This argument is at least based on truth, though its intended conclusions are deeply flawed. It is thought that humans have been consuming meat since we were recognisably human and it was a necessity for our survival. It is also thought that it played a significant part in our neurological development. That being said, the undoubtedly significant role that eating animals has had on our evolutionary development is not a convincing ethical reason to continue to consume animals today.
When humans began incorporating meat into our diets, this was largely due to necessity. When a particular action is a necessity, as in, the moral agent could not have acted otherwise and still continued to survive, the action in question is not subject to our normal moral considerations. Now, however, the situation has changed. It is almost universally agreed that humans can survive without animal products; the vegan diet is nutritionally adequate, healthful and may even provide health benefits. This means that the original justification for consuming animals, that it was a matter of survival, has ceased to be relevant for the vast majority of the population.
Given that this is the case, it is now very possible for the vast majority of people to survive without consuming any animal products at all; doing so is therefore a choice. Ethically speaking this makes an enormous difference; the decision to consume animal products in the modern day cannot be compared to the same action being performed when it was a necessity for our survival. The context of an ancient human stalking prey with rudimentary tools and the modern consumer buying their meat pre-packed from the supermarket could not be more different, and modern hunters who kill for sport are no closer to our hunter gatherer ancestors than the rest of us are.
The premise being presented here is that because we have always done something, this is a good enough reason to continue performing that action today. The issue with this is that there are many harmful traditions that humans have always perpetuated, for example, humans have committed infanticide, rape, murder and incest for thousands of years and yet the longevity of such actions are rarely cited as reasons to keep doing them. It seems that people are generally quite happy to dismiss this argument as an attempt to justify harmful behaviour, except where eating animals is concerned. Humans have always eaten animals, but that is absolutely irrelevant when the decision that is left to us is whether we can continue to practice this environmentally destructive, profoundly harmful action in a society which offers us a wealth of truly humane and sustainable alternatives.