A recent paper on Y-chromosome phylogeny found that a big fraction of Y-chromosomes fall into a few star-cluster lineages that are a few thousand years old. You’ve already heard of some of these (R1a and R1b, for example).
I might be wrong, but I get that the impression that some people have gravely misunderstood what this means. For example, something at Slate writing about this work seems to think that back in those old times, only one man passed on his DNA for every 17 women. Others apparently got the same impression.
Let’s think about it. If only 1 in 17 men reproduced, Y-chromosome diversity would be reduced, all right – but overall genetic diversity, autosomal diversity, would also decrease, which apparently has not actually happened, as Alan Rogers has pointed out. More than that: is such a society, one in which 94% of men never reproduce, very plausible? Would it be stable? Reminds of a case in which some drunken soldiers wandered into a hard-core Baptist church service, and after an inspired sermon on the torments of Hell, objected: “There couldn’t be no sich place! People wouldn’t stand for it !”
Such a society would be like the famous car-wash scene in Cool Hand Luke – all the time.
Here is a more plausible scenario, one that fits the facts. Some conqueror has an inordinate number of kids (because he can). His sons, and his sons’ sons, rule for a long time – eventually most of the aristocracy are their male-line descendants. That first generation had a huge reproductive advantage, but we’re only talking one guy: if our hero had 100 wives, that doesn’t make much difference in the overall fraction of men that reproduce. As the generations pass, his patrilineage gets bigger but their average reproductive advantage becomes smaller (they can’t ALL be kings). The conqueror’s autosomal contribution is cut in half each generation ( at least while this patrilineage doesn’t make up much of the total population) , unlike his Y chromosome: his genes never make up much of the overall autosomal ancestry, even when most of the men in the population have his Y-chromosome. Autosomal genetic diversity is hardly reduced, effective Ne does not drop noticeably, while the Y chromosome is almost fixed – would be, except for continuing mutations.
Let’s suppose that this process (one Y chromosome becomes dominant in this Genghis-Khanish way) takes place over a thousand years. I think at the worst point, the fraction of guys having offspring is probably 80% as large as it was back in the egalitarian days of old.
Genetic inequality may have increased some. The rich may well have out-reproduced the poor, although with cities as population sinks, you can’t be sure. In Europe, this process surely involved conquest, likely with a lot of indigenous old-farmer men getting whacked.
But only 5 0r 10% of guys fathering kids in a given generation? Never happened.