CoolBeans writes:
Don't gastric acids dissolve the food but without the protection it would burn the stomach and for them to exist we would need something to be protected from.
Yes, this is correct.
How did the stomach evolve?
The stomach evolved a long time before humans were around. By the time humans came around, the stomach had already evolved to the point where it had gastric acids and mucous protection.
The mucous would be present first... although not like the mucous we have. A much more basic version.
The mucous would work with a simple in/out system just helping things move around (before there's even a "stomach!").
Eventually, a "digesting pouch" evolves that we can sort of call a stomach.
Once this basic stomach exists, small amounts of gastric acids would help with digestion.
At this point, the acids and protection would evolve together.
As one gets a bit stronger... so does the other. Not necessarily either one over the other at any specific time.
And then... one day... a stomach!
And then... after a lot more days... humans with a stomach!