Ocean tides could have driven ancient fish to walk

By Alexandra Witze

Tides that left fish high and dry hundreds of millions of years ago could have kick-started the evolution of land-walking vertebrates.

New calculations suggest that, around 400 million years ago, many coastlines experienced two-week tidal cycles that varied in height by four metres or more. Such a huge range could have stranded fish in tidal pools for a couple of weeks. Only the ones with fins strong enough to muscle themselves out would have been able to journey back into the ocean and survive. Fossil evidence for the earliest known land vertebrates comes from places that had such wide tidal ranges.

Hannah Byrne, who led the work while at Bangor University, UK, and is now a doctoral student at Uppsala University in Sweden, reported the findings on 15 February at the Ocean Sciences meeting in Portland, Oregon.

The idea that the first land-walking animals could have evolved from those stranded in tide pools is generally well accepted and dates back decades. “What we’re suggesting is the actual driver of why the pools formed and why they were drying out,” says team member Mattias Green, an oceanographer at Bangor University.

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