User blog comment:Supahbadmarine/Official Canon Policy/@comment-4608429-20130220004829/@comment-4608429-20130220112013

5. I'd say no for humans, but yes for Xenos (several alien species in 40k come from that time).

6. Hmm, not sure really. It is possible (Space Marines do fall to Chaos), but the circumstances need to be right to do so, so extensive explanation would be required.

Again, anything can be justified provided it has a good reason.