The oxygen on Earth has changed dramatically over time — here’s how our data can help us discover alien life
Are we alone in the universe? This is a question that has intrigued humans for centuries and inspired countless studies and works of fiction. But are we close to finding out? Now that the James Webb Space Telescope (JWST) is up and running, we may have taken a giant leap in our ability to answer this one day.
One of the JWST’s four main goals is to study exoplanets – planets outside our solar system – and to identify the gases that make up their atmosphere. Now, our new research on the variability of oxygen on Earth over geologic time has provided clues about what to really look for.
To try to understand how, when, and why life might evolve on other planets, it makes sense to look at the only planet we currently know of of any host for life: Earth. Understanding our planet’s complex evolutionary history may provide the key to finding other life-supporting planets.
life and oxygen
We know that animals require oxygen to survive, although some, like sponges, require less than others. However, while oxygen is readily available today, making up 21% of the atmosphere, we also know that this has not been true for the majority of Earth’s history.
If we travel to the depths of our past, some 450 million years later, we’ll need to carry a handy supply of oxygen tanks with us. But what we are less certain about is the absolute amount of oxygen in the atmosphere and oceans over time and whether increases in oxygen levels fueled the evolution of animal life, or vice versa. These questions have actually sparked many discussions and decades of research.
Current thinking is that oxygen levels rose in three broad steps. The first, called the Great Oxidation Event, happened about 2.4 billion years ago, transforming Earth from an oxygen-free planet in the atmosphere and oceans to one with oxygen as its permanent feature. The third occurred about 420 million years ago and is called the Paleozoic Oxygenation Event, which saw atmospheric oxygen rise to today’s levels.
But in between, about 800 million years ago, lies the second step: the “modern oxygenation event,” or NOE. Initially, information extracted from sedimentary rocks that formed on the ocean floor indicated that during this time the oxygen rose to what looked like modern levels.
However, more data collected since then points to a more interesting history of oxygen. Importantly, NOE occurred before the first animals appeared, which appeared about 600 million years ago.
Oxygen level modeling
We set out to explore and reconstruct atmospheric oxygen levels during NOE to find out the conditions under which the first animals appeared. To do this, we built a computer model of the Earth, integrating knowledge about the different processes that can bring or remove oxygen into the atmosphere.
We have studied carbon-bearing rocks, deposited all over the world, to calculate ancient photosynthetic rates. Photosynthesis is the process by which plants and microbes use sunlight, water, and carbon dioxide to produce oxygen and energy in the form of sugars – the main source of oxygen on Earth.
Carbon is naturally found in many isotopes – atoms that have a different number of neutrons in their nuclei (the nucleus consists of protons and neutrons). So the different isotopes have slightly different sizes and masses from each other.
We looked at carbon isotopes known as carbon-12 and carbon-13, which do not undergo radioactive decay. Plants prefer to use carbon-12 – the lightest isotope – during photosynthesis, leaving seawater and the rocks that form on the ocean floor rich in carbon-13 instead.
When we analyze these rocks, after millions or even billions of years, if we find more carbon-13 than carbon-12, we can expect more photosynthesis to occur, and therefore more oxygen production. Then we modeled volcanic activity, which can release gases that react with oxygen, removing it from the atmosphere.
This approach may seem a little strange, and you may ask why there was nothing more direct for us to measure. This is because most geological evidence from this time has not been preserved, and these carbon isotope ratios are one of the few well-defined data sets we have during this time period.
What we found is that, rather than a simple jump in oxygen levels during the Cenozoic Era, the amount of oxygen in the atmosphere changed dramatically, and on geologic time scales, very quickly. Whereas 750 million years ago oxygen made up 12% of the atmosphere, in a few tens of millions of years it fell to about 0.3% – a tiny fraction – before rising again a few million years later.
Our research shows that atmospheric oxygen may have continued this dance between high and low levels until plants gained a foothold on Earth about 450 million years ago.
Searching for a strange life
These results are interesting for a number of reasons. We have often thought that the relative stability that the Earth has experienced over the past 4.5 billion years is necessary for life to flourish. After all, when big events occurred, like asteroid collisions, they didn’t go well for some Earth dwellers (sorry, dinosaurs).
But if the first animals did indeed evolve against a background of highly variable oxygen levels, this suggests that some dynamic changes may instead be required in order to foster ecological innovation.
Our results suggest that periods of low atmospheric oxygen levels could be important for the development of more complex life by driving the extinction of some simple organisms and allowing survivors to expand and diversify when oxygen levels rise again. Therefore, we should not rule out taking a closer look at exoplanets with an oxygen-poor atmosphere.
Of course, this is a view centered on the Earth and even the animals. Alien life may be very different from life on Earth. For example, it can be found well on planetary bodies like Titan – one of Saturn’s moons – which contains seas of liquid methane and ethane. But as a starting point in our search for extraterrestrial life, understanding the history of atmospheric oxygen on Earth is a useful guide.
Written by Alex Krause, Research Fellow in Earth System Modeling, UCL; and Benjamin JW Mills, Associate Professor of Biogeochemical Modeling, University of Leeds.
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