Michael Collins was an astronaut on Gemini 10 and Apollo 11. As command module pilot for Apollo 11, he was alone while Armstrong and Aldrin were galavanting on the moon. He took this photo, and thus is not in it. Matter can not be created or destroyed. That means that every human that lived up to the point of this photo being taken still exists, at least in some form, and every human that has been born since then was also is in this photo, at least in some form. Except Michael Collins.
When you look at it like that this photo is pretty mindblowing.