See below a documentary on YouTube about the first horse riders in history; the Botai (who had no successors) and then the Yamnaya (one of the most successful people ever).
It is simplified in the way that documentaries are when compared to books.
But it has genuine experts – eg David Anthony and Dorcas Brown and Alan Outram – and the content does reflect their academic work.
David Anthony wrote the bestselling book “The Horse, The Wheel, and Language” which is excellent. His wife Dorcas Brown has led excellent archaeological work on the steppe and written many great papers. And Alan Outram is a world renowned expert in early horse domestication.
One point to bear in mind watching this is the actors they have in the reconstructions are not always accurate.
They use some modern Mongolian / East Asian people to represent the Yamnaya (along with Europeans).
This is wrong.
The Yamnaya were fully European people.
When people today think of steppe nomads living in yurts on the plains they think of Mongols and Kazakhs and the like. In fact it was only in the early medieval era when the ethnic groups on the far eastern end of the steppe (created by European steppe peoples mixing with Eastern people to create Turkic and other ethnic groups) began to move west across the steppe, replacing those that had inhabited it before.
These new ethnic groups retained the “steppe cultural package” of horses, wagons, tents, etc that had been created millennia earlier.
The Botai featured in the first half of this documentary were descended from the Ancient North Eurasians – a people of the stone age. So they were isolated aboriginal hunter gatherers who invented horse riding and domestication before anyone else. And then they disappeared without leaving any descendants. A really remarkable people.
The Yamnaya are discussed in the second half. See my post about the Yamnaya. In this documentary they say they brought a plague with them which wiped out the people they came into contact with. There is very limited evidence for that and it was different in different places and times.
There is no single cause for this people’s astonishing expansion in the early Bronze Age.
Anyway, take everything in this with a pinch of salt but it’s worth a watch:
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Thank you very much for reading. I do have a mailing list for the fans of my fantasy and science fiction novels. If you’re interested in that my newsletter signup is the main page on this website https://dandavisauthor.com/