Alternate theory

Monkeys hornier then their brothers started mating over longer seasons than the customary august september. Being active longer, they needed extra fat on their red asses. There could be days without food trying to impress a femonk.
Their gathering skills started to improve. So did the related parts of the brain. And they began to change. Not their features , but their attributes.

Their heads got bigger. They stayed young longer. The life expectancy increased from 25 years to 35.The offsprings of these 'mutants' also had these attributes. Gradually their population started to increase. As predators found them hard to pray upon. And they had excellent survival techniques.
With each generation they became raunchier.

They took over.

The world became their mating dance.

So dance on.