(Inside Science) -- Terrestrial animals may owe a special debt to the sun and the moon. It may have been their combined pull on ancient Earth's oceans that helped primitive air-breathing fish gain a toehold on land, new research suggests.
In a new study, published in the journal Proceedings of the Royal Society A, physicist Steven Balbus argues that the gravitational forces generated by the sun and moon would have been conducive to the formation of a vast network of isolated tidal pools during the Devonian Period, between 420 to 360 million years ago, when fish-like vertebrates first clambered out of the sea.
"By the end of the Devonian, there were vertebrates that were quite at home moving around on land," said Balbus, who is at the University of Oxford in the United Kingdom.
According to Balbus, a rather remarkable confluence of cosmic, geological and biological events occurred during the Devonian period that helped jump-start life on land. First, when viewed from the Earth, the sun and the moon appeared to be almost the same size, as is true today. This is called having the same angular diameter.
"The sun is much bigger than the moon, but it's also much farther away, so the two bodies look to be about the same size to us. This is extraordinary," Balbus said. He added that it's very unusual for an Earth-sized planet to have such a large moon.
One consequence of this arrangement is that the "tidal force" of the sun and the moon on our planet are similar. The tidal force is a side effect of the force of gravity and is responsible for ocean tides. Because the Earth is a sphere, gravity doesn't act equally on all parts of it.
"The part of the surface that's closer to the sun is pulled a little bit more strongly, and the part that's farther away is pulled a little less strongly," said Balbus. "The same thing happens with the moon."
Because the tidal forces of the sun and moon are similar, the timing and size of Earth's ocean tides can change depending on whether the solar and lunar tidal forces are opposed to one another or acting in synchrony. The variety of ocean tide patterns generated by the sun and the moon would have been especially noticeable during the Devonian due to the arrangement of our Earth's continents, Balbus said.
"The positions of Earth’s continents have changed over time because of [plate tectonics]. They had a rather special orientation in the Devonian," explained Balbus.
Watery safe havens
Specifically, there were only two "supercontinents," Gondwana and Euramerica, at the time, and the two land masses were separated by an expanse of water known as the Rheic Ocean. The Rheic Ocean had a unique tapered shape, so that its eastern side was narrower and shallower than its western end, which would be conducive to the formation of large tides.
"This would have helped create a very extensive and very complex network of inland tidal pools," Balbus said.
The final part of Balbus' proposal is biological. The Devonian period also happened to be when scientists think stubby-legged fish with primitive air-breathing lungs known as tetrapods first ventured onto land. If strong tides stranded animals in shallow pools, they could be trapped and perish if unable to scramble to larger bodies of water.
If, as Balbus suggests, tidal pools were plentiful during the Devonian, early tetrapods that were mobile would have had an easier time surviving out of the water because they could have dipped back into one of the many refuges scattered across the landscape.
From there, it would have been a relatively short leap to a full-time terrestrial lifestyle, Balbus said.
A bold idea
Per Alhberg, an evolutionary biologist at Uppsala University in Sweden, praised Balbus's original thinking.
"I love the boldness of this paper ... and the way Steven has dared to link together wildly different scientific disciplines in an attempt to understand a unique evolutionary event," said Alhberg, who was not involved in the study.
Alhberg also thinks the scenario envisioned by Balbus is very plausible.
"There is direct evidence that many of the earliest tetrapods and their fish ancestors lived in deltaic or marginal marine environments, so one way or another they must have passed through the tidal zone on the way from water to land," Alhberg said. "It could hardly have failed to have an effect on them, and we know that there are plenty of animals today that exploit the intertidal environment and live partly in, partly out of, the water."
Balbus said that developing his theory has made him skeptical of the notion that complex terrestrial life might be common in the universe. "A lot of things had to come together in a strange way on the Earth," he added.
Alhberg, on the other hand, thinks alien life could still be plentiful, but that its makeup might be different from Earth's.
"An Earth-like planet without a moon might have a rich diversity of life in the oceans but rather simple microorganism-dominated ecosystems on land," he said.
Ker Than is a freelance writer living in the Bay Area. He tweets at @kerthan.