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.
The right timing
Hundreds of millions of years ago, the Moon was much closer to Earth than it is now. Steven Balbus, an astrophysicist at the University of Oxford, UK, has explored how the Moon’s proximity to Earth might have affected its gravitational pull and influenced life on the planet. In 2014, he suggested that Earth’s tidal ranges would have been greater around the time the first four-limbed vertebrates, or tetrapods, appeared on land1.
Byrne and Green took that idea and simulated how far the tides would have run up at shores around the world, based on the arrangements of the continents at the time and the shapes and seawater depths of their coastlines. They studied two time periods: 430 million years ago, around the time the first animal lungs evolved, and 400 million years ago, roughly the time of the first known land tetrapods.
For both periods, the team found large variations in many locations for both the twice-daily and fortnightly tides. The two-week tidal cycles, which are generated as the Moon orbits Earth, are important because they control how long a fish might be stranded, Green says. If a fish got deposited in a rocky pool at the highest possible tide, it might have taken another 14 days before water returned to wash it out. “That’s a long time to be stuck,” Green says.
The team speculates that fish that could have made their way out of the tide pool, and back to the water, would have been more likely to survive. Fossils of some of the earliest known terrestrial tetrapods, such as the Tiktaalik lobe-finned fish from Canada's Ellesmere Island and trackways in Poland's Holy Cross mountains, have been found in places that had these high tidal variations.
Some researchers are sceptical about the idea, however. “It’s only one of a plethora of ideas for the origin of land-dwelling tetrapods, any or all of which may have been a part of the answer,” says Jennifer Clack, a palaeontologist at the University of Cambridge, UK.
Matthew Huber, a palaeoclimate modeller at Purdue University in West Lafayette, Indiana, would like to see more evidence that the correlation between the timing of large tidal ranges and the evolution of animals that walked on land isn’t a coincidence. But the work is intriguing, he says: “The connection seems worth pursuing.”