This article presents a theoretical framework for the evolution of light element nuclides in the Galactic disc. The authors use two key assumptions: infall of metal-poor gas to the disc at an increasing rate, and destruction of fragile nuclides in hot relatively dense supergiant envelopes. Light nuclides are accelerated by supernova shocks, and many are confined to hot interstellar zones by magnetic fields. Their repeated passage through the hot envelopes causes depletion which peaked during the star main forming phase of galaxy evolution around z ≍ 1 (Hubble Deep Field). This mechanism reduced the D/H abundance from its primordial value of ≅2×10-4 to its Solar System value of ≅2.5×10-5, and subsequently more gradually to the current ISM value of 1.5×10-5. The model accounts well for Solar System and the current ratios of 7Li/6Li and 11B/10B. It fits extremely well an SBBN model with baryon density ≅0.05.