I can't speak directly to ancient avocado cultivars, (or, I guess, pre-cultivars), but I can say that a shopper might not recognize two of today’s varieties as being the same basic fruit when put side by side like on this page here.

There are three basic categories that the common cultivars fall into today – West Indian, Mexican, and Guatemalan. The one on the left is a West Indian avocado – they’re quite large (they get bigger than the one pictured), smooth-skinned, and tend to be a paler, brighter green. The one on the right is a Mexican variety – about 2/3 to ½ the size, dark green to purplish-black, with a rougher, thicker skin.

Guatemalans are kind of halfway in between, with more of a “fruity” look to them (blush of brownish red, not as big as the West Indians) but even bumpier skin – practically a rind. Haas avocados, the ones you probably get at your market, are descended from a Guatemalan strain.

Not based on any research, but just speculation from knowing how plants grow, I’d be highly surprised if a 10,000-year-old ancestor were as large as any of today’s commercial varieties. I’d guess, too, that it’d have a darker peel, thicker, with a larger seed (some of today’s trees occasionally bear seedless fruits that look a little like cucumbers; Mexican varieties have smaller seeds than West Indians as a general rule).