So, I was thinking. If Edward was forced to make a homunculus, he would have to add Al's soul to that body. The only way we've seen that so far is through the blood seals on the armor.
Which would mean that, to affix a soul to a homunculus body, you'd need a seal. And all of the Sins sport those nifty seals. So, that makes sense...Envy (rather painfully) correcting Ed in saying they were born. They were once people, the homunculus bodies were made, and the souls were attached through the seal.
Which would also mean that the so-called perfect humans have a very visible weak spot. Destroy the seal on the body and the body would be lifeless, right?
Just a weird thought. I'm probably way off (usually am @_@).