Let's imagine that a tribal war erupts in Tierra del Fuego around 1000 BCE. The losing tribe, having been kicked out of its hunting grounds, finds itself forced to seek other lands beyond the stormy waters to the south. Taking to the sea aboard flimsy rafts, many unsurprisingly fail to make it, but, by sheer luck, some manage to reach the South Shetlands. Living conditions are harsh, but not that much worse than in Tierra del Fuego, and the outcasts adapt to it, learning among other things to clothe themselves in sewn seal pelts. Within a few generations the tribe has grown back to respectable numbers, and discovered the South Orkneys and the Antarctic Peninsula. Little by little, adventuring groups begin to settle these new territories, making it as far as Ellsworth Land and Thurston Island. In the following centuries, a cultural split develops between the islanders, whose lifestyle is still fairly similar to what it was in Tierra del Fuego, and the mainlanders, who have become an Antarctic analog to the Inuit, living off the hunting of seals and other sea mammals.
By the time European explorers show up, the whole periphery of the Antarctic is home to nomadic tribes, whose total population is in the 100/200,000 range.