The idea is that eating meat helped us in the past evolutionarily, so it is justified today.
This is a contradiction with the claim that it's OK because animals are bred for a specific purpose because those animals at the time were not. So is it then that the outcome of intelligence justifies carnism (i.e. what comes after the killing) or is it that breeding justifies carnism (what comes before it)?
You're not surviving off the land on the Serengeti. You can't talk about hunting animals with spears having helped us evolve to justify putting frozen bags of chicken wings in their shopping cart. Freezers and shopping carts didn't exist. Furthermore, the animals we're killing now aren't the ones humans would have killed back then.
What was ethical then isn't now. Xenophobia and tribal warfare, for instance, was a major player in keeping tribes together and affected human migration patterns that had us populate the globe, but that does not give us the freedom to do whatever we want to people that look different from the members of our small ingroup.
You can't respond to the claim that "it hurts animals, other humans, the environment, and yourself when you consume animal products for your own entertainment" with "yeah, well it helped us evolve". That is just an argument about the ends - but that doesn't justify the means and therefore does not counter the argument.