Do we actually know that early humans were fit? A couple of big things to consider are that we mostly see depictions of early humans made by marketing teams trying to get people to go to their museum, or see their movie, or eat their new fad diet. That’s not likely to be realistic. And that natural selection would have culled the weak, slow, and otherwise deficient. If we picked a sub-population of humanity right now that would be similar to the humans of the past that survived to 30, most of us would not be in that group. Furthermore, it is absolutely possible that all of the humans in a given region were malnourished because they didn’t have access to a varied diet. They might likely die out and be later replaced by humans that had learned to grow crops, bring cattle, or otherwise provide for the deficiencies in diet that the surrounding area couldn’t give them.
This is why they got rid of price zombie