Thank you all for your replies.
I am not able to eat red meat, it makes me very sick. Can someone tell me other things which I can eat to get the 8 essential amino acids?
Like brikis said, there are essential amino acids required for protein synthesis. We don't have the enzymes required to synthesize all our necessary amino acids from plant material, like plants do. Also, I think you have it wrong on apes: many ape species eat meat as often as they can. It may infrequent in their diet since it requires a great deal of energy compared to foraging/gathering, but they definitely eat meat.
Not being able to eat red meat isn't a problem. Other animal sources are chicken, turkey, tuna, most other fish, pork, milk, cheese, eggs, etc.
To maintain a diet with all essential amino acids, a vegetarian must eat both legumes (beans, peas, etc) and grains (anything made from wheat, oats, rice, etc) daily. Soy products are also an option, since soy is a complete protein, but it has some drawbacks such as being hyperallergenic, high in phytoestrogens, and having poor environmental connotations. Being a vegetarian, it may be hard to get enough protein in your diet unless you turn turn to animal products like milk and eggs.