Final answer:
Hydrogen was the first element formed right after the Big Bang, followed by helium and a small amount of lithium during a period known as Big Bang nucleosynthesis. The heavier elements were created in the cores of stars or during supernovae much later in the universe's history. The CMB is evidence of the universe's early state when neutral hydrogen atoms first formed.
Step-by-step explanation:
The original element formed moments after the Big Bang was hydrogen. After that, the processes that occurred in the early universe allowed for the fusion of hydrogen nuclei into helium and a small amount of lithium. This period of nucleosynthesis occurred within a few hundred seconds of the Big Bang. Heavier elements were created much later in the cores of stars and during supernova explosions.
During the first few minutes after the Big Bang, conditions were ripe for nuclear fusion due to the extremely high temperatures. Protons and neutrons combined to form deuterium (a stable isotope of hydrogen), which then fused into helium. Only about 5% of the current universe's ordinary matter was created during this brief period of Big Bang nucleosynthesis.
As the universe cooled and expanded, fusion became less viable and only the fusion within stars continued the process of creating heavier elements. The Cosmic Microwave Background (CMB) radiation that we observe today is a remnant from the time when the universe cooled enough for neutral hydrogen atoms to form, making the universe transparent to radiation again.