Anyone who has read Charles Darwin's origin of species will have a difficult time arguing that we have evolved from brutes. Over a long period of time, our intellect has developed. It is also clear that man adapts to his environment. In the land of brutes and at the given time, the environment did not allow for much contemplation, species were forced to ensure of their physical survival.
At this point man's environment is much conducive to reflection, and that is why we have become smarter than our ancestors.
It is clear that among us, some are more like the evolved species, and some are more reminiscent of our predecessors. Some of us are 'better made' for action like apes and other animals out of which we have evolved, yet others are better made for contemplation and more intellectual endeavors.
The human race could be divided into 2 classes. The passionate and the intellectual.
Extroverts, sensors and Feelers are best categorized as the passionate. David Hume persuasively argued that it is not intellectual notions that make us act, but our passions. This is a truism as an abstract idea, if it is not relevant to me in any way will not inspire me to act. I will only act, consciously or unconsciously if in some way I think it is relevant to me. Extroversion is in closer affinity with action. Feeling for the same reason. Feeling is what makes all action possible because if we did not process emotion, it would not be possible for us to survive. Jung however calls Feeling a rational function. One may see this faculty as an intellectual assessment of emotions. That is false. When Feeling is backed up by thinking, such an assessment is possible, but feeling as an intellectual faculty is only capable of allowing our minds to clearly focus on emotion. Sensation is better identified with the passionate because it aims us at the concrete, external world. Our physique is what first and foremost fuels our bodies into action. Our physical needs play a great role in our need to act out externally.
Intuition, Thinking and Introversion are to be better identified with the intellectual. Intuition is an abstract collection of perceptions. Primarily concerned with imagination and vision. Introversion is concerned with allowing us the necessary down time to properly think. Introversion therefore in itself is not a faculty of the intellect or contemplation, but only an accomplice. It is the platform for intellectual endeavors, much like Extroversion is the platform for the passionate endeavors. However, Introversion is better identified with the former because it conduces to the intellectual pursuits more than to the passionate.
Hence, the passionate and those primarily concerned with action are in closer affinity with our primates than with the evolved modern man. One may argue that despite that they are this similar to animals, they have enough of the intellectual aspect within them to be more comparable to people than to animals. As even the most retarded of us, or the most outgoing and least analytical are superior in intelligence to animals.
Thus, we have the most passionate of all beings, as the ESFJ, this is likely the type the primitive man inhabited. Intensely externally focused, intensely feeling oriented and intensely sensation oriented.
ESFP is less externally focused, and less feeling oriented, thus it is secondary.