The best part of Bridget Wade’s job, she says, is 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. The other part is the microscope. Every day is different, and on most of them she bends over the eyepiece and looks at extraordinary fossil plankton from millions of years ago. 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 notes that she still spends most of her time reading and writing, so perhaps the two worlds are not so far apart.
The things 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. When the animals die, the shells settle into marine sediment 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 reads the oxygen and carbon isotopes locked in the shells to reconstruct sea temperatures, ice ages and warm periods, 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 a many-chambered shell, and more than 50,000 species are recognised, fossil and living. Most are smaller than a millimetre. Most live on or in the seafloor; only about forty living species float in the water, carried by currents, and it is these that rain down everywhere. Their shells blanket the seabed across the world, an archive running back some 560 million years.
Somewhere in that archive is a puzzle that has been sitting unsolved for seventy years. Many planktonic forams build snail-like coiled shells, and some species strongly prefer to coil one way, left or right. In the early 1950s, when seafloor coring first allowed researchers to work layer by layer through stacked sediment, the Swiss micropaleontologist Hans Bolli noticed that the preference was not fixed: in several species, it changed through time. In 1959 the marine geologist David Ericson, a core specialist at Columbia University’s Lamont Geological Observatory, sorted hundreds of shells of Neogloboquadrina pachyderma from the North Atlantic and found what looked like an answer. During the ice ages the shells coiled left; during warmer intervals they coiled right. He could not say why a spiral should care about climate, but he suspected temperature was doing the steering.
The temperature idea survived for decades and then came 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 forms of N. pachyderma are not forms at all but two distinct species, each with its own coiling direction. In 2013 the evolutionary paleobiologist Yurika Ujiié, now a professor at Kochi University in Japan, examined coiling in several foram species collected from several oceans and found that it did not track temperature. There was also a plain difficulty of logic: it is hard to see what advantage a direction of coiling could give a single-celled organism with no obvious left or right side. Even earlier, work on a South Pacific drill site had warned that the coiling of N. pachyderma did not settle into its present-day pattern until the early Pliocene, so it could not be used to read temperatures in older faunas at all.
What drew Wade in was geography. After decades with sediment cores, her team noticed that several foram species seemed to flip their shell direction at roughly the same time at different latitudes in the Atlantic, the Indian and the Pacific. In one species the switch looked almost instant in the tropics and in high latitudes alike. Whatever was happening was not confined to one ocean basin. So the team gathered five decades of published coiling counts and analysed the patterns in several planktonic species reaching back 56 million years. In every case they found flips spread across multiple basins and climate belts. Paragloborotalia siakensis went from mixed to left-coiling 15 million years ago. Globorotalia scitula 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 exist for millions of years coiling one way and then suddenly reverse for no apparent reason.
The best case was Pulleniatina obliquiloculata, which has an unusually detailed fossil record, is still alive today and lives across tropical oceans everywhere. For the last 860,000 years it has coiled almost exclusively 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.
Higher-resolution counts from two cores on either side of the Indonesian through-flow, one in the Pacific and one in the Indian Ocean, showed something stranger still. Measuring isotopes in single shells, researchers found that left- and right-coiling shells from the very same sediment sample could differ significantly in both carbon and oxygen. That implies that the two populations were living apart ecologically; in one case they also differed in size. The proposed explanation was that big swings in coiling ratio are caused by hidden populations replacing one another in competitive sweeps, a mode of evolution more usually linked to organisms that reproduce asexually.
That is essentially the hypothesis Wade’s synthesis advances. Oceans look uniform and are not: they are divided into habitats differing in temperature, currents, ultraviolet light, chemistry and oxygen, and an apparently global species can conceal several genetically distinct hidden ones. If one of those hidden species gained some broad advantage, it could spread on currents and on its own success in a gigantic population sweep. That would carry a single coiling direction to dominance and stamp the event into the fossil record. “Flipping, in my opinion, means they’ve speciated,” said Darling, who was not involved in the study. Wade reached for a more recent comparison: it is a bit like the way a Covid-19 variant became abundant in one place and then, with a slight advantage over another genetic variant, swept to dominance almost everywhere.
Seen that way, the direction of coiling is not an adaptation at all but a marker: a visible tag on an invisible event. Replacements of this kind would normally leave no trace. They become readable only when the competing groups happen to differ in coiling preference, which is why the flips argue for speciation driven by the division of habitats in the open ocean rather than by reproductive isolation alone. There are hints of the same division elsewhere: in Globorotalia truncatulinoides, left-coiling shells form deeper in the water than right-coiling ones, and periods when the left-coilers dominated point to a deepening of the permanent thermocline.
Not everyone is ready to turn shells into species. Coiling direction does appear to have a genetic basis, Ujiié cautions, but that does not necessarily mean right-coiling and left-coiling individuals belong to separate genetic populations. And the older question remains untouched: why should a whole population coil one way to begin with, 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 forams in culture and finds that her coiling ratios do not always match what fossil work would predict. “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.