得非常普遍，”韦德说，“然后比另一个遗传变种具有轻微的适应度优势”，并迅速传播，成为地球上几乎每个国家的主导毒株。

然而，氏家由利香告诫说，不要假设物种的遗传身份与其壳体手性之间存在任何简单关系。“卷曲方向似乎有遗传基础，”她说，“但这并不一定意味着右旋和左旋个体代表不同的遗传种群。”

目前仍不清楚为什么整个种群一开始会朝同一方向卷曲；这只发生在具有螺旋壳体的部分浮游有孔虫物种中。在其他物种中，壳体以大约各半的比例向两个方向卷曲。那么，为什么有些物种具有强烈的手性偏向？科学家们似乎唯一达成共识的是：他们还不知道。

梅扬研究活体培养中有孔虫的繁殖，她观察到培养物中的卷曲方向比例不一定与根据化石研究在野外预期发现的比例相符。她认为，卷曲方向的驱动因素可能相当复杂。“我认为可能与基因、重组、它们试图演化、环境有关，也与运气和生命本身有关，”她说。

虽然全球有孔虫翻转的近乎同步性可能看起来令人费解，但达林和氏家由利香指出，在化石记录中看似瞬间发生的事情，在真实时间中可能历经1000年甚至更久。在这段时间里，海水环绕整个地球循环流动。水团也会变化和移动，这可能加速新变种的传播。

其他海洋生物的形状或物种也有可能以这种方式席卷全球。但如果没有明确的化石标记——比如有孔虫的壳体手性——就很难观察到。因此，这些卷曲翻转为了解演化过程如何在全球尺度上展开，提供了一个难得的窗口。

英文来源：

Why Do These Fossil Shells Flip Their Spirals Every Few Millennia?
Introduction
For thousands and sometimes millions of years, marine plankton all over the world built their spiral shells in one direction. Then, suddenly, the spirals switched direction at the same time, everywhere, only to switch back again later.
These microorganisms are types of foraminifera, or forams. Found in all oceans, from the tropics to high latitudes, they’re among the most abundant eukaryotic organisms on Earth. 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. 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.
By studying the composition of species, or measuring isotopes or trace elements, scientists use accumulated foram shells to reconstruct past climates and ocean conditions. Over the years, they have noticed a strange phenomenon. 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. And sometimes, a new coiling direction dominates seemingly everywhere across the oceans all at once in the fossil record. 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.
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. Some researchers speculated that the shell flips were connected to changes in temperature or other climate factors. 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.
“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. 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. “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.
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.
A Twisted Hypothesis
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. 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.
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. Ericson observed that in cold climates during the ice ages, the shells tended to coil left, while during warmer periods they turned right.
He wasn’t sure why coiling direction would relate to climate, but he speculated that temperature was this species’ determining factor. However, as more cores were retrieved from around the globe, and with advances in genetics, the temperature hypothesis didn’t hold up.
In 2006, Kate Darling, now an honorary professor at the University of Stirling, published genetic work showing that the variants of N. pachyderma are, in fact, two distinct species, each with its own coiling direction. 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.
Each study countered Ericson’s hypothesis in a different way. Plus, it was hard for many researchers to imagine what advantage coiling direction would offer a single-celled organism with no obvious handedness. Half a century after Ericson’s initial observation, the driving force behind the flips once again became a mystery.
It wouldn’t take long to pique another researcher’s interest. Bridget Wade, a micropaleontologist at University College London, had been studying sediment cores for decades when her team noticed a curious pattern. Several foram species seemed to flip their shell direction around the same time at different latitudes in the Atlantic, Indian, and Pacific oceans. In one species, the flips seemed almost instantaneous in the tropics as well as in higher latitudes. This evidence that the phenomenon extended far beyond a single ocean basin suggested a global process with more than temperature at work.
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. For each species, they found evidence of flipping across multiple ocean basins and climate belts. Paragloborotalia siakensis changed from mixed to left-handed coiling 15 million years ago. Globorotalia scitula flipped twice: from mixed to left-handed 15 million years ago, and then to right-handed 10 million years ago. “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.
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. For the past 860,000 years, its shell has coiled almost exclusively to the right. But before that, it went through a sequence of rapid shell-coiling flips that occurred globally every few thousand years.
The shifts were far too sudden and widespread to be explained by gradual evolution. “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. The fact that it was happening everywhere suggested a different process at work.
Cryptic Meaning
What could explain the worldwide sweep of a chirality switch? Wade knew that despite their apparent uniformity, oceans hide many distinct habitats that differ in temperature, currents, ultraviolet light, chemistry, and oxygen. Likewise, an apparently global population of a foram species can hide cryptic species. 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.
What if, her team hypothesized, one of these cryptic species developed some broad adaptive advantage. 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.
“Flipping, in my opinion, means they’ve speciated,” said Darling, who was not involved in Wade’s study.
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. “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.
However, Ujiié cautioned against assuming any simple relationship between a species’ genetic identity and its shell chirality. “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.”
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. In other species, shells coil in both directions in a roughly 50-50 split. So why do some have a strong chiral bias? The only thing the scientists seem to agree on is that they don’t know yet.
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. She thinks that the drivers of coiling direction could be rather complex. “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.
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. Over that period, ocean waters circulate around the entire globe. Water masses also change and move, which could speed up the spread of a new variant.
It’s possible that traits or species of other marine organisms can also sweep the globe this way. But it would be difficult to observe without a clear fossil marker, such as the forams’ shell chirality. These coiling flips are therefore a rare window into how evolutionary processes can play out on a global scale.