Phosphorus is a key element in biological systems, acting in cell replication as RNA and DNA, in cell structure as phospholipids, and in metabolism as ATP. Given its ubiquity in biochemistry, phosphorus was likely present in the origin or early evolution of life. I will discuss sources of phosphorus on the early earth, concentrating primarily on extraterrestrial sources of reduced oxidation state phosphorus compounds, and evidence that these sources were used by early biochemical systems. Additionally, I will show how these reduced oxidation state phosphorus compounds could act in prebiotic or early biochemical systems to generate both key biologic compounds and metabolic energy. This presentation is part of the Planets, Life, and the Universe: Astrobiology Lecture Series 2009/2010, for more information please visit the series website (http://astrobiology.stsci.edu/).