Plant sources are no longer regarded as secondary because it's denigrating to vegetarians. I think that's rather subjective. Rather they are more highly regarded as it becomes clear that plant sources of protein are more readily assimilated. What meat has is a broader base of amino acids in larger quantities. We don't have the stomach acids that carnivores have, nor the fangs, and our jaws do not move like carnivores. From what I can conclude, we evolved to where we are adapted to eat meat, perhaps even need it to some extent, but we adapted rather imperfectly. We certainly never ate as much meat as we do now. Our ancestors ate meat only occasionally. A deer only goes so far when shared between several dozen tribe members. Getting back to vegetable being secondary protein. I have read that there's research showing that how much of a nutrient you consume and how much you assimilate are two different things. Who would think for instance, that carrots would have a higher glycemic index (which measures how much Sugar goes in the blood stream) than fruits or even soda beverages! Or that Celery is a good source of calcium that quickly enters the blood stream.