Matter in the early Universe (103 < z < 1012) could have been extremely inhomogeneous. Photon diffusion and neutrino diffusion alter the density contrast of matter, which can lead to collapse or disruption of primordial clouds. Nucleosynthesis proceeds differently in dense regions, so observed abundances provide a clue to the lumpiness at T ∼ 109 K. Clouds which eventually collapse can form stars, but not one of the visible populations, because no choice of parameters yields correct C, N and O abundances. Neutrino diffusion effects limit the range of possible nuclear abundances in clouds which disperse and mix with visible matter: gas cannot emerge from the big bang with a higher metal content than metal-poor halo stars. A wide range of conditions at recombination is consistent with available evidence.