Early humans ate meat, yes, but neither humans nor animals just popped onto the scene fully formed, one craving the other to eat.
Humans *are* animals, by the way. Human beings evolved from a common ancestor with the apes, about six million years ago. That hominid ancestor, Orrorin tugenensis, ate some - though probably not much - meat, as evidenced by its teeth.
At some point in the journey from the simplest forms of life to becoming a hominid, the creature that would evolve to become man found it increased it's chances of living long enough to reproduce if it added meat into its diet - initially it was probably in the form of insects or something of that ilk, and gradually, over millions of years, natural selection favoured those that ate meat and plant material.
Imagine a rainbow. Imagine the red end is the very earliest ancestor of Homo sapiens. Imagine the other end, the violet end, is modern day Homo sapiens. Each colour (red, orange, yellow, green, blue, indigo, violet) in the rainbow represents an intermediate stage - Homo erectus, Homo Habalis etc.
It's easy to look at a particular colour and say, 'that's colour x', but when you look at it as a whole, you see that there's actually a continual gradual change with each colour incrementally different from its neighbours. It wasn't like one day there wasn't Homo sapiens then the next there was; there was a gradual, incremental change over time, and in the same way that anyone can look at the rainbow and see Indigo is different from Violet, it's nevertheless difficult to say when one colour becomes the other.