Take a deep breath: how oxygen shaped the Earth

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).

Oxygen levels were certainly much lower than today in Triassic-Jurassic times, estimated between 12 per cent to 15 per cent, and Prof McElwain wonders whether some plants harbour genetic traces of adaptation to this low oxygen world. This is of interest to Nasa, which tested plants in low-oxygen conditions in space to produce breathable oxygen. “We are basing our work on these space experiments; there really is nothing else out there,” says Prof McElwain. Her work may show some plants are better disposed to low oxygen.

High on oxygen
This summer Prof McElwain is running an experiment in her specialised UCD plant chambers with the atmosphere tuned to when animals and plants first moved on to land (400 million years ago). Oxygen levels were lower than today at 11-16 per cent, compared with our 21 per cent.

Later this year Prof McElwain will recreate the high oxygen of a period before dinosaurs, rewinding the dial to 300-275 million years ago. This is the Carboniferous, a time when Earth hosted tropical swamps that gave us coal deposits and air is believed to have held 30-35 per cent oxygen.

What lived in at this time was otherworldly, with lots of giant fern-like trees with three-metre leaves. Plants were not alone in this gigantism trend. “You have dragonflies with one-metre wingspans,” says Prof McElwain, as the raised oxygen levels allowed organisms take up the gas easier via diffusion. Prof McElwain’s experiments will run at 25 per cent oxygen for safety reasons, as raised oxygen poses a combustion hazard.

She is collaborating with geologist Prof Isabel Montanez, an expert on the Carboniferous at University College Davis, California. "We know that oxygen levels go up and up through this period, with rainforests diversifying," says Montanez. "Carbon-dioxide levels rose during this time from low values we had 100 years ago up to 900 to 1,100 parts per million, which is where we could be by the time we finish burning fossil fuels," she explains. She argues this ancient world deserves far more attention from climate change science, as it was similar to our own in having polar ice, which is unusual. What happened then should serve as a warning, she says.

The big ice sheets melted, and rainforests retreated, facilitating a domino effect on climate. A world that looked like ours changed. Vast deserts and acid lakes spread; the ice disappeared for 200 million years. Some argue this has no relevance to today, but the fundamental processes are the same, says Montanez.

The UCD scientist’s four experiments in time and air are funded via a prestigious ERC grant. See more about OXYEVOL at ucd.ie/plantpalaeo/oxyevol.html

FINE-FEATHERED FOSSILS
The secrets of the dinosaur layer
Understanding how animals fossilise will help locate feathered dinosaur fossils, says Dr Patrick Orr, fossil expert from University College Dublin. Orr has just returned from China, where he studied the distinctive conditions that allowed China's famous feathered dinosaurs and early birds be exceptionally preserved.

“There are these beautifully laminated mudstones that were deposited in a lake and, critically, were undisturbed by animals. They are dark and it seems that these places were low in oxygen, with no animals scavenging to break down tissue,” says Orr. This kind of work will help scientists zoom in on precise horizons which hold the treasure chest of more complete fossils, the so-called “bird layer” and “dinosaur layer” referred to by local excavators.

Irish scientists are also resurrecting fossil colours. Dr Maria McNamara of University College Cork, collaborating with Orr, has subjected insects and feathers to high temperatures and squashed them at high pressure to simulate the fossilisation process in the lab.

Colours are rarely preserved in fossils, so in the past scientists guessed what animals looked like. Recent discoveries of colour-producing structures in fossil insects and feathers are helping solve this mystery and learn about the evolution of colour and its role in communication in animals.

“Published reconstructions of fossil dinosaurs are probably not quite correct,” says McNamara. “If we want to reconstruct the colour of fossils feathers we need to understand more about fossilisation.”

In the case of dinosaur feathers, tiny fossilised granules in their fossils give clues as to their colours.

McNamara recently discovered that insect colour changes when fossilised, so that blue beetles were really a green to yellow colour in life. Her research also unearthed toxicity in a fossil moth for the first time: a 50-million-year-old moth with a striking colour pattern similar to their modern relatives with a liking for cyanide compounds. Her work was exhibited at the Royal Society summer science exhibition in London in July.

Palaeontologists are interested in what happens to organic matter in the geological record, but so are others. “We could use insect colour as an independent estimate of how sediments have aged,” she says. “That can tell us about the thermal history of sediments,” and that can be of interest to the oil and gas industry.

There are exceptionally preserved animals in Ireland too, though they tend to be marine or estuarine creatures, not dinosaurs. Deep within the coal fields of Kilkenny, tiny shrimp-like creatures have been found perfectly preserved, having lived in a swampy delta 315 million years ago, says Orr.