I was hoping that you would get to this question!
In @Swamidass's prior postings.... readers have gleaned the following:
1) Most special creation timelines put Adam & Eve and 4000 BCE.
2) That's 4000 years to the approximate year of the arrival of Jesus.
3) Virtually all the studies on the exponential growth of a mated pair's offspring describe a time frame at or less than 2,000 years for one pair to become the universal ancestors (but not the only universal ancestors) of everyone alive on the Earth today!
The most influential factor is the assumptions involved with human migratory patterns, intentional and accidental.
Isolated areas, like the rooftop nation of Nepal, or more importantly, Australia and New Zealand. Rather than assume any consistent flow of human traffic to-and-from these isolated regions, I think the researchers just assume a single trader or lost vessels, once every centurty, or some such minimum factor.
This is the sentence you write, @Jay313, that gives me some pause:
"So, again, if the process must have been completed by the first century, then you are making a historical claim. Show me some historical evidence that at least makes the idea plausible. Math won't get someone across the ocean."
Two factors should be considered here ...
A) If a Christian believes God can make humans out of dust, I think a Christian could accept the idea that God led someone to travel (intentionally or not) to a new land and procreate!
B) Australia was already populated once! I don't believe we need special evidence to show that it could be reached at least once more time between 4000 BCE and 800 CE.
I've read some estimates of 50,000 years ago. All we would need is one lost boat in the waters of Indonesia, captained by an offpsring of Adam's lineage (say 1000 years after Adam's creation or the year 3,000 BCE), to be delivered by a storm to the coast of Australia.