At any given moment in time, let n be the number of people who are an ancestor of everyone alive in 2021 (or 2022!). Right now, n = 0 because there is nobody o

At any given moment in time, let n be the number of people who are an ancestor of everyone alive in 2021 (or 2022!). Right now, n = 0 because there is nobody on earth that is the anscestor of everybody else. If you go back far enough, you'll get n = 1. That's a single person that is an anscestor for everyone alive today. That's the most recent common anscestor. Other people then were anscestors to only some people alive today. If you go back further, you'll get n = total number of people on the planet at the time. Everyone on earth is an anscestor of people alive today. Except, wait, that's not true! There will always be some people alive who aren't anscestors to anyone alive today. Maybe they simply didn't get lucky at all in their lifetime, or were infertile. Maybe a collection of people only bred within a community (tribe, city, etc) which then got wiped out in a war. Maybe something more complex happened. So n=1 is possible, but n=everyone never happened. So, totally ignore all the people who aren't anscestors to anyone in 2021, leaving only people that still have at least some anscestors today. Then n = 1 is (still) the most recent common anscestor. And the earlier date, n = everyone (that we care about) is the identical anscestors point.