Most days, Bridget Wade bends over a microscope and looks at fossil plankton that died millions of years ago. The best part of the job, she says, is not the eyepiece but the students: a lab group of master’s and doctoral researchers, many of them from abroad, working on climate change, evolution and the repair of the geological time scale. She had not planned any of it. She applied to university to study English literature and her grades were not good enough, which she now counts as the turning point of her life. She still spends most of her time reading and writing, so perhaps the two worlds are not so far apart.
The creatures under the lens are planktonic foraminifera, single-celled marine organisms about the size of a grain of sand. Their shells are made of calcium carbonate, and when the animals die the shells sink into the sediment on the seafloor and pile up, year after year, into a long and readable record. Wade works from cores drilled through that record by the International Ocean Discovery Program. She counts species to measure rates of evolution and extinction, and she reads the oxygen and carbon isotopes locked inside the shells to reconstruct sea temperatures, ice ages and the productivity of ancient oceans.
The group is enormous and very old. Foraminifera means hole bearers in Latin, after the pores that connect the chambers of the shell, and more than 50,000 species are known, fossil and living. Most are smaller than a millimetre, and most live on or in the seafloor. Only about forty living species float in the water, carried by the currents, and it is these that rain down everywhere when they die. Their shells cover the seabed all over the world, an archive that runs back some 560 million years.
Somewhere in that archive is a puzzle that has been waiting seventy years for an answer. Many planktonic foraminifera build coiled shells, like a snail, and some species strongly prefer to coil one way, left or right. In the early 1950s, when seafloor coring first let researchers work layer by layer through the sediment, the Swiss micropaleontologist Hans Bolli noticed that the preference was not fixed: in several species it changed over time. In 1959 the marine geologist David Ericson, a core specialist at Columbia University’s Lamont Geological Observatory, sorted hundreds of shells of one North Atlantic species and thought he had the answer. During the ice ages the shells coiled left; during warmer periods they coiled right. He could not explain why a spiral should care about climate, but he suspected temperature was steering it.
The temperature idea survived for decades, then fell apart from two directions. In 2006 Kate Darling, now an honorary professor at the University of Stirling, published genetic work showing that the left- and right-coiling forms were not two versions of one species but two different species, each with its own direction. In 2013 the evolutionary paleobiologist Yurika Ujiié, now a professor at Kochi University in Japan, studied coiling in several species from different oceans and found it did not follow temperature. There was also a simple logical problem: it is hard to see what advantage a direction of coiling could give a single-celled organism.
What drew Wade in was geography. After decades of work with sediment cores, her team noticed that several species seemed to flip direction at roughly the same time at different latitudes in the Atlantic, the Indian and the Pacific oceans. In one species the switch looked almost instant in the tropics and in high latitudes alike. Whatever was happening was not limited to one ocean. So the team gathered five decades of published counts and analysed patterns in several species going back 56 million years, and in every case the flips appeared across several ocean basins and climate belts. One species went from mixed to left-coiling 15 million years ago. Another flipped twice: to left-coiling 15 million years ago and to right-coiling 10 million years ago. It seems truly puzzling, the authors wrote, that a species could coil one way for millions of years and then suddenly reverse for no apparent reason.
The best case was Pulleniatina obliquiloculata, a species with an unusually detailed fossil record, still alive today and found in tropical oceans everywhere. For the last 860,000 years it has coiled almost only to the right. Before that, it flipped globally every few thousand years. “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. It was not local.
Wade’s explanation is that oceans only look uniform. They are divided into habitats that differ in temperature, currents, ultraviolet light, chemistry and oxygen, and one apparently global species can hide several genetically separate ones. If one of those hidden species gained some advantage, it could spread on the currents and on its own success, carrying a single coiling direction to dominance and stamping the event into the fossil record. “Flipping, in my opinion, means they’ve speciated,” said Darling, who was not involved in the study. Wade offered a more recent comparison: it is a bit like the way a Covid-19 variant became common in one place and then, with a slight advantage over another variant, spread almost everywhere.
Seen this way, the direction of coiling is not an adaptation but a marker: a visible sign of an invisible event. Replacements like this would normally leave no trace, and only become readable when the competing groups happen to prefer different directions.
Not everyone is ready to turn shells into species. Coiling direction does seem to have a genetic basis, Ujiié warns, but that does not necessarily mean left- and right-coiling individuals belong to separate genetic populations. And the older question is still open: why should a whole population coil one way at all, when plenty of species split roughly fifty-fifty? Julie Meilland, a researcher at the Cerege in France, who was not part of the study, grows foraminifera in culture and finds that her coiling ratios do not always match what the fossil work predicts. “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.