Fossil shells flipped their spirals worldwide, and scientists have argued for 70 yearsWhy Do These Fossil Shells Flip Their Spirals Every Few Millennia?

In the ocean live single-celled creatures called foraminifera. They build themselves a spiral shell, and when they die the shells sink and pile up on the seafloor, forming an archive of Earth's history going back hundreds of millions of years.

The odd part: in some species nearly all the shells coil the same way, up to 97% of them. Odder still, that direction can flip everywhere at once in the fossil record, then flip back later.

The 1950s answer was temperature. Then it got dismantled

In 1959 the geologist David Ericson counted hundreds of shells from the North Atlantic and found left-coiling in ice ages, right-coiling in warm periods. Temperature, he guessed, was deciding.

Genetics later killed that idea. In 2006 Kate Darling showed the differently coiled forms were two separate species. In 2013 Yurika Ujiié found coiling direction did not match temperature. The mystery was back where it started.

A new guess: not adaptation, but a global species swap

Bridget Wade's team at University College London stitched five decades of data together and found the flips happening in several oceans and climate belts at once, too fast and too widespread to be gradual evolution.

Their proposal: what looks like one species hides genetically distinct cryptic species. One of them happened to gain an edge, rode the currents around the globe, and dragged its coiling direction into dominance along the way. Much like a Covid variant emerging in one place and ending up dominant in nearly every country.

So a flipped shell may just have been a passenger, not a creature responding to anything. Why some species strongly prefer one direction at all, nobody can yet say.

Put plainly: which way a shell twists hardly matters. What matters is that it left an accidental attendance record of a worldwide changing of the guard.