It's a fascinating thought experiment to consider a desert island populated with newborn babies, growing up to be individuals, forming a societal structure, forming culture, without any pre-existing human input.

Obviously there's a "chicken and egg problem" that human babies require human adults.

Raising chickens, however, doesn't have this "chicken and egg problem". You can hatch baby chicks from eggs, and despite them having never seen an adult chicken before, they're pre-programmed to behave exactly like chickens. Every newborn chick is fully programmed from birth.

What would humanity look like after a "hard reboot"?

(Obviously the way to answer this question is that we must send a rocket full of babies to Mars and live-stream their evolution.)

Not exactly the same, but there was the case in Fiji of 'Fiji chicken boy' - when his mom passed way, his dad abandoned him in a chicken coop, and he was basically living with the chickens. When he was later discovered, he was behaving like a chicken, making clucking sounds and pecking at his food.