They had breathing space. Australian researchers have found 60 sand-sized flecks embedded in limestone that are about a billion years older than other spacedust samples. The virtual time capsule reveals a surprising clue about the chemical evolution of Earth’s upper atmosphere: It was about 20 percent oxygen, like today. Scientists had assumed it wouldn’t contain much of the gas, since the lower atmosphere was virtually oxygen-free at that time. More study is needed to figure out Earth’s multilayered prehistoric atmosphere, which could inform the search for life on other planets.