A Billion-Year-Old Piece of Sky Locked Within Ancient Salt Crystals

Explore

BAccording to the fossil record, animals exploded onto the scene nearly 600 million years ago. Although it may seem like a long time, the Earth has already existed for almost 4 billion years. By examining the conditions that led to the evolution of animals, scientists better understand how complex life arose. In a recent study published in PNASgeoscientists have obtained the most precise snapshots ever of Earth’s early atmosphere during the Mesoproterozoic, and it has opened more questions than it answered.

Nicknamed the “boring billion,” the Mesoproterozoic covers the period from about 1.8 to 0.8 million years ago, when things appear to have been relatively static, without major physical or biological changes. Researchers at Rensselaer Polytechnic Institute (RPI) and Lakehead University in Ontario have obtained samples of rock salt crystals dating back 1.4 billion years. They analyzed pockets of fluids and gases trapped in halite rocks to determine the chemical composition of the atmosphere at the time.

“It’s an incredible feeling to open up an air sample that is a billion years older than the dinosaurs,” Justin Park, lead author of the study and an RPI graduate student, said in a statement.

Read more: »The bacteria that revolutionized the world»

The data showed that the Mesoproterozoic atmosphere was rich in oxygen, containing several times (3.7%) the current level of oxygen. Carbon dioxide was about 10 times more abundant than today, which would create a warm climate, even if the sun was young and not yet as hot. The study authors estimate that, with the sun at about 70 percent of its current strength, temperatures would have been around 88 degrees Fahrenheit.

So, if oxygen levels and climate were suitable for animal life, one might wonder why animals did not appear on Earth until hundreds of millions of years later. “[This data] may reflect a brief, transient oxygenation event during this long period that geologists jokingly call the ‘boring billion,'” Park explained.

On the other hand, note the authors of the study, red algae appeared during this period, adding oxygen to the atmosphere via photosynthesis. The high oxygen levels could reflect the fact that photosynthetic algae were diversifying and becoming more abundant, an insight into the role they still play in the global oxygen cycle today.

Thus, the tiny time capsules that preserved the ancient conditions of halite rocks provided evidence of the evolution of the atmosphere and life on Earth. Knowing what Earth looked like 1.4 billion years ago could also give us a better understanding of what to look for when searching for life on other planets.

Enjoy Nautilus? Subscribe for free to our newsletter.

Main image: Kuttelvaserova Stuchelova / Shutterstock

• Devin Reese

  Published on January 12, 2026

  Devin Reese is the editor-in-chief of Natural history and a science writer based in Alexandria, Virginia.