My main question is why do people eat meat?
Can we not survive without meat?
I recently became a vegetarian and I have not yet felt any impact on my health. Can any one explain this phenomenon from the perspective of nutrition?
There are many reasons for why people eat meat. Some of them are to be discussed here and some of them have nothing to do with biology.
Reasons that have nothing to do with biology
There are many reasons that are not to be discussed on Biology.SE. These include cultural reasons, the reasons related to taste and appreciation, farming and availability, price and economy and of course for reason related to political ecology (incl. water and energy consumption of meat production).
Reasons that has to do with biology / medicine
It is not impossible to get all of these nutrients without meats. However, when becoming vegetarian, some people need to be careful and rethink their diet to ensure not to suffer from the lack of these nutrients.
Tribal peoples throughout the world have revered meat since the times of cave paintings, over 30,000 years ago and the oldest stone arrowheads are 66,000 years old.
Chimpanzees also eat meat. They can hunt like wolves and devour small prey. (see videos)
Meat is a highly useful or essential nutrient source to those who cannot source enough local protein for the level of activity.
The importance of meat depends on geographical factors. Eskimos live in snow and eat more than 90% meat. There are no plants there. Equatorial people like Indians have plentiful plant growth and can live fine as vegetarians on replenishing foods like lentils and greens.
Northern people have less choice of vegetables and fruits, whereas tropical fruits like melons grow year round, and northern plants grow slowly, whereas northern grazers can grow fast.
Energy budget in the north is limited by the sun, which is deficient to grow grains and oils. Fat is the highest energy source, and it is very easily sourced from livestock.
Disclaimer: I am not an anthropologist, this attempt at an answer is only my crude 2 cents, I haven't looked for references to support this supposition.
Nowadays we can survive without meat, but this has not always been so. We know that early human groups were hunter-gatherers. Hunting is obviously much more difficult than gathering: it takes more energy because you have to run, it is more dangerous because you can be hurt in the process. Therefore, the primary reason why early humans hunted and ate meat must be because they had no choice; likely because they could not gather fruits and plants in enough amounts or variety to support a healthy diet. Later on, when they transitioned to agriculture they also naturally transitioned to raising domesticated animals for meat production because that's what their diet was already like.