Most days, Bridget Wade bends over a microscope and looks at fossil plankton that died millions of years ago. She also leads a group of master’s and doctoral students, many of them from other countries, who study climate change and evolution. She says the students are the best part of the job. She had never planned this career. She applied to university to study English literature, but her grades were not good enough, and she now calls that the turning point of her life. She still spends most of her time reading and writing, so perhaps the two worlds are not so different.
The creatures under her lens are called planktonic foraminifera. They are single-celled sea organisms about the size of a grain of sand, and they build shells of calcium carbonate. When they die, their shells sink and pile up in the mud of the seafloor, year after year, making a long record that scientists can read. Wade studies long tubes of this mud, drilled from the ocean floor. By counting species, she can measure how fast life evolved or died out. By studying the chemistry of the shells, she can work out how warm the sea was in the past.
The group is huge and very old. The name foraminifera means “hole bearers” in Latin, because the shells have pores between their chambers. More than 50,000 species are known, living and fossil, and most are smaller than a millimetre. Most live on the seafloor, and only about forty living species float in the water. Their shells cover the seabed everywhere, in an archive that goes back about 560 million years.
Inside that archive there is a puzzle that is seventy years old. Many of these shells are coiled like a snail, and some species prefer to coil one way, left or right. In the early 1950s the Swiss scientist Hans Bolli noticed that this preference could change over time. In 1959 the marine geologist David Ericson, who worked with sediment cores at Columbia University, sorted hundreds of shells of one species from the North Atlantic. During the ice ages the shells coiled left, and in warmer periods they coiled right. He could not explain it, but he thought temperature was the cause.
That idea lasted for decades and then failed. In 2006 Kate Darling showed with genetic work that the left and right forms of Ericson’s species are in fact two different species. In 2013 Yurika Ujiié, now a professor at Kochi University in Japan, studied shells from several oceans and found that the direction did not follow temperature. There was also a simple problem: it is hard to see how coiling in one direction could help a single cell.
What interested Wade was geography. Her team noticed that several species seemed to change direction at about the same time in the Atlantic, the Indian and the Pacific. So they collected fifty years of published counts and studied species going back 56 million years. Everywhere, the changes crossed several oceans. Their best example was a species that still lives today in tropical seas. For the last 860,000 years it has coiled almost only to the right, but before that it changed direction, globally, every few thousand years. Wade said this was a surprise, because a local event would be easier to explain.
Her explanation is that one species may really be several hidden species living in different ocean habitats. If one hidden species gains an advantage, it can spread and take over, bringing its coiling direction with it. Darling, who was not part of the study, thinks the flipping means new species formed. Wade compares it to a Covid-19 variant that became common in one place and then spread almost everywhere.
Not everyone agrees. Ujiié says coiling probably has a genetic basis, but that left and right shells need not be separate populations. And nobody knows why a whole population should coil one way at all, when many species are about half and half. Julie Meilland, a researcher at the Cerege in France, grows these organisms in her laboratory and finds that her results do not always match the fossil record. She said there could be something with genes, with the environment, “and also with luck and just life”.