@Swamidass's discussion of this Genealogical concept takes some getting used to!
Several studies have shown that All the People Alive, some 3000 or 4000 years ago, are divided into just two groups:
(A) Those whose genealogical lineage ran into sufficient blockage to descent, that not a single one person alive today descends from the (A) Group of ancient humanity. Not too surprisingly, the more time elapses, the more likely that a lineage will be cut short by some combination of bad circumstances and global calamity!
(B) The other group, (B) Group, is the ancestor of not just some of the people alive today, but all of the people alive today.
How is this possible? There are two reasons:
1) There are no places on Earth that doesn't have some people leaving or some people arriving. And given enough time, even the tiniest factor of immigration and emigration triggers a tumbling waterfall of genealogical expansion into any territory inhabited by humans.
2) Secondly, the mathematical progression tells us that within a thousand years, Charlemagne should have more descendants than the number of people who have ever been born! Obviously, this is too many descendants to be considered credible. The reason nobody ever gets that many descendants is because of the inevitability of distant cousins to eventually find each other and pair up!
Each time distant cousins mate, a section of Genealogical Chart collapses in on itself.... instead of having "X number" of ancestors in the 20th generation, you now only have "X-1" of ancestors in the 20th generation. And this "collapsing" house of cards extends backwards in time, geometrically expanding with each generation, all the way back to Africa!
Indeed, this inevitability is repeated many times in any one person's genealogical chart. Mathematical projections going back into the mists of pre-history have measured the approximate time when someone's genealogical embrace of ancestors exceeds the actual number of people estimated to be alive in, say, 3000 BCE - - or 3500 BCE - - or 4000 BCE...
This creates the mathematical certainty that everyone alive in that ancient slice of time, say 3200 BCE, is the ultimate source of all people alive today - - once you eliminate those individuals who were unable to bring a single descendant of theirs into the modern period.
Now, @Richard_Wright1, back to your question, how is it possible regarding Adam & Eve?
All Adam and Eve had to accomplish was to get a single descendant down to, say, 3200 BCE, and by God's will, not into one of the lineages that was inevitably going to go extinct.
In so doing, Adam & Eve, as well as hundreds if not thousands of other pairs, become the inevitable ancestor of all humanity alive just before the birth of Jesus.
A) Adam & Eve are not unique in being the ancestor of all humans alive today. There are hundreds of other pairs that can lay claim to the same honor - - if we knew their genealogical charts.
B) Adam & Eve's descendants would have been guided by God (knowingly or not) to avoid the genealogical dead-ends.
C) It only takes a very small amount of immigration/emigration into the world's most isolated regions to accomplish the Universality of the genealogical expansion.
D) Some famous people of history were genealogical failures -- even if they had many children and grandchildren while they were still alive. Somewhere down the tree ... every descendant became blocked by early death, poor fertility, or global catastrophe.