everything reduced to its constituent parts is a sloppy atomic mess, every constituent part as old as the universe, and in our system originating from the nebula that was once our young star.
billions of divergent variables forces the observer to take the baseline as starting point, ie. the organic human machine and the innate behaviors that have been formed via millions of years of evolution.
shared traits across all divergent human genetic lines, such as the asymmetric snarl over fangs that no longer exist, the arched-eyebrow greeting, the sideways glance while eating etc... All human cultures share such traits, proven by ethnologists and their study of tribes who had no contact with 'modern' humans.
what then is human nature? Human nature is, simply, what it is. Divergences from common bio norm occur(both in body and mind) and with such a complex species working just off the baseline is sure to create errors in judgement. On the whole, looking at things from an alien perspective, the human is little different from other organism-consuming, self-perpetuating creatures. Complexity enters with three simple elements: Imagination, Environment, and Socializing. Without these elements, humanity as it is today would never have come to be. It takes mastery of environment, ordered imagination and a social structure for the human collective to achieve feats such as building pyramids.
Thus in the collective view, the human is a planet-changer by employing imagination and/or convincing his fellow beings to exert energy on behalf of an unrealized ideal. Crusades, moon landings etc. are all collective endeavors that differ vastly from anything that any other species, at least on earth, can achieve.
Surpassing us in this is nature, for while we can build drydocks and launch spaceships, we are still far too far in our infancy to cause supernovas or winnow down mountain ranges--though with nuclear power we have gained, in a way, mastery which natural forces planetside cannot recreate. Such capabilities pale when compared to the heart of even the smallest star, however.
Individual basis: Complexity enters and there are approximately a hundred billion different theoretical case studies, going on current estimates of how many humans in toto have existed. Figure in that humans change from childhood to late life, and it's very easy to state that this 100 billion could be expanded greatly to, say, 1000 billion or more as you consider that each baby is different, each child, each adult, and even year by year and month to month the human can vastly differ mentally and physically depending on diet, social milieu, health etc. etc. Simpler to take the 100 billion individuals, or the 8 alive today, but it's easy to see how the complexity compounds itself.
As said, baselines exist, and some humans will adhere frighteningly close to them, others not so. Think of twenty rats inserted into a maze; eleven are smart enough to get the cheese, four remained at the starting point to fuck, two killed each other when the one trod on the other's tail, two only went searching for the cheese after the others found it, making sure to bully their way to a good chunk, and one wears a tophat, hums Beethoven's 9th, and has escaped from the maze and found the larder before the experiment even began.
Interesting book, though outdated, is The Human Animal by Hans Hass. Great view, Hass expresses amongst other things that technology is so very very intrinsically part of humanity, it is our 'artificial organs'.
There are definite patterns, and immutable biological requirements and mechanisms---other than that, it's a free-for-all freakshow and we're all part of it.
What is human nature? Fuck if I know.
***Edit: kek just found out thanks to this thread that a copy of above book as hardcover, which I bought for USD 0.73, goes for USD 471+ on amazon. Sho : P
