All terrestrial vertebrates owe their existence to that unlikely moment 350-odd million years ago when an ancestral species of fish hauled itself out of the water. But perhaps the real surprise is that it happened only once.
“Intuitively, we might think that moving from an aquatic to terrestrial lifestyle is very hard to do – hence the cliché ‘a fish out of water’,” said Terry Ord of Australia’s University of New South Wales.
But his survey suggests otherwise. Thirty-three families of modern fish contain species capable of moving and breathing on land. Among blennies alone, amphibiousness has evolved on seven separate occasions.
Why, then, haven’t any of these species that have adapted to a partially terrestrial life gone the whole hog, like their ancestor did all those millions of years ago?
One possibility is that preventing desiccation is such a massive challenge. Or perhaps the fact that one fish managed to conquer land made it harder for others to follow.
“Making a complete transition to land today could be more difficult, not because of the physical challenges, but because there are many other organisms already on land competing for resources,” said Ord.
Main image: Amphibious mudskippers are able to walk on land using their pectoral fins. © EarnestTse/iStock