Earth Breathed Oxygen Billions of Years Before Atmospheric Rise

• Scientists have found evidence that oxygen was reaching deep into the Earth's mantle billions of years before it built up in Earth's atmosphere

• The discovery, from rocks up to 3.4 billion years old, is the most complete timeline yet of how Earth's early oxygen and deep interior evolved together

• The findings bring us a step closer to answering one of science's biggest questions: what made Earth capable of supporting life

Why is there life on Earth? It's one of the oldest questions humans have asked and as far as we know, our planet is the only one that holds the answer.

Earth is also the only planet we know of with plate tectonics, the slow-motion churn of its outer shell that recycles rock, reshapes continents, and cycles the nutrients that could feed life.

A new international study, published in Nature Geoscience and led by Dr Hugo Moreira , Royal Society University Research Fellow at the University of Portsmouth, is helping us understand the origins of life better by tracing the deep history of Earth's oxygen back billions of years, to a time when the planet, and its atmosphere, looked nothing like they do today.

A time capsule in a crystal

To look that far back, Dr Moreira's team turned to one of the toughest minerals on Earth known as zircon. These tiny crystals can survive largely unchanged for billions of years, and using lasers at the University's Mass Spectrometry and Laser Ablation Lab, scientists can date them with remarkable precision - in this study, pinpointing some samples to exactly 3.4 billion years old.

Some of these zircons contain a second, smaller mineral called apatite, trapped inside like a fly in amber. Because it's sealed inside the tougher zircon, this apatite is protected from change, meaning it's exactly the same age as its host.

These minerals are really exciting to scientists, as they are essentially time capsules."

Dr Hugo Moreira, Royal Society University Research Fellow at the University of Portsmouth

"These minerals are really exciting to scientists, as they are essentially time capsules," said Dr Moreira from the University's Institute of the Earth and Environment . "Because they're so well protected, we can analyse their chemical fingerprints to get an untouched snapshot of a moment billions of years in the past. We're looking into the early Earth, when it was a very different planet, and figuring out what we can still see of it today."

From a Portsmouth lab to a giant X-ray machine in France

The team collaborated with an international network of researchers and investigated ancient rock samples from around the world - including Australia, Finland, South Africa, Brazil and Scotland - which were sent to Portsmouth for initial analysis.

The samples were then taken to the European Synchrotron Radiation Facility (ESRF) in Grenoble , France, one of the most powerful X-ray facilities in the world, where researchers used a specialised technique to measure the chemical state of sulphur trapped inside each tiny apatite crystal.

The form that sulphur takes acts as a highly sensitive chemical fingerprint. It reveals how much oxygen was present when the crystal formed, at the point where the planet's outer shell is dragged down and recycled into its interior - a process called subduction.

"These crystals are tiny, often narrower than a human hair, and they contain only small amounts of sulphur," explained co-author Marine Cotte , the scientist in charge of ID21, the ESRF beamline where the measurements were carried out.

"So, we needed an X-ray beam that was very fine, very sensitive and fast enough to measure lots of crystals. That way, we could be confident the results reflect the bigger picture and not just one or two samples."

Earth's first breath

For most of Earth's early history, there was almost no oxygen in the air or oceans. That changed permanently around 2.3 billion years ago, in what's known as the Great Oxidation Event. This is often described as the moment Earth "started breathing".

Scientists have also long suspected there were brief, localised bursts of oxygen even earlier - small oxygen oases appearing between 3 and 3.5 billion years ago, at a time when there was barely any life on Earth to produce it.

The sulphur trapped in these ancient crystals shows that Earth's deep mantle was already responding to oxygen from the surface back then. The mantle is the thick, mostly solid middle layer of Earth located between the crust and the outer core.

The chemical signature of oxygen in the mantle layers affected by recycling increased between 3.4 and 3 billion years ago and stayed consistently present for the whole 3.4-billion-year period studied, reaching today's levels by around 2.1 billion years ago.

This suggests that some form of subduction - the process constantly recycling surface material into the deep Earth - was already carrying oxidised material downward as the atmosphere itself was changing above. In other words, the deep mantle world and the surface world were connected, and reacting to each other, far earlier than previously realised.

It's also the first-time scientists have used the same type of rock across this entire timespan to study the question, rather than comparing different rock types from different periods.

Dr Moreira explained why this is important: "Using the same rock type throughout gives us a much more robust result.

"We're comparing like with like across billions of years, rather than trying to bridge the gap between different kinds of evidence."

The bigger picture

Dr Moreira recognises that the research won't change anyone's daily life any time soon. Instead, it's driven by some of the oldest questions humans have asked.

"This isn't about changing how we live tomorrow," he said. "It's about answering the philosophical questions that are in our nature as human beings - where did we come from, and why are we here? It's curiosity-driven research, coming from a simple drive to know things. It won't affect a person on the street immediately, but it may help humankind in its own understanding of itself as a species.

"I would argue that plate tectonics has played a major role not only in the emergence of life, but in its continued development. We're trying to trace back to when life was made possible, and when this system was introduced to our planet, to understand the origins of a habitable Earth."

The study is a collaboration between the University of Portsmouth, Université de Bretagne Occidentale, European Synchrotron Radiation Facility, Université de Montpellier, and Geo-Ocean CNRS Ifremer.

It builds on Dr Moreira's Royal Society University Research Fellowship, worth £1.83 million over eight years , awarded in 2024 to investigate the origins of plate tectonics and its links to the emergence of life on Earth, as well as on the cutting-edge facilities at the University of Portsmouth.

The research follows earlier work published in Nature Geoscience in 2023, which first revealed a connection between Earth's ancient atmosphere and deep-Earth chemistry across the Great Oxidation Event.