So AGAIN I ask:
How does they know what ADAM's was to begin with?
"How does they know what ADAM's was to begin with?"
Again, the "Y-Chromosome Adam" is is the LAST shared ancestor of all males. Now, there could be an EARLIER shared ancestor (if the population looped back on itself, such as if a cousin married a counsin), but this shows the LAST person.
They can tell what "Y-Chromosome Adam's" DNA is because the male Y-Chromosome does not combine DNA with anything (it matches up against the X and does not swap DNA).
Because the Y-Chromosome is unmatched with a like Chromosome, the Y-Chromosome has passed largely unchanged through the ages, albeit with distint, and easy to spot changes.
By taking samples, of males throughout the world, they can compare the shared, unchanged, portions --- the DNA of the "Y-Chromosome Adam."
The only relevance here is to show that there was --- at least once ----- ONE MAN, per evolutionary theory ----- whom all living people are decended.
It's only relevance is to answer your concern that evolution is inconsistent with ORIGINAL SIN.
It's not inconsistent.
Indeed, if you read more, you'll find that evolutionary theory and the Bible work quite nicely together --- because --- it appear there were AT LEAST two shared male ancestors of all living humans.
Sounds a lot like Adam and Noah? Does it not?
"Speak truth in love." (Ephesians)