Why this shift occurred is hard to say. Since the beginning describes all that was created in the Garden of Eden as good, including Adam and Eve, their genes may have been well-developed to the point that their kids could marry siblings and produce children with no genetic defects. This would fit the shift we see in Genesis from early generations with long lives producing children past their 100s to humans with shorter lives and shorter fertility rates. We go from Adam and Eve, who were still having children when they...