(Since Steve Yegge's back blogging again, it kicked me just enough to take another crack at it :-)
This is in the "ridiculously and totally obvious" department, but still quite astounding when you really think about it.
Every single one of your ancestors lived long enough to either father a child or birth a child. Two parents, four grandparents, eight great-gandparents, sixteen..., etc. Every ancestor of yours, across the centuries and millenia parented a child.
Despite famine, disease, accident, war and every other depredation throughout history that killed off humans, your and my ancestors all survived "long enough".
The human race is tough.