 The end of neutrino decoupling marked the beginning of nucleosynthesis. The universe was around two seconds old, with five protons for each neutron. Here we track the predicted ratios of each kind of nuclei as these processes evolved. At the start, there was sufficient energy for protons and the remaining neutrons to combine into deuterons, the nucleus of deuterium. As the deuterium density climbed, significant amounts of helium and hydrogen isotopes were formed along with lots of tightly bound hydrogen. This diagram illustrates the various nuclear interactions along the way. Hydrogen resists combinations with protons and neutrons, so the creation of beryllium and lithium went more slowly, producing far less of these elements than helium. By the time the temperature of the universe had fallen to 300 million degrees, when it was around 1,000 seconds old, the nucleosynthesis energies were no longer available and the epoch was over. The vast majority of the baryonic matter wound up in the form of hydrogen and helium nuclei. The percentages of deuterium and tritium were much smaller. There were just traces of beryllium and lithium and the remaining free neutrons decayed over time into protons. The best way to measure the primordial percentages of these elements is to look at the spectra of distant quasars like this one. The actual amounts of hydrogen, helium, beryllium and lithium in the early universe does match the predicted levels. This represents significant evidence that the Big Bang Nucleosynthesis process did occur.