According to scientists, in the very early universe, there were particles, which became hydrogen atoms after protons were developed. (It makes sense, right? Hydrogen in its usual phase has a single proton.) When stars were formed, nuclear fusion caused hydrogen to convert into helium. During the later phases of stars, carbon was also formed from radioactivity. To make it a short story, all materials in this universe- including the things that make up humans- are born from stars.
The substance that we are made up of is actually ancient: they have been through five stars. (In case you didn’t know, stars are constantly being created and destroyed.) I know that perhaps it appears that I’m just repeating what Carl Sagan said, in a much less sophisticated manner, but I swear I thought of this before I knew about Carl Sagan!