MRCA: Take every current person's family tree, who is the most recent person who is in all of them? This doesn't mean there was only one ancestor around at the

MRCA: Take every current person's family tree, who is the most recent person who is in all of them? This doesn't mean there was only one ancestor around at the time, though if all life came from the same cell that is of course true if you go back that far. IAP: For any given time X, every person at time X is either an ancestor of all current people, or none. Take everyone's family tree, and since it can't be size 2^x generations back, the overlaps eventually mean that all those people at time X who have a current descendent actually have everyone alive currently as their descendant, and everyone else at time X is the ancestor of nobody currently alive.