Why the one million light year boundary?
Because the majority of the mass of our bodies is from elements heavier than hydrogen, helium, and lithium. For example, pure water is only 11% hydrogen by weight. None of the heavier elements were formed in the big bang. On the other hand oxygen, nitrogen, and carbon are all formed in the cores of stars, but for the most part, those elements are locked up within stars that do not go super kablooey. Only supernovas distribute some of their guts out into the universe at the end of their lives.
For those reasons, it is interesting to talk about where the star dust in our bodies comes from and just how far it may have traveled between the time it was formed and the time it became incorporated into the terrestrial planets forming our particular solar system. You are correct that the hydrogen needs no such explanation, but some of it too is released from the dying stars and supernova.
Edited by NoNukes, : fix miscalculation.
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