Babies of humans and other species have been shown to have a sense of morality that is not learned.

https://campuspress.yale.edu/karenwynn/files/2015/10/WynnInn...

@bholzer put it nicely. Google "chimps and bonobos demonstrate morality". Cooperation increased survivability and that generally has affected our acceptance of what is intrinsic to what feels right. Later religion slapped on meaning to it via super natural explanation which was the best bullshit they could come up with at the time. I get up voted on PC answers by the way but I should have cited some examples.

Seeing a chimp and bonobo demonstrate morality doesn't exactly assure me that a born sense of morality is "adequate enough by itself".

Pretty sure there is a zoo in Cincinnati that agrees with me on that one.

Prison isn't known for bringing out the best in humans either.

> How exactly did you arrive at that conclusion?

You're here aren't you? Had it not been sufficient, the human animal wouldn't have survived for 200k years. Morality is innate, instinctual, and is present across the animal kingdom.