The problem that remains is larger. In a Brief Report published by PNAS on 27 May 2026, Bridget Wade, Paul Pearson and David King of University College London and Valeria Luciani of the Università di Ferrara (University of Ferrara, Italy) line up the cases of global reversal: not shifting water masses, but changes of coiling that appear everywhere, from the tropics to high latitudes, in warm periods and in ice ages. The paper brings no new measurements. It gathers those published between 2021 and 2025 and proposes a reading. It is worth discussing now because that reading is reaching the general press, often under the formula "scientists cannot explain it". The formula is wrong: the explanation is the paper's thesis.

The cases begin in the Eocene. During the Early Eocene Climatic Optimum, 53 to 49 million years ago, the extinct genus Morozovella shifted from dextral to sinistral soon after warming began; the sinistral shells carry lower carbon isotope values, a sign of less reliance on symbiotic algae. In the Miocene, at nine sites across different basins and climate belts, Paragloborotalia siakensis went from mixed to sinistral coiling 15.37 million years ago; Globorotalia scitula flipped twice, mixed to sinistral at 15.14 million years and sinistral to dextral at 10.02. Five million years one way, then a reversal, for no visible reason. Pulleniatina obliquiloculata, which arose 4.2 million years ago and lives today in every tropical ocean, has been almost entirely dextral for 860,000 years; before that it changed coiling several times, worldwide, each time within a few thousand years. Oxygen and carbon isotopes measured on hundreds of individual shells show, in some intervals, different mean values for dextral and sinistral specimens of identical size and shape: they lived at different depths, or bloomed in different seasons.

The explanation comes from the genetics of living populations. Coiling is inherited. The species a microscope can tell apart hide genetic variants a microscope cannot see. Pulleniatina obliquiloculata has several, all dextral, one confined to the warmest water of the west Pacific, the others more widespread; Globorotalia truncatulinoides includes a genotype with frequent sinistral forms, and its genotypes divide the water masses between them. Water masses act as barriers in the open ocean: they limit gene flow and keep the variants apart, coexisting and slowly diverging. Now and then one variant gains an advantage and replaces the others across the whole ocean. This is a population sweep, the wave-like replacement biologists know from bacteria and everyone saw in the variants of the Covid virus. Almost always the replacement leaves no trace in the sediment. If the two variants differ in coiling, the sediment records it as an abrupt reversal. Coiling itself is good for nothing, the authors write: it is an accidental marker of a hidden dynamic. And with populations of ten million billion individuals per species, the record is complete.

If the reading is right, then for foraminifera reproductive isolation alone does not make a species: a niche is needed, a different way of living. This is ecological speciation, and finding a new way of life is harder than ceasing to interbreed. The authors also declare the gap: nobody knows why a lineage should favour one coiling. The cell has no handedness, the shell by geometry must turn one way, and what decides which way is unknown. They are waiting for molecular biology. The method has a limit the paper does not hide: the cases are few, six taxa in fifty-six million years, and they enter the count only because the fossil record shows them. Replacements that do not change coiling cannot be seen and are not counted. The paper is a synthesis, not an experimental study; the authors declare no competing interests, and the work was funded by the UK Natural Environment Research Council.

References
Wade B. S., Pearson P. N., King D. J., Luciani V., «Flipping plankton», PNAS, 123, 22, e2603416123, published online 27 May 2026, open access CC BY. doi: 10.1073/pnas.2603416123
Pearson P. N., Penny L., «Coiling directions in the planktonic foraminifer Pulleniatina: A complex ecoevolutionary dynamic spanning millions of years», PLOS One, 16, e0249113, 2021, open access. doi: 10.1371/journal.pone.0249113
Pearson P. N., Young J., King D. J., Wade B. S., «Biochronology and evolution of Pulleniatina (planktonic foraminifera)», Journal of Micropalaeontology, 42, 211–255, 2023. doi: 10.5194/jm-42-211-2023
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