What if some civilisation back then knew how to modify DNA?
In the old days we were living up to a thousand years, according to the Bible records. Now there is barely anyone reaching 120 years, max. Barely the time to grow up and make babies and it's already the end of the line.
If so, then methinks those who tampered with human genes wanted short-lived, obedient slaves or soldiers. And kept the females with the 1000-year DNA for themselves, but died in some accident. Research agrees we are all descending from the same, late on the line, woman.