The truth about the natural diet is that our ancestors were eating whatever they could get their hands on. They could digest meat as well as vegetables. Some humans have been fed mainly bread their whole life. Some people were eating only meat like some game-following nomads in the north of Canada, and some people were strictly vegetarian (maybe in India?). Some people like in Japan were eating a lot of fish.
If you think back before civilization, more than 20 000 years ago, before Ancient Egypt, among others, people were digging up onions, scavenging dead animals (we can eat the bone marrow that most predators cannot get out of a body), eating insects and small critters, as well as fruits, roots, nuts, some leaves, mushrooms, and we were also foraging through the feces of bigger animals. Hunting has also been with us for many thousands of years.
Hunting was not our thing in the beginning, it came more with advanced intelligence, use of fire and traps and tools.
If we want to look further back in the past: by logic, a monkey would be mostly frugivore, with supplements of nuts and insects and a lot of other random things. Bear in mind that Homo Sapiens is thought to be at least 300 000 years old. And the genus Homo, 7 millions years. (https://en.wikipedia.org/wiki/Homo_sapiens) Still according to wikipedia, usage of fire by humans could be as old as 1.7 million years, while more dedicated fireplaces and creation of fire by humans could have appeared between 700 000 and 300 000 years ago. Fire then allowed cooking.
Eating non-human milk products is kind of recent in history but has also been extensively used. People were crossing deserts, drinking camel milk, goat cheese and such. Alcoholic beverages have also been very popular, and not only to get drunk: alcohol doesn't spoil like water does, so it's useful for long trips.
The luxury of chosing what foods you would eat came very recently in our history. An optimal nutrition should be very diverse to be sure your body has plenty of possibilities and is not missing anything. Every body is different and needs different foods. The most important is to have enough food to survive, for the rest the body can adapt a lot.
Another answer could probably expand on all the types of vitamins and minerals, but I think that's just looking at the same issue but with a microscope, the answer will still be to eat a variety of food.
To answer your question “Is there anything in meat that isn't available from vegetables?”, there is one vitamin that is only exclusively found in animal products: B12. See there: What is a good source of Vitamin B12 for a vegan?