BBC - Earth - Why our ancestors drilled holes in each other's skullshttp://www.bbc.com/earth/story/20160826-why-our-ancestors-drilled-holes-in-each-others-skulls
Chalcolithic Europe - Wikipediahttps://en.wikipedia.org/wiki/Chalcolithic_Europe
Domestication of the horse - Wikipediahttps://en.wikipedia.org/wiki/Domestication_of_the_horse
Ghassulian - Wikipediahttps://en.wikipedia.org/wiki/Ghassulian
Mesopotamia: The Ubaid Period (5500–4000 B.C.)https://www.youtube.com/watch?v=UhzdBaNtN2I
Tell al-'Ubaid - Wikipediahttps://en.wikipedia.org/wiki/Tell_al-'Ubaid
Ubaid period - Wikipediahttps://en.wikipedia.org/wiki/Ubaid_period
Urban revolution - Wikipediahttps://en.wikipedia.org/wiki/Urban_revolution