In Romans 5 we are plainly told in verse 19 it is through one man by birth...I would support it a bit more ...but its is a simple answer it would seem compaired to other questions I have seen on here so far.
Judging only from the title it sounds like it was a truth question, which is something we don't do around here. Your answer may seem simple enough to you but with it you are representing what you think in the truth, not some aspect of Christianity as a religious entity. Questions that are not about Christianity –and instead are about the spiritual issues that Christianity is about– are often closed (as often as we spot them).
I think the question deals with the concept of original sin, which may be covered already, since that is a very popular topic among Western Christians. You may want to look into the origin, history, and evolution of original sin, and if you are not clear on any theological concepts, you may ask here. Consider this Stack Exchange a Question-and-Answer website for experts, by experts.
I asked that question but it seems it is not suited for Christianity.SE. I asked again in Hermeneutics.SE. You may try to answer it there.