Say, the Aryans migrating from the Pontic Caspian steppes of Central Asian to the Indus Valley or the SA Hunter Gatherers going ALL the way to Australia?
Or the Siberian/North East Asian tribes that crossed the Bering strait into North America?
This was literally thousands and thousands of years ago before modern medicine, travel, shelter and mapping. How the hell did they cross thousands of miles of land and ocean without perishing in the attempt? How did these migrations take place? What motivated them to even leave (since they had no idea about the local geography)?
Most of all, how on earth did they cross oceans in say 3000 BCE? (Aboriginals of australia)
I appreciate all answers, cheers!
There are several different questions here.
The answer to the major question is that it all happened a lot more slowly than you imagine. It wasn't a matter of one tribe migrating several thousand miles in a few years. Rather it was a matter of generations of people migrating only a few hundred miles per generation, and ultimately migrating across vast distances over centuries. People walking for just a solid week straight can easily cover 100 miles distance. If a group migrates that much only once or twice a generation then it can easily cover 500-1000 miles a century, and thus many thousands of miles over a millenium.
Most of the major migrations are thought to have occurred over land. During the ice ages prior to the holocene era large ice sheets covered a lot of land area and consequently the ocean levels were lower. A lot more of the continental shelf was above water, which connected many places that are now separated by water. For example, the British Isles were connected to the European continental mainland, and an area in the North Sea called "Doggerland" was inhabitable. Land bridges connected Eurasia to North America across what is now the Bering Strait and most of the island chain of what is now Indonesia et al were connected by land as well.
Australia, for example, was initially inhabited roughly 40-60 thousand years ago, back when most of the area between Australia and South East Asia was above water. However, there are areas between Asia and Australia that were always under water, such as the Lombok Strait. So humans must have traveled on seagoing vessels at some point in order to make the journey to Australia. But the distances involved would not have been great, people could have seen the other shore quite plainly and even a simple raft might suffice to make the journey. So it's not crazy to imagine ancient peoples doing so.
Oceangoing colonization did occur during in ancient times during the holocene, however, by the Polynesians and Austronesians, starting about 5000 years ago. These people did not colonize Australia (an even that happened tens of thousands of years previously, as described above) but they did colonize Polynesia and New Zealand. Polynesians developed the technologies necessary to explore, colonize, and trade amongst the Pacific Islands. These included: boat building (generally multi-hull or outrigger ocean going sailing canoes); navigation (celestial navigation, the ability to recognize patterns in ocean currents that indicated the presence of islands, the ability to record the locations of islands and sea routes between them); and agriculture (foods that could be brought on ocean voyages and cultivated on islands to provide sustenance, in addition to fishing).
If you want to learn more about the feats of the Polynesians and how they managed them I'd suggest reading Vaka Moana, Voyages of the Ancestors: The Discovery and Settlement of the Pacific by Howe.