[Retrieved from, let's say, a dimensional portal.]
Don't be absurd. Intelligent, loyal followers offer a massive reproductive advantage; parenting offers the best chance to indoctrinate the children of the Tribe. As the Jesuits used to say, “Give me a child for the first seven years and I will give you a man.” Thus, while in many species (lions, for example) the leader of the pack will <i>kill</i> his vanquished rivals' children, and replace them with as many of his own as possible; in humans the advantage of retaining these children as your own was too great to give up.
Indeed, even today … many third-world cultures organize themselves into extended families, governed by a central "Allfather" and his wives. But as prehistoric humans clustered into bands and eventually villages, it became increasingly difficult for a single individual to raise the children; first childrearing was split along gender lines, with most of the work being done by his harem, and eventually delegated to trusted lieutenants and their families. (This is likely the reason behind Robin Hanson's interesting discovery [sure, he exists in this universe] that a village is a “family of families”, numerically, and a city is a “village of villages”.)
The emergence of cities is often cited as straining even these extended bonds, with subordinates splitting off into many different patrons with no official tie to central government; but this is too simplistic. In truth, most cities and large towns did not actually begin as ever-larger, expanding villages; so this is simply untenable as an explanation. A glance at any map will provide the real solution … most cities remain divided into various ghettos, regions, and slums depending on each area’s purpose and inhabitants. City centres, Chinatowns, or simply "bad parts" or "good parts" of town … all hint at the city as an inherently different, divided form of habitation. And indeed, looking at older cities such as London, we see that many of their place-names and regions are those of older towns and villages, swallowed by the City.
Quite simply, when economics and the emerging new technologies of mass production demanded it, these familial tribes and their adopted children moved to centralized overlapping locations … while retaining their power structure. Leadership, then, became less a matter of personal power and more a question of brokering deals between these larger players. This, presumably, is the reason the Renaissance was rapidly followed by a disintegration of monarchy and kingship … while the churches, overwhelmed by the poor, lost their previously firm footing on the sons and daughters of peasants and the dispossessed. Their place was taken by captains of industry, new players who quickly seized upon the opportunity left open by the now-departed Divine Right of Kings. Without parenting concentrated in the hands of one dictator and his cronies, the path was clear to democracy - at last, the most able could rise to the task of training the next generation.
Without highly-paid experts raising each of us, would we have seen technology make such great strides - curing diseases, shrinking distances, even carrying this humble comment to your eyes? Without rags-to-riches success stories parenting each of us - would we have seen modern Social Justice movements ending slavery, granting women the right to vote … tearing down social barriers of every kind? Without the tribal chieftains of yore giving us a central place of learning, passing on stories and culture and simple tips for survival … would humanity have <i>ever</i> become something more than a simple upright ape with delusions of grandeur; discovering everything anew only to have it wiped away with every generation? Without adoption, would we even exist?
I suppose we’ll never know. It seems implausible.
[This is deliberately a touch sexist in places, my apologies. I think it holds together as worldbuilding, though.]