Milutin Milankovitch is generally credited with the theory of "cycles" - which are complex - oscillations and changes in orbit are involved in a complex "dance" which leads sometimes to more and sometimes to less radiation falling on the Earth's surface.

However, there are so many other factors at play that it is not just down to cycles. How cold the summer is has a big effect because the colder the Spring and Summer the more snow lies on the ground and the more radiation is reflected back. This can enter a feedback loop eventually resulting in an ice age.

Another factor is the position of the continents and therefore the flow of oceanic currents - the current disposition allows the Arctic to freeze, reflecting back a lot of solar radiation. However, that era may soon be over and nobody really knows what happens then. Best guess is a feedback loop with a positive effect on temperature - going back to a more normal (geologically speaking) phase of hot and dry. However, primates did not exist at those times.
Source(s):
Bill Bryson (book) A Short History of Nearly Everything.


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