New genetic analysis indicates that Australian Aboriginals, at least those in the northern part of Australia, picked up about 11% of their ancestry from India, relatively recently – about 140 generations ago. 4000-5000 years, depending on generation length.
Lots of interesting things happened in Australia around that time. Previously Australia had a Middle Stone Age technology, no better than the Neanderthal toolkit. But around that time, microlithic stone tools showed up, along with new food processing techniques and the dingo. Very few dingos – they’re genetically similar, very much so. There might have been a single pregnant bitch. Population density went up by something like a factor of five. Altogether, the transition is similar to what happened when anatomically modern humans replaced Neanderthals in Europe.
Let assume that this new analysis is correct. How could this have happened? There was never any significant colony -archeologically, it would stand out like a sore thumb. India had all kinds of useful technology that never made it to Australia, above all agriculture.
So the problem is this: what historical process ends up contributing some key technology and 11% of the genome, while failing to transmit the vast majority of available culture and technology?
Here’s a scenario. A ship from some Indian civilization about 4000 years ago [maybe Indus] loses its way and ends up crashing on the shore of Arnhem Land. Like most ships, its crew is all male. They survive the landing, and end up warring with a local tribe. Eventually, they kill off most of the men and annex the sobbing women. The new tribe has 5x higher carrying capacity, essentially due to better technology, and they’re militarily superior as well – better weapons and tactics. They expand and expand, picking up more and more old Australian genes as they do so. Not as an empire – they’re too simple for that – but the tribes that descend from them keep winning. They speak the ancestor of the Macro-Pama-Nyungan languages, which originates as a mix between their old-Dravidian language (from the men) and some local Australian tongue (from the women).
In the post-Columbian world, there have been events that could have gone this way, but the players always ran into Europeans again before anything really interesting had time to happen.
One funny thing about this scenario: the sailors lose almost all of their civilization, and probably thought they were descending into savagery. On the other hand, the locals probably thought of the castaways as incredibly advanced, magicians or gods. They were both right.