The ocean's most abundant shell-builders have been playing a strange trick on the fossil record for tens of millions of years. At irregular intervals, populations of planktonic foraminifera in every ocean basin flipped the direction their spiral shells coil — often within a few thousand years — and later flipped back. A synthesis in PNAS titled "Flipping plankton," by Bridget Wade of University College London with Paul Pearson, David King and Valeria Luciani, argues the reversals are not climate responses at all but the fossil fingerprint of a hidden evolutionary process: cryptic speciation followed by global population sweeps. Quanta Magazine reported on the work today.
Foraminifera, or forams, are single-celled protists found in all oceans from the tropics to high latitudes, among the most abundant eukaryotes on Earth. They secrete hard, perforated shells that blanket the seafloor when they die, building a natural archive of Earth's history going back some 560 million years. Among the many planktonic species with snail-like coiled shells, one direction — left, or sinistral, versus right, or dextral — often dominates, with as many as 97 percent of individuals in a species coiling the same way.
The phenomenon was first described in the 1950s, as seafloor coring advanced. The Swiss micropaleontologist Hans Bolli noticed the directional preference; in 1959 the geologist David Ericson read the left coiling of Neogloboquadrina pachyderma in North Atlantic cores as a cold-climate indicator, launching a climate proxy used for decades. Genetics undercut that reading. In 2006 Kate Darling, now an honorary professor at the University of Stirling, showed the two coiling forms of N. pachyderma are in fact two distinct species. In 2013 the evolutionary paleobiologist Yurika Ujiié, now at Kochi University, found that shell chirality across foram species did not track temperature. What could move quadrillions of single-celled organisms in unison became a fresh mystery.
Wade's team took a different route: they synthesized five decades of published case studies and analysed coiling changes across several planktonic species spanning the last 56 million years. The pattern that emerged was global and abrupt — flips in the tropics and at high latitudes, in warm periods and ice ages, across the Atlantic, Indian and Pacific oceans, far too sudden to be explained by gradual trait evolution. Shortly after the Early Eocene Climate Optimum, the long-extinct genus Morozovella shifted from dextral to sinistral. Paragloborotalia siakensis changed from mixed to sinistral coiling 15.37 million years ago. The Globorotalia scitula lineage flipped twice: mixed to sinistral at 15.14 million years, then back to dextral at 10.02 million years.
The best-documented case is Pulleniatina obliquiloculata, which evolved about 4.2 million years ago and still thrives in tropical oceans worldwide. For the past 860,000 years its shells have coiled almost exclusively to the right. Before that, it went through a series of rapid flips that occurred globally within a few thousand years each. Isotopic measurements of hundreds of individual shells show occasional divergence in average values between dextral and sinistral specimens even though their size and shape are indistinguishable — a possible sign of subtle habitat differences, or of peak abundances at different times of year.
The authors' explanation: oceans that look uniform hide distinct habitats and water masses, and an apparently single species often conceals several genetically distinct but morphologically identical forms. Such cryptic populations might continually throw off variants that are largely isolated in a reproductive sense and that prosper or dwindle with circumstances. Every so often, one variant could gain a global selective advantage and replace the rest — a "population sweep," familiar from bacteria and, more recently, from Covid-19 variants. Most such replacements would leave nothing visible; only when the competing forms differ in coiling preference does a geologically abrupt flip appear. On this reading, chirality confers no advantage of its own and is merely an accidental marker, and speciation in these plankton is driven less by reproductive isolation than by the partitioning of habitat niches in the open ocean.
Not every specialist endorses every step. Kate Darling, who was not involved in the study, puts it plainly: a flip, in her view, means the organisms have speciated. Ujiié cautions that although coiling direction appears to have a genetic basis, that does not necessarily mean sinistral and dextral individuals are separate genetic populations. Julie Meilland, who studies reproduction in living cultures at the French institute Cerege, has observed coiling ratios that do not match what fossil studies would predict, and suspects the drivers involve genes, recombination, environment — and luck.
The deepest question remains open. Why a population should favour one direction in the first place is unexplained, and only some coiled foram species have a strong bias; others coil roughly 50-50. One important caveat: what looks instantaneous in the fossil record may unfold over a thousand years or more, time enough for ocean currents to carry water masses and variants around the globe. Even so, the flips offer a rare window into evolution playing out on a planetary scale — a process that, without a marker as simple as the direction of a spiral, would leave no visible trace at all.
