[jwegan]

The problem is just 1 or 2 or even 10 people washing ashore on an island wouldn't be enough genetic diversity to sustain a population.

[jandrese]

On the other hand, the population wasn't sustained. One or two generations of people on that island would still leave behind a good number of artifacts.

IMHO concern that the population wasn't large enough for multiple people to attempt the trick ignore the timescales involved. These events could span tens of thousands of years--longer than all of recorded history.

[s_m_t]

Is that actually true? Surely they'd encounter some problems but why would they absolutely be doomed?