Humans can survive on just about anything, compared to most animals. We can be 100% carnivores, 95% vegetarian (100% with modern biotechnology), or anything in-between. We can survive on seafood, foraging for insects/plants in the desert, or just hunt big game.

Now, a "healthy" diet might get you an extra 5% or so of lifespan, or an extremely "unhealthy" diet might take 15% away. It's a tricky thing to study since it takes decades to see a small difference & we don't force people to eat an experimental diet for that long. If you exclude diseases with only a modest correlation to diet (e.g. diabetes, obesity, coronary artery disease -- lifestyle, genetics, and such play an equally large role) then the difference is very small.

One also has to recognize that a modern diet is too nutritious for the amount of exercise we do, it's actually fairly rare to develop a vitamin deficiency in a first-world country. The FDA recommendations are simply the average of what healthy people eat, we aren't doing starvation experiments to determine the true minimums.