Yeah? And how long have homo sapiens been around for? How long have they been practicing monogamy? And the real killer is: did Australopithecus afarensis practice having a nuclear family? Evidence suggests not. You know why? Because of the high degree of sexual dimorphism. Modern equivalents like the gorilla, which is a highly sexual dimorphic species, and other examples show that the more dimorphic a species, the more likely it is to practice the alpha male, multiple female groupings. Given that homo sapiens have only been around for a fraction of the time that other hominids have been around, all I can say is Pfffffft! to your nuclear family being the foundation of everything. That's the biological precept. As for the cultural one: don't be so fucking westernocentric.
And just so we're clear on the definition:
Main Entry: nuclear family
Function: noun
: a family group that consists only of father, mother, and children
I'd just like to add that in polynesian societies there are many examples of extended families, and matriarchal based families that do not support your nuclear family hypothesis. And that's not a modern basis. This was practiced before missionaries came over to convert them to the christian concept of marriage and 2.5 kids.
edit: must remember to finish sentences when I'm furiously typing.