> Muscle is not a reliable source of ascorbic acid, and we need it to manage correctly our urea metabolism.
Let's be clear, firstly... I think we should stick to talking about the skeletal muscle of animals that our ancestors primarily adapted towards, which would be large ruminants (aka red meat with legs). I can't comment on a hypothetical animal that lacks vitamin C in its tissue.
What you're saying simply isn't true. There is enough vitamin C in meat, and the idea that you need to eat liver to get enough of any vitamin is largely mythical. But let's stick with vitamin C.
The human body needs very little vitamin C, and orders of magnitude less so on a diet of meat. Vitamin C and glucose both depend on the insulin pathway to enter cells, thus they "compete" to do so. With even a modest amount of exogenous carbs, glucose out-competes vitamin C for update because it has a greater affinity for the GLUT-1 receptor. Thus, one would need a lot more vitamin C on such a diet. Without exogenous carbohydrates, skeletal muscle provides plenty of vitamin C.
https://www.cell.com/cell/fulltext/S0092-8674(08)00204-3
We all know that hard tack was once a staple on sailing ships, and that those who subsisted on hard tack too long were prone to scurvy. What people either forget or weren't taught is that not all sailors were eating hard tack on any given ship; the captain and his officers were more likely to be eating dried or cured meat, meaning the prevalence of scurvy was far greater among the regular crewmen who were receiving the lesser food.
> Your bold claim that "Skeletal muscle from cattle alone has everything one needs to thrive" would led us directly to die by scurvy.
Completely false. It takes as little as a month to develop scurvy, yet there are many people who have been eating meat exclusively for years and none of them have scurvy. When should they expect to get scurvy? After a year? 5 years? 10 years? I myself have experience with this and, well, it's been far, far longer than a month... still no scurvy. I don't eat liver. Never have and have no desire to start doing so.
> Muscle is not a reliable source of ascorbic acid
Again, not correct. A meat based diet requires substantially less vitamin C.
> So we need to add a source of Vitamin C, that comes typically in fruits.
No, you don't need to add fruit if you're eating a diet solely of meat.
> Yes, can be found also in liver of some animals
Yet many carnivorous animals, including humans historically, don't bother with the liver under most circumstances. Liver can have benefits, but eating liver too often or in too great a quantity can lead to types of poisoning and electrolyte imbalance.
> but liver is not skeletal muscle, so your previous claim is incorrect.
Yeah, it's not, which is why carnivorous animals consume mostly skeletal muscle and don't just eat the liver, despite how some think that liver is a cornucopia of health. Liver is not a cure for anything, except perhaps carbohydrate-induced scurvy.
> so your previous claim is incorrect.
Not really. It's quite correct. You really should reevaluate your view of vitamin C in the human diet.