The first nucleosynthesis is believed to have taken place during the Big Bang Nucleosynthesis, which occurred approximately 3 minutes after the Big Bang.

During this brief period, the universe was incredibly hot and dense, allowing protons and neutrons to collide and fuse into the first light nuclei, primarily hydrogen (¹H) and helium (⁴He). Trace amounts of deuterium (²H), tritium (³H), and lithium (⁷Li) were also created.

This event is crucial because it laid the foundation for the formation of stars, galaxies, and eventually, all the elements we see today.