Atheists: What is the source of "I"?

For eons prior to your birth you did not exist (according to most of you). Then for a span of less than a century there exists what you call "I" or "me", a sense of self-identity that is obliterated at the time of bodily death (according to most of you). Where did this sense of "I" originate? What is it's source? Since your sense of self came into existence out of non-existence and then will return to non-existence, can this happen again?