I don't think that Engels quote can be the final word. Engels' writings on pre-history were based on the best available literature of the late 19th century, but those writings have been superseded. I don't think it's convincing that there was a clear transition from being an animal in to being a meat eater and so in to being a human. Hunter gatherer diets were mixed and a lot of meat would come from foraging kills made by other animals. Isn't fire a much bigger deal in terms of unlocking 'a far richer flow of the materials necessary for its nourishment and development?'
Add to this that farming led to a narrowing of our diets and a reduction in the amount of meat people ate. What sustained our humanity when we started eating less meat? I don't think it's possible to attribute humanity to meat-eating as Engels does here. He didn't have access to modern-day archaeological methods, but if he had I think he would have revised his argument.