Evan studied physiological sciences at Wadham College, Oxford, and then medicine at the Oxford University Clinical School . He began his medical career at the Royal Liverpool University Hospital in 1991 as a House Officer, then went on to the John Radcliffe Hospital, Oxford, becoming a senior house officer in 1992.

From 1992-994 Evan served as a local BMA representative and negotiator and proceeded to represent doctors in the Region on the National Council of the BMA. In 1994 he became a specialist registrar in public health with the Oxfordshire Health Authority where he remained for three years.

Evan as elected in 1997 as the MP for Oxford West and Abingdon and held the positions of Liberal Democrats' Shadow Science Minister and Shadow Secretary of State for Health. During his time in Parliament he was on the House of Commons Innovation University Science and Skills Select Committee and the Joint Commons/Lords Select Committee on Human Rights.

As a well known campaigner for evidence based policy, Evan contributes to the science policy blog for The Guardian.