I'm working on a planet design which makes it feasible for several human-like societies to emerge and evolve separately, up to a point where they are capable to interact with one another directly with relative ease (in a pre-industrial stage). Imagine Europe, Asia, America and Oceania being remotely aware of one another until the 1500s where in the span of 200 years direct contact becomes the rule.
I'm looking for help infidning means to reconnect a world that has been disconnected by topology. I'm a scientist myself, so I'm not afraid of technical details.
My current best candidate is a world with no oceans or large water masses (in liquid form). It has two icy poles and a desert belt. The rest of the planet is rich with biodiversity. The whole planet is carved by mountain ranges of huge altitude (+10km) that mostly transit in the direction of poles. Such topology creates isolated regions (those within mountain ranges that cant be crossed regularly) where different societies emerge and evolve.
The water cycle I assume for such world would be: clouds drop all water on high mountains (orographic lift), which leads to rivers and lakes here and there, which go into the direction of the poles, where water is frozen and clouds are generated and loaded again with moisture. The desert belt is the result of water not reaching the equator because of the high mountains. The icy poles I assume will be massive in both surface and height, since they must hold most of the water of the planet.
The problem is how a planet which is disconnected by topology can become connected in a relatively short time span. So far I have the discovery of new routes or mountain passes (hardly a breakthrough for advanced societies). The discovery of new flying technology (too much of a game changer). And the discovery of tunneling tech. The latter is my favorite, and I'm leaning towards mountains made of minerals particularly sensitive to temperature changes. This way, a primitive device applying ice and fire could act as a tunneling machine. Unfortunately, it is hardly enough to turn +10km mountains into easy routes.
Question: Is it feasible that pre-industrial societies perform large scale tunneling? Is there an alternative for reconnecting such a world?
P.D: I'm also happy to hear about alternative planets that could cause such a disconnected-connected social pattern. However, I'm not interested in sea centered worlds (I've read a couple of great posts on this forum in that regard). How to avoid boats on a mainly oceanic world? How to hinder the development of sea travel until the industrial age?