Look at the very short section titled Big Bang nucleosynthesis on https://en.wikipedia.org/wiki/Cosmic_neutrino_background https://en.wikipedia.org/wiki/Cosmic_

Look at the very short section titled Big Bang nucleosynthesis on https://en.wikipedia.org/wiki/Cosmic_neutrino_background https://en.wikipedia.org/wiki/Cosmic_neutrino_background. Based only on the relative abundance of He-4 and D (H-2), one gets an estimate of 3.14 for the number of neutrino species. You can find more detail on http://darkuniverse.uni-hd.de/pub/Main/WinterSchool08Slides/CosmologicalNeutrinos.pdf http://darkuniverse.uni-hd.de/pub/Main/WinterSchool08Slides/.... Basically, the more light neutrino species there are, the faster the expansion occurred in the early universe, and the less time there is for making other nucleus from protons and neutrons. After a while, the matter is to diluted for fusion to occur and the ratio of various nuclei is "frozen" .... until stars are formed much later on.