The key to the evolutionary success of fish – and their possible survival in future – may lie with a molecule that they ultimately bequeathed to humans: hemoglobin, the precious carrier of oxygen into our brain, heart, muscles and other organs.

In a paper in the latest edition of the journal Science, Dr Jodie Rummer of the ARC Centre of Excellence for Coral Reef Studies and colleagues from the University of British Columbia report a groundbreaking discovery about how fish manage to survive in hostile water conditions.

“Four hundred million years ago the oceans were not what they are today. They were low in oxygen, high in CO2 and acidic,” says Dr Rummer. “Yet the fishes not only survived in these unpromising circumstances, they managed to thrive. Their secret weapon was a system for unloading huge amounts of oxygen from the hemoglobin in their blood, whenever the going got really tough. 

“Hemoglobin in the blood takes up oxygen in the gills of fish and the lungs of humans. It then carries it round the body to the heart, muscles and organs until it encounters tissues that are highly active and producing a lot of CO2.” “The acid is a signal to the hemoglobin to unload as much of its oxygen as possible into the tissues,” she explains.