Dolmens

I wasn't expecting 40% of the world's dolmens to be in Korea. They have several varieties, and theories about whether the notion was from southeast asia or northeast asia or home-grown.

The patterns seem simple enough that parallel invention seems plausible--however, note where dolmens aren't found in significant quantities.

Maybe there's sampling bias at work. Do sub-Saharan Africa and the Americas have menhirs?

The wikipedia list is not complete. It leaves out Korea, there are deer stones in Mongolia/Siberia, and others. However, it mentions Ethiopia, though that's close enough to the MidEast to have had a lot of influence from there. And there's a tribe in South America that erected some menhirs--clearly independently of anything in the Old World.

Of course there are plenty of examples of South American stonework--far more sophisticated than simple megaliths--but not as old.

My intuition was that kids building things out of rocks would be inspiration enough for adults to build their own: medium-sized first for keeping critters out of the cairn and then big for monuments. If you built earthworks first the assembly wouldn't be so bad--the headache would be finding and moving the appropriate stones. But making the big ones--the ones that last--takes a minimum amount of manpower. If dolmens are too ugly, or the religion doesn't honor stones in relevant way, or if the local culture is too small or too poor--no, you'd not build them. If you had tools for shaping stones to your liking, you'd not bother with anything that crude.

Finding none in such wide areas? I have to guess that my intuition was wrong: they spread with the "idea" of dolmens, and possibly even then only with information about technological tricks for making them. And there'd be a window of opportunity. Too early and you don't have the manpower, and too late and you'll have nicer ways to make monuments.

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When you try to cross-check things, you find ever expanding temptations to rabbit-track. I ran across claims of ancient Indian stone structures in New England--some of which seem to be attested and others seem to be wishful thinking. There's an interesting site nearer to me that unfortunately has to have its location kept secret. And some things look extremely fake.