A 56-million-year synthesis pins worldwide foram shell flips on an evolutionary sweep

Synchrony is the part that needs explaining. A trait that begins in one small subpopulation has to end up dominating shells in every ocean basin, and the fossil record shows a new coiling direction taking over seemingly everywhere at once ^[5]. The synthesis proposes that the flip is an accidental marker of such an event ^[9]. It does not identify what carries the variant from one basin to the rest, or how long the crossing takes.

Coiling preference is strong without being absolute. In some species as many as 97 percent of individuals coil the same way ^[4], which leaves up to three in a hundred going the other way ^[1]. A core that records a flip is recording a change in which direction dominates.

David Ericson gave the phenomenon its first explanation in 1959, after going through hundreds of Neogloboquadrina pachyderma shells from North Atlantic cores at Columbia's Lamont Geological Observatory: left-coiling in the cold of the ice ages, right-coiling in warmer periods ^[10]. He was not sure why direction should relate to climate, and he suggested temperature was the determining factor for the species ^[11]. Forty-seven years later, Kate Darling's genetic work showed that the two coiling variants are in fact two distinct species, each with its own direction ^[13][2]. On that result, an ice-age flip in a North Atlantic core is a change in which species was living there ^[4].

Yurika Ujiie, now at Kochi University, then reported in 2013 that chirality in foram species collected from multiple oceans did not correspond to temperature ^[14]. The temperature hypothesis had already been weakening as more cores came in from around the globe ^[12].

What is unusual about the new work is the pairing: coiling data from case studies across millions of years, read alongside insights from modern foram genetics and biology ^[7][19]. Julie Meilland, a researcher at Cerege in France who was not involved in the study, said "It was very refreshing to see these worlds connect because very often people doing more modern research don't necessarily connect to people doing deep-time research" ^[15][16].

Fifty-six million years is about a tenth of the roughly 560-million-year shell archive on the seafloor ^[2][3]. The thing this does not tell you is how evenly those published case studies sampled the world ocean, or whether the other species in the compilation hide the same split that Darling found in N. pachyderma ^[7][13]. The isotope and trace-element measurements that carry most reconstructions of past climate and ocean conditions are separate readings of the same shells ^[3].