The genes book I mention in the post talks about Y-Chromosomal Adam and Mitochondrial Eve. Super cool stuff. The directed acyclic graph nature of lineage totally makes sense why the number of ancestors is smaller in reality than the theoretical numbers. I don't follow how Adam and Eve relate, though. I know this example is crude but bear with me; hopefully it's valid enough to have discussion around. If we follow the lineages back to the point where we have 1000 ancestors each, it seems like Adam just represents the 1 we all have in common, and by extension the other 999 are the ones not in common.
From the Wikipedia article:
> Although the informal name "Y-chromosomal Adam" is a reference to the biblical Adam, this should not be misconstrued as implying that the bearer of the chromosome was the only human male alive during his time.[7] His other male contemporaries may also have descendants alive today, but not, by definition, through solely patrilineal descent; in other words, none of them have an unbroken male line of descendants (son's son's son's … son) connecting them to currently living people.
(Reading the Eve article left me with even more questions than answers, but I'm still getting the sense that Adam and Eve are orthogonal to the "number of ancestors in 1600" topic.)