Wes Traub, of the Jet Propulsion Laboratory in California, reminded the AAS audience today of how an Earth-centric view can help us understand other worlds. Somehow I missed this back in September when it came out – but he and colleagues have done a nifty piece of work looking at Earth’s evolution as seen from space. In other words, how has Earth’s atmosphere changed composition through time? More specifically, how would aliens looking at Earth interpret life on our planet depending on when they spotted us?