Well, it’s one of those things the multi earth-age-ers believe. Essentially, that there was an earth age prior to this one that was wiped out. That people (? Whatever they were called) living then didn’t have physical bodies, etc. One reason they haven’t and won’t find physical remains of them and such.
Nothing to do with Adam though.
I think that a belief and faith in God is more important than the little things that people don't understand or agree on. I'm sure he'll answer all these things for us someday.