The oxygen in our atmosphere should be a telltale sign to any alien scientist that our planet harbors life. But our scientists might have more luck if we look for the gases that were present on Earth billions of years ago, Scientific American reports. Our planet was once dominated by methane-producing microorganisms, resulting in an unusual combination of methane and carbon dioxide in the atmosphere, which would not have arisen naturally. This mixture of gases may be a better signature of life than oxygen, which was fairly scarce for most of Earth’s history, the researchers argue this week in Science Advances. Given that new telescopes such as NASA’s James Webb Space Telescope could soon be used to study exoplanet atmospheres, scientists may be able to heed that advice in their quest to detect life.