Take a deep breath: how oxygen shaped the Earth
The see-sawing oxygen content of air through history helped dinosaurs and mammals to thrive and contributed to mass extinctions
Natural disasters such as meteorite impacts or volcanoes have devastated life on Earth. But shifting oxygen levels have also had critical – yet overlooked – impacts, say scientists.
The oxygen content of air see-sawed through Earth’s history, directly assisting dinosaurs and mammals to have their day in the sun, according to new thinking. It also had a role in mass extinctions.
It is timely, then, that Prof Jennifer McElwain is to recreate snapshots from critical periods in Earth’s history in her plant growth chambers in UCD. This is the first time scientists have focused on oxygen in such experiments. Prof McElwain will help pin down levels of this gas at four critical evolutionary crossroads by growing plants at different oxygen levels and then comparing them to fossils.
“There is huge debate over oxygen levels. People have argued that the only reason mammals radiated and were successful is because oxygen concentrations have been increasing over the last 65 million years,” she says. “Some have argued birds are adapted to low oxygen and perhaps dinosaurs were also adapted to low oxygen conditions.”
While we struggle to breathe in low-oxygen conditions on high mountains, birds such as geese can exert themselves flying at more than 20,000ft.
Birds breathe more efficiently because they take in and stash air in hollowed-out bones, before sending it to their lungs. This saves space and allows an efficient two-way flow, whereas mammals operate a one-way breathing system – into lungs, then out. Birds are reckoned to be three times better at extracting oxygen than us at 5,000ft. They inherited this breathing system from their ancestors, the dinosaurs, which evolved on a low-oxygen Earth, according to palaeontologist Prof Peter Ward of the University of Washington, Seattle.
“The lowest oxygen that the planet has seen since animals first evolved takes place within a million years of the appearance of the first dinosaur,” he says. Fossilised dinosaur bones reveal they too breathed air like their bird relatives, adds Ward, and this offered advantages when competing with or preying on groups that did not have superefficient lungs.
If you have a lung system capable of living in 12 per cent oxygen, then when oxygen levels go up you can become larger, says Ward, and dinosaurs did scale up when levels rose. “So bigness in dinosaurs is a direct consequence of having evolved this supremely efficient lung system in low oxygen.”
The models are uncertain exactly how much oxygen there was during prehistory, especially during the Triassic and Jurassic dinosaur periods, and this is something Prof McElwain will address in her chamber experiments. She will recreate the air from this ancient world, growing relatives of its plants (living fossils).