We have seen in preceding chapters that molecular lines are excellent tracers of interstellar gas, of star-forming regions, and of the interactions of stars on their environments in the Milky Way Galaxy and in external galaxies. Observations of molecular emissions, supported by detailed modelling, allow a rather complete physical description to be made of the regions where these molecules are located, even when the galaxies are not spatially resolved. But what about pregalactic situations in the Universe? These include some of the most active areas of research in modern astronomy. Did molecules have a role to play in pregalactic astronomy, and if so could molecular emissions help to trace processes occurring very early in the evolution of the Universe? When did molecular processes begin to play an important role? What are the best tracers of the first galaxies in the Universe?

In this chapter we show that molecules were likely to be present from the era of recombination after the Big Bang and certainly played an important role in the formation of protogalaxies and of the first stars. Whether molecules generated detectable signatures of those very early events is problematic, at least with our present range of astronomical instrumentation. However, it seems likely that we shall soon find molecular signatures of the post-recombination era. Once the first stars appeared and seeded their environments with metallicity, the formation of the first galaxies was modulated by molecules and it should certainly be possible to trace their formation using molecular emissions.