Is it possible that a "genetic mutation" ocurred altering the characteristics, and the genetically-driven causes of mortality, from that of our ancestors (Adam)?
Maybe "environmental pressures" created a change in the population related to these characteristics?
It is impossible that Adam was the ancestor of all humanity, if he lived in the Middle East a mere 6000 years ago. Humans have been in the New World at least 10,000 years, and probably longer. Aborigines have been Australia at least 40,000 years.