Mahdi Kouretchian Posted about 2 years ago Evolution: "just a theory". Scientific caution is sometimes confusing. Walking ostriches is another proof for revolution. Because they can observe predator's existence on their foot more easily.That's why they equipped with long necks. Revolution modifies ostrich's shape in this way which is absolutely compatible with their surrounding and increases chance of survive. That is exactly what has happened to humans through 2 or 3 million years. When our ancestors lived on trees, After eating all foods they had to descend from the tree and wend a little on the ground to find another food resources. But the ground was filled with pernicious animals and the distance between trees was very high due to climate condition of that time. Therefore, nature forced them stand on their foot to have a better picture about their surrounding. Through thousands of year and revolution they gained the ability of running to evade predators. About the birds, I think their foot's shape helps them in landing or taking-off process. Having extra foots or hands increase a bird's weight which makes flying impossible for them.