Dirac wrote it in 1928 so an electron could obey both quantum mechanics and special relativity; the γ matrices mix the components precisely as Lorentz transformations demand, while the mass term couples the two chiralities.
A first-order equation for a four-component spinor that simultaneously carries energy, momentum, spin and the seed of antimatter.
Gauged, the same compact line becomes the starting point of quantum electrodynamics and, with the appropriate internal symmetries, of the weak and strong interactions as well.