Why O2 Is Required by Complex Life on Habitable Planets and the Concept of Planetary "Oxygenation Time" Catling, David C. ; Glein, Christopher R. ; Zahnle, Kevin J. ; McKay, Christopher P. Abstract Publication: Astrobiology Pub Date: June 2005 DOI: 10.1089/ast.2005.5.415 Bibcode: 2005AsBio...5..415C