As for “slavery” and the impact on the Italian genome…where to begin? For one thing, slaves were shipped to every part of the empire. For another, I always find it interesting that people who raise this issue often seem to focus only on slaves from the Middle East or North Africa, when so many thousands of Celts and “Germani” were enslaved by the Romans. I’ve seen an estimate that one third of the population of Gaul was enslaved during the Gallic Wars. That may be an exaggeration, of course, but it’s pretty clear that Caesar’s fortune was mainly derived from the sale of Gaulish slaves.
Regardless of where the Roman slaves came from, if the latest IBD studies are correct, the Italian genome has experienced minimum inflow from other groups since around 500 B.C. (See Ralph and Coop et al), so it doesn’t seem that these Roman slaves, or the slaves bought in the Crimea during the medieval era, for that matter, had all that much influence. I’d be more than interested in knowingthe specific y dna sub clades that can be precisely pinpointed as “slave” lineages, versus, say, Roman legionnaires recruited in far flung parts of the empire, including, of course, Gaul and Germania.
It appears that perhaps laboring in the galleys or the mines or as virtual farm animals on vast latifundias didn’t leave much time for procreating. There were women slaves also, of course, but it seems that many ended up in brothels…the number of such establishments in a small town like Pompeii is rather astounding…and, as some recent discoveries around a brothel in Britain show, the progeny of slaves were not exactly valued. Slavery is a brutal, inhuman business, no matter who is the master.
As for the fact that our written records of these peoples are by either Romans or Greeks, it could hardly be otherwise; the Gauls and Germani were illiterate. For an analysis of the interaction between the Gauls and the Romans, a book that will soon be available from the Cambridge Classics Series, “Southern Gaul and the Mediterranean: Multilingualism and Multiple Identities in the Iron Age and Roman Periods”, looks as if it will be both interesting and nuanced.