Sydney: A massive plunge in global oxygen levels, not freshwater frolicking, could have led to the rise of air-breathing animals, argues a new study.
Researchers made the claim after analyzing the fossilized remains of a new lungfish species from Gogo in northern Western Australia that lived roughly 375 million years ago.
Doctoral candidate Alice Clement from Australian National University (ANU) and John Long, its adjunct professor, now based at the Natural History Museum in US, have just conducted this study.
It linked low global oxygen levels in the mid-Devonian period with the fossil Rhinodipterus, a marine lungfish species discovered in 2008 that is believed to be the first of its kind.
"The Rhinodipterus specimen has a number of features that suggest it was air breathing, including a long mouth cavity and articulations of its cranial ribs, which are important in the living forms of lungfish air-gulping behaviour," said Clement from the Research School of Earth Sciences at ANU.
"Yet Rhinodipterus lived in the ocean, not in freshwater, which runs counter to the standard theory that fish evolved the ability to breathe air once they moved to freshwater habitats."
In order to explain the existence of air-breathing adaptations in a marine lungfish, researchers looked to environmental factors other than habitat.
They turned to existing knowledge about global oxygen levels in the Devonian, which fell to as low as 12 percent of the total atmosphere. Today oxygen levels are around 20 percent.
"This plunge in global oxygen levels would have been a strong selection pressure on lungfish and other animals, including the tetrapods - the fish-like ancestors of land animals," explains long.
"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."
The researchers say this discovery adds an important piece to the puzzle of how life on Earth evolved. The next step will entail searching for more specimens similar to the Rhinodipterus in order to bolster the theory, said an ANU release.