  Theories of evolving quintessence are constructed that generically lead to
deviations from the w = -1 prediction of non-evolving dark energy. The small
mass scale that governs evolution, m_\phi \approx 10^{-33} eV, is radiatively
stable, and the ``Why Now?'' problem is solved. These results rest crucially on
seesaw cosmology: in broad outline, fundamental physics and cosmology can be
understood from only two mass scales, the weak scale, v, and the Planck scale,
M. Requiring a scale of dark energy \rho_{DE}^{1/4} governed by v^2/M, and a
radiatively stable evolution rate m_\phi given by v^4/M^3, leads to a
distinctive form for the equation of state w(z) that follows from a cosine
quintessence potential. An explicit hidden axion model is constructed. Dark
energy resides in the potential of the axion field which is generated by a new
QCD-like force that gets strong at the scale \Lambda \approx v^2/M \approx
\rho_{DE}^{1/4}. The evolution rate is given by a second seesaw that leads to
the axion mass, m_\phi \approx \Lambda^2/f, with f \approx M.
