In the early days of the Human, life was much more basic as people had to learn about the natural world around them. Early Humans had to learn to know which plants and berries were edible and which were toxic but it is not a purely vegetarian diet that has made the Human species as intelligent as it is today.
Early hunter-gatherers would have fished in the rivers and streams close by and hunted animals that lived around them in order to sustain themselves. However, things today are very different with packaged products in supermarkets and the vast array of convenience food available, numerous people that eat meat tend to have no idea what animal it comes from.
What is worse though, is the fact that the majority of western households don't want to even think about the fact that they are eating something that had a life before it appeared on the dinner table, often refusing to eat anything that actually looks like an animal (which is in all regards disrespectful to the animal in question).
Those who chose to eat meat should acknowledge that it is our modern-day lifestyles that has made it so easily convenient to gain all sorts of animal products, and that the meat and fish consumed did once have a life despite what it may look like in vacuum packed plastic.
We have lost touch in the developed world with what it is we are actually putting into our mouths to sustain ourselves and this needs to be given more thought. Animals today are commercially farmed on a vast global scale to provide people with food so if you can't accept that you are eating an animal then you shouldn't really be eating it.