Timeline of Himalayas/Tibetan plateau formation Note: edited based on helpful comments.

In Walking with Cavemen, Robert Winston claims an African rainforest existed 8 mya, but by 4 mya it was gone. He attributes this to newly Himalayas-triggered Indian monsoons, which prevented rainfall from reaching Africa. However, it's my understanding that India collided with the rest of Asia circa 50 mya. So why did the monsoons take so long?

Multiple comments have noted that Winston should have credited the monsoons to the Tibetan plateau. Either way my assumption is that, until the mountains had been pushed up far enough, they didn't threaten the forest's rain supply. On a linear approximation, by 8 mya the Himalayas' mean height would be approximately 85% of its present value, which bounds the "critical value" of the altitude as a percentage of its current one. A similar calculation applies for the Tibetan case.

Why did the motion of India lead to a multi-stage mountain formation? How did the timing of these effects influence Africa?
Drying out of North Africa is likely the result of something far more local. I've never heard of the either the Tibetan Plateau or the growing Himalaya as causing changes to Africa's monsoons and always understood the connection between Milankovich cycles and the consequences of glaciation. According to this study, which modelled weather patterns of the time:

Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene

The drying of Africa started much earlier and was likely related to orbital cycles of glaciation-induced drying of the Mediterranean region.


  Through a set of experiments, we demonstrate that the African summer monsoon was drastically weakened by Tethys Sea shrinkage, allowing arid, desert conditions to expand across north Africa. Not only did the Tethys shrinkage alter the mean climate of the region, it also enhanced the sensitivity of the African monsoon to orbital forcing, which subsequently became the major driver of Sahara extent fluctuations.