Repeatedly history has shown that in dire circumstances, we help each other. Across time and cultures, in times of need, people get less selfish. When a natural disaster hits, or when a terrorist attack happens, people open their doors and pull out the spare mattresses and share food and clothing and volunteer to help and start patrolling the streets, even risking their own lives while doing so.
It's a well studied effect, and when you think about it for more than 5 seconds, we are a social animal first and foremost so of course "reverting back to out instincts" means building communities and helping others. Civilization makes us more selfish, not the other way around.

Reality makes for terrible fiction though because most fiction is written through the lens of character conflict (especially in Hollywood) and "people help each other but some people die anyway from lack of resources or zombie attacke" is boring, so we have been "taught" to expect the opposite of what actually happens when shit hits the fan (actually I'd love a TV show to delve into that, like TLoU kinda did, but Hollywood is notoriously terrible at writing socio-cultural commentary).