I am, myself, of Ashkenazi stock : my paternal Great grandfather’s name was Nathan Cohen.
Jewishness passes down through the female – NOT the male.
With the Diaspora circa 70 AD, my understanding is that it was the Jewish men who were driven out into mainly eastern europe, whilst the Jewish women were left behind to be raped by the Romans and/or inter-marry with Palestinians.
The wandering Jewish men were forced to marry non-jewish women and convert them.
If this is the case wasn’t True Judaism lost way back in 70 AD.
The answer would seem to lie in a massive program of DNA analysis.