The search for life on planets outside our solar system will use spectroscopic identification of atmospheric biosignatures. The most robust remotely detectable potential biosignature is considered to be the simultaneous detection of oxygen (O₂) and methane (CH₄) at sufficiently high levels. Life has been present on Earth for more than 3.8 billion years, yet the atmosphere contained no free oxygen until 2.3 billion years ago – and levels may well have been below spectroscopic detection limits until much later. In this talk, I will introduce the history of life and oxygen on Earth and highlight some of the critical steps in biological evolution that may be unique to Earth's biosphere. The record shows an apparent co-evolution in which bigger and more advanced organisms have evolved in concert with increasing oxygenation of Earth's atmosphere. By correlation, the search for life seems better suited to find planets with advanced, complex life than simple, unicellular biospheres reminiscent of the early Earth. This effect should lead to false negative detections of life beyond Earth.