Quanta Magazine · Science

Foram Shell Spiral Flips Linked to Evolutionary Events, Not Climate

Marine plankton called foraminifera (forams) exhibit a puzzling phenomenon: their spiral shells, usually coiled in one direction for millennia, suddenly and globally switch to the opposite direction, then back again. This has been observed in the fossil record for decades.

Scientists study foram shells, a vast archive of Earth's history, to reconstruct past climates. They noticed certain planktonic foram species predominantly coil one way, and this preference can change globally and simultaneously.

While early theories linked coiling direction to climate, recent research integrates genetic and fossil data spanning 56 million years. The new hypothesis suggests shell direction changes mark evolutionary events, not environmental adaptations.

The study proposes a new trait emerges in a subpopulation and sweeps across oceans due to a fitness advantage, bringing its dominant coiling direction with it.

This global sweep, like a dominant virus variant's spread, is preserved in the fossil record as a sudden, worldwide shift in shell chirality.

It remains unclear why only some foram species show this chiral bias, with others maintaining a 50-50 split. The exact triggers for these flips are still unknown.

What appears instantaneous in fossils may have taken thousands of years. This phenomenon offers a rare glimpse into how evolutionary processes spread globally, marked by the foram's shell chirality.

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