Think the Earth has always enjoyed its lush, breathable atmosphere? Think again. According to a report from Science, a new study from scientists at the University of Washington reveals that the atmosphere on Earth was once only half as thick as it is today.

Air pressure is one of the key factors influencing the makeup of the atmosphere. It helps lock in water vapor and solar energy, both crucial components for life to survive here on Earth. Life requires a very specific atmospheric makeup to survive on Earth today, but researchers know relatively little about the ancient atmosphere and how life interacted with it.

Scientists analyzed gas bubbles locked in ancient lava samples to get a picture of what the atmosphere looked like some 2.7 billion years ago. Taking samples from basalt lavas that oozed out of the center of the earth over an area covering thousands of square kilometers, scientists examined the dissolved gases trapped inside the lava samples.

Air trapped inside the lava as it cooled can help scientists determine the air pressure of the times. According to University of Washington geologist Roger Buick, the team measured the size of the bubbles on the top of the lava sample and compared them with the size of the bubbles on the bottom. The bubbles at the top only bore the weight of the atmosphere pressing down, and the bubbles at the bottom had additional pressure acting on them from the weight of the rock itself. By subtracting the effect of the rock’s weight, scientists determined the approximate air pressure of the atmosphere 2.7 billion years ago.

The study offers some fascinating insights into the ancient atmosphere and how the lower air pressure could have affected key chemical processes necessary for the development of life. One key puzzle remains, however – scientists are still unsure how the Earth maintained its habitable temperature despite the lack of air pressure, and thus the lack of heat-trapping forces of the greenhouse effect.

“It’s really bold, really ambitious, and fraught with difficulties,” says geologist Dork Sahagian. “But you’ve got to try it. It’s as good a proxy of the pressure as you can hope to find.”

A press release from the University of Washington describing the details of the study can be found here.