Sisoon,
I read two books during my formative years that I still reflect on.
One was "The Naked Ape" by Desmond Morris. He tried to show how much of human behavior, and physical characteristics, are driven by nature, by biology, by survival of the fittest. (Nature trumps nurture)
The other was "Beyond Freedom and Dignity" by B. F. Skinner. He was very much in the "nurture trumps nature" camp. He espoused "Cultural Engineering" to solve humanity’s problems.
Morris’s book has been somewhat discredited; Skinner’s is still debated.
But I would say that "nurture trumps nature" has largely prevailed. Most psychologists agree with your statement that human behavior is so variable that it cannot be tied to biology.
This stance, of course, creates room for psychologists.
I find myself a little more in the "nature" camp. But I acknowledge the variability of human behavior.
Nonetheless, when somebody acts consistently with "survival of the fittest", (promiscuity) I am content to blame nature.
And when we act inconsistently with "nature" (monogamy), I credit our big brains for overcoming our dna.