I'd argue you've got things mixed up, actually.

Humans are social animals. We are individually physically weak and defenseless. Unlike other animals, we are born into this world immobile, naked, starving and helpless. It takes us literally years to mature to the point where we wouldn't simply die outright if we were abandoned by others. Newborns can literally die from touch deprivation. We develop huge brains not only to allow us to come up with clever tools but also to help us build and navigate complex social relationships. We're evolved to live in tribes, yes, but we're also evolved to interact with other tribes - we created diplomacy and trading and even currency to interact with those other tribes without having to resort to violence or avoidance.

In crises, this is the behavior we fall back to. Yes, some will self-isolate and use violence to keep others away until they feel safe again. But overwhelmingly what we see after natural disasters and spaces where the formal order of civilisation and state is disrupted and leaves a vacuum is cooperation, mutual aid and people taking risks to help others - because we intrinsically know that being alone means death and being in a group means surviving. Of course the absence of state control also often enables other existing groups to assert their power, i.e. organized crime. But it shouldn't be surprising that the fledgling and atrophied ability to self-organize might not be strong enough to withstand a fast moving power grab by an existing group - what might be more surprising is that this is rarely the case and often news stories about "looting" after a natural disaster turn out to be uncharitable descriptions of self-organized rescues and searches.

I think a better analogy for human selfishness would be the mirage of "alpha wolves". As seems to be common knowledge at this point, there is no such thing as an "alpha wolf" hierarchy in groups of wolves living in nature and the phenomenon the author who coined the term (and has since regretted doing so) was mistakenly extrapolating from observations he made of wolves in captivity. But the behavior does seem to exist in captivity. Not because it's "inherent" or their natural behavior "under pressure" but because it's a maladaptation that arises from the unnatural circumstances of captivity (e.g. different wolves with no prior bonds being forced into a confined space, naturally trying to form a group but being unable to rely on natural bonds and shared trust).

Humans do not naturally form strict social hierarchies. For the longest time, Europeans would have laughed at you if you claimed the feudal system was not in the human nature - it would have literally been heresy to challenge it. Nowadays in the West most people will say capitalism or markets are human nature. Outside the West, people will still likely at least tell you that authoritarianism is human nature - whether it's the boot of a dictatorship, the boots of oligarchs or "the people's boot" that's pushing down on the unruly (yourself included).

What we do know about more egalitarian tribal societies is that they often use delegation, especially in times of war. When quick decisions need to be made, you don't have the time for lengthy discussions and consensus seeking and it can be an advantage to have one person giving orders and coordinating an attack or defense. But these systems can still be consent-based: if the war chief is reckless or seeks to take advantage of the group for his own gain, he is easily demoted and replaced. Likewise in times of unsolvable problems like droughts, spiritual leaders might be given more power by the group. Now shift from more mobile, nomadic groups to more static, agrarian groups (though it's worth pointing out the distinction here is not agriculture but more likely granaries, crop rotation and irrigation, as some nomadic tribes still engaged in forms of agriculture) and suddenly it becomes easier for that basis of consent to be forgotten and the chosen leaders to maintain that initial state of desperation and to begin justifying their status with the divine mandate. Oops, you got a monarchy going.

Capitalism freed us from the monarchy but it did not meaningfully upset the hierarchy. Aristocrats became capitalists, the absence of birthright class assignment created some social mobility but the proportions generally remained the same. You can't have a leader without followers, you can't have a ruling class without a class of those they can rule over, you can't have an owning class without a class to rent that owned property out to and to work for that owned capital to be realized into profits.

But just like a monarch despite their divine authority was still beholden to the support of the aristocracy to exert power over others and to the laborers to till the fields, build the castle and fight off foreign claims to power, the owning class too exists in a state of perpetual desperation and distrust. The absence of divine right means a billionaire must maintain their wealth and the capitalist mantra of infinite growth means anything other than growing that wealth is insufficient to maintain it. All the while they have to compete with the other billionaires above them as well as maintain control over those beneath them and especially the workers and renters whose wealth and labor they must extract from in order to grow theirs. The perverse reality of hierarchies is that even those at the top of it are crushed underneath its weight. Nobody is allowed to be happy and at peace.