The ecosystem of Earth evolved over hundreds of millions of years. It flourishes because it is always in balance. Plants grow in the ground, herbivores eat the plants and carnivores eat the herbivores. This process evolved because it works. We evolved into exactly what we are today because our ancestors ate a largely carnivorous diet for a couple of million years. How can we be sure this is true?
The simple answer is no. A quick internet search reveals that parents from all over the world have been convicted in court of causing either the death, or severe malnutrition, of their own young children as a result of their vegan diet.