The equality of all mankind is not a Jewish idea. It is a Greek/Roman/Western-democratic idea that Jews have assimilated over time. Humanity does not all exist on one level. Let's look at how Rabbi Moshe Chaim Luzzatto, ztz"l explains it with commentary provided by Torah.org's Rabbi Yaakov Feldman...
Derech Hashem, Part II, Chapter 4 - Israel and the Nations, Paragraph 3
 The Highest Wisdom deemed it fitting that this effort be divided into a period for the roots, and another for the branches. The original effort would thus be that of the roots, while what would come later would involve the branches.
The human race initially had a chance to permanently regain its original state and rectify the spiritual damage that had been done. The proper procedure would have been for the roots and heads of Adam’s descendants to first elevate themselves to the rectified level. Once this was accomplished, both the roots and their branches would remain in this state forever, since the branches always follow the roots.
The time provided for generations to function as roots, however, was limited. During this period, the gate was open and the opportunity existed for any individual to properly prepare himself and permanently become a good and worthy root. He would then be prepared for a high degree of excellence, appropriate for man in his original state, rather than that of man in his fallen state.
Since this individual would perfect himself as a Root, he would attain this for his deserving descendants as well as for himself. They would all receive what he attained, and would therefore all be able to remain on the level and state attained by him as their root.
The period during which this was possible extended from the time of Adam until the Generation of Separation [when the Tower of Bavel was built]. During this period there never ceased to be some righteous people who preached the truth to the multitudes, warning them to correct themselves. These included such individuals as Chanoch, Mesushelach, Shem and Ever.
Man's measure was filled, however, in the Generation of Separation. God's Attribute of Justice then decreed that the time when men could be considered roots should some to a close. Until this time, things could become a permanent part of these roots, depending on what had transpired previously. With the Generation of Separation, this period came to a close.
God then scrutinized all mankind, perceiving the levels that should be made permanent in that generation's members according to their deeds. These things then became a permanent part of their nature in their aspect as roots. It was thus decreed they each should bear future generations, all possessing the qualities that were deemed appropriate for their root ancestor.
The descendants of each of these individuals were thus divided into permanent groupings, each with its own characteristics and limitations. They were destined to father future generations who would inherit these characteristics of their forebears. According to the Highest Judgment, it turned out that none of them deserved to rise above the degraded level to which Adam and his children had fallen as a result of their sin. [Not a single one had risen above it all.]
There was, however, one exception, and that was Avraham. He had succeeded in elevating himself, and as a result of his deeds was chosen by God. Avraham was therefore permanently made into a superior excellent Tree, conforming to man's highest level. It was further provided that he would be able to produce branches [and father a nation] possessing his characteristics.
The world was then divided into seventy nations, each with its own particular place in the general scheme. All of them, however, remained on the level of man in his fallen state, while only Israel was in the elevated state.
After this, the gate was closed on the era of roots. Things would then be directed and brought about upon individuals as branches, each one according to his nature.
Even though it may seem that man was originally the same as he is now, there is actually a great difference. Before the Generation of Separation, man existed in the age of roots, and was dealt with accordingly. When this period ended, things were judged and made permanent, and a new era began. This is the age of branches, which still exists.
Torah.org - Rabbi Yaakov Feldman
Many of us work on the assumption that "human nature is human nature" and that our inner beings are no different from what our ancestors' were in antiquity, but that's not so. Let's see how.
Recall that originally there were "root souls" and "offshoots". The root souls were to have regained Adam and Eve's original high spiritual stature, their offshoots were to follow in their wake (i.e., to have "inherited" perfection, if you will), and all of humanity was to have remained on that exalted plane forever.
There were two stipulations, though. One was that there was to have been a fixed period in antiquity for this to have happened by, as we said: from the time of Adam and Eve themselves up to the time of the destruction of the Tower of Babel (see Genesis 1:26 to 11:1-8). The second stipulation was that *anyone* within that time period who strove to could have perfected himself and become a root soul, and all his descendants would have reaped the benefits of his efforts.
Now, this is a very important point for our purposes. There were indeed people at that time who strove for personal perfection -- like Enoch, Methuselah, Shem, and Eber. What we're to realize, though, is that they could very well have become roots souls to *their particular descendants*, who'd then have served as bearers of G-d's message for mankind, as we Jews do. But none of those individuals did. Only Abraham, the father of the Jewish Nation, did. Which is why only we, his descendants, came to inherit his spiritual bounty.
Thus when the time-limit was up and there proved to be no one other than Abraham worthy of being a root soul in a spiritual sense, G-d saw to it that other individuals would serve as root souls to their offshoots only on a genetic, cultural level.
G-d is thus said to have *chosen* Abraham to be the sort of ideal root soul depicted above as a consequence of his having succeeded at elevating himself. And we Jews, his offshoots, have thus been "chosen" -- chosen to glory in Abraham's inheritance and to carry out his mission.
There came to be 70 primal nations in all (with many subsequent subdivisions). And each plays its own particular role in the larger scheme -- while yet remaining on the level of man in his fallen state.
Thus while mankind may seem the same as it always has been, there's actually a profound difference. For, again, up to the time of the Tower of Babel all of mankind existed in the age of potential root souls and was dealt with accordingly; while afterwards a new age began -- the age of offshoots, which we're still in the midst of.