  We use the chemical evolution predictions of cosmological hydrodynamic
simulations with our latest theoretical stellar population synthesis,
photoionization and shock models to predict the strong line evolution of
ensembles of galaxies from z=3 to the present day. In this paper, we focus on
the brightest optical emission-line ratios, [NII]/H-alpha and [OIII]/H-beta. We
use the optical diagnostic Baldwin-Phillips-Terlevich (BPT) diagram as a tool
for investigating the spectral properties of ensembles of active galaxies. We
use four redshift windows chosen to exploit new near-infrared multi-object
spectrographs. We predict how the BPT diagram will appear in these four
redshift windows given different sets of assumptions. We show that the position
of star-forming galaxies on the BPT diagram traces the ISM conditions and
radiation field in galaxies at a given redshift. Galaxies containing AGN form a
mixing sequence with purely star-forming galaxies. This mixing sequence may
change dramatically with cosmic time, due to the metallicity sensitivity of the
optical emission-lines. Furthermore, the position of the mixing sequence may
probe metallicity gradients in galaxies as a function of redshift, depending on
the size of the AGN narrow line region. We apply our latest slow shock models
for gas shocked by galactic-scale winds. We show that at high redshift,
galactic wind shocks are clearly separated from AGN in line ratio space.
Instead, shocks from galactic winds mimic high metallicity starburst galaxies.
We discuss our models in the context of future large near-infrared
spectroscopic surveys.
