A global drop in oxygen levels may have been the driver that led ancient fish to leave the water and evolve into the first air-breathing animals on land, suggests an Australian study.
The finding, published today in the Royal Society's journal Biology Letters, challenges the accepted wisdom about the environment from which tetrapods - the fish-like ancestors of land animals - first moved on to land.
Doctoral student Alice Clement, from the Australian National University's Research School of Earth Sciences, and Museum Victoria researcher Professor John Long, now based at the Natural History Museum of Los Angeles County, make the claim based on the fossilised remains of a fish that lived about 375 million years ago.
The new species of lungfish, known as Rhinodipterus, was found in the Gogo formation in northern Western Australia.
Clement says a number of features found in modern lungfish that are important to its air-gulping behaviour were found in the fossil.
These included a long mouth cavity and cranial ribs that are attached to the base of the skull.
She says in modern lungfish the longer mouth cavity enables them to hold a bubble of air in their mouths, while the cranial ribs anchor the pectoral girdle during air gulping.
Yet while modern lungfish exist in freshwater environments, the Rhinodipterus lived in the ocean.
"This runs counter to the standard theory that fish evolved the ability to breath air once they moved to freshwater habitats [because marine water is more oxygenated than freshwater]," says Clement.
The researchers suggest low global oxygen levels during this period, known as the Devonian, may explain the evolution of air-gulping characteristics.
Gulping for air
Previous studies have shown oxygen levels fell as low as 12% of the total atmosphere. Today global oxygen levels are about 20%.
Long believes this plunge in global oxygen levels would have been a strong selection driver on some animals, such as tetrapods, to become air-breathers.
"In previous work, scientists thought fishes invading freshwater habitat would encounter pockets of low oxygen water - due to rotting plants - so this was thought to be the main driver for breathing air," he says.
"Now, because we have found a fossil lungfish in a marine environment that is an air-breather, it shows that entering a freshwater environment wasn't the main cause, it was global oxygen levels."
"This makes us believe that breathing air arose twice at this early time in vertebrate evolution: once in lungfishes, and once in the fish lineage leading to land animals, and ultimately to us."
Clement says their study is another step in explaining the evolutionary progression from ocean to land.
"It is a piece of the puzzle that contributes to the story of life [that is] tied so closely to our past, as well as to when fish moved out of the water to a terrestrial environment."