The five being one pregnant woman, the good white guy, a black man (which probably was a bit edgy for 1951!), a kindly old man, and an extremely douchy foreigner.
Ironically not much has changed since 1951. You could use the same 5 people in any post-acapocalypse movie now and this formula will still work (for example: now, the foreign would either be an Indian or Arab instead of a Russian or Asian).
P.S. They did forget one thing in the 1950s: a dog!