In 1952, a study was done by William Rose to determine the requirements in humans of the eight essential amino acids. They studied the values of these eight as found in unprocessed complex carbohydrates (as opposed to pure protein sources) and found that all eight existed in each plant food, in more than the minimum requirements set at the time. In fact, some plant sources have a higher protein content (per calorie) than meat does. It's important to distinguish nutrients per weight, versus nutrients per calorie...two completely different things. Spinach is 49% protein per calorie compared to chicken which is actually lower per calorie (the rest being fats) 100 calories of broccoli supplies about 12 grams of protein, while 100 calories of steak supplies about 6 grams of protein. Broccoli has twice the protein content of steak PER CALORIE. So why aren't people more aware of the high protein qualities of plants? Much has to do with the brainwashing and spin put out there by the meat and dairy associations. Another more deceptive reason, is that all our charts are by weight (grams, ounces) which show more protein in animal products than plants. (as opposed to nutrients per calorie) A small or "normal" portion of meat , might be 6 ounces, which would have more protein in it than a normal portion of broccoli, which might only be 2 ounces, since it is so light in weight compared to animal flesh.
Vegetables are clearly the winners when it comes to having the most nutrients per calorie! Mother Nature obviously planned it that way. Another point to remember is that many of the largest animals on the planet, all being vegetarians, get all the protein they need from plants. Elephants, giraffes, rhinos, hippos, great apes, horses etc, all do fine with a plant based diet and get all the protein they need. And note, that humans tend to eat land animals that are vegetarian in nature. We certainly do get more nutrients from them than from carnivorous animals like the large carnivorous cat family for instance.