If one was to digitize the rhythm of Puerto Rican plena, one would have a cliché reggaeton beat.

Then again, every once and a while I will hear a bomba or cumbia lick and see the roots of a reggaeton beat.

From an anthropological perspective, I would have to say that reggaeton is a music developed in Puerto Rico but derived from Panama. It's original shape must have been heavily influenced by cumbia and Jamaican dance-hall. Once it arrived to Puerto Rico, the rap influence accelerated and the beat became much more clearly defined.

My personal theory, by the way.