[MD] Social Imposition ?

[Case]

[Arlo]
Interestingly, there is enormous evidence to suggest that certain "advances"
were achieved at more-or-less simultaneuous times across the planet. Case
mentions writing. A personal interest of mine is megalithic construction. 

[Case]
Allowing for a pretty big span of 'more or less,' how would you account for
this? The megaliths I mean? They seem to have been for astronomical
observations but why not sooner or more spread out in time?