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,
[N ii]/Hα and [O iii]/Hβ. 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 interstellar medium conditions and radiation field in galaxies at a given redshift. Galaxies
containing active galactic nucleus (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.