All evidence (throughout the history of humankind) shows man to be an omnivore.
Depending mostly on climate conditions, the ratio
of plant to animal intake varies. In colder and
more severe environments, the meat in-take is usually
much larger. This primarily occurs for two reasons:
1) plants won't grow. You can't eat what isn't there. (Ask an Eskimo. He'll know.)
2) the concentration of nutrients is much higher in meat than in vegetables. This means you can harvest less tonnage of food. It also means the culture can have less of an impact on the environment (then if they cut their meat in-take and became more herbivore-like.)
The reason humans have always been omnivores is because they have to be omnivores. The genetic make-up of the human is that of an omnivore. For the human body to properly function, it must have chemicals found in plants and animals. You could say humans have a chemical addiction to being omnivores.
Because of "survival of the fittest," there has never been a culture of herbivores. A human that tries to become a herbivore will cause physical damage to their neurological (and other) systems. A culture of herbivores would eventually wipe itself out.
The easier the catch, the more an omnivorous culture will thrive. Fishing tribes are often good examples of what plentiful meat can do to better society.