After the Torah's account of Avraham's other sons, Zimran, Jokshan, Medan, Midian, Ishbak and Shuah, being sent to the East with gifts, do we have any tradition about what became of them? Did they play any significant role in the continuation of history?
They possibly were responsible for the synthesis of Hinduism. This can be found in two back-to-back verses in Genesis, Gen. 25:5 and 6. The translation of the first is “And Abraham gave everything he possessed to Issac.” The very next sentence is “And to the sons of Abraham’s concubines, Abraham gave gifts, and he sent them away… eastward, to the land of the East.” The obvious question, of course, is “If Abraham gave everything he possessed to Issac, what gifts did he have left to give to the sons of his concubines?” The answer is gifts of knowledge. (“Gifts” here is also spelled strangely to indicate that there is something unusual about them.) A short while (100-200 years) after they were sent out eastward to the land of the East, the Brahmins entered India from the northwest (traveling eastward). The name of BRaHMa is the same as aBRaHaM. The name of Brahma’s consort is Sara, as Abraham’s wife. (She is commonly called Sarasvati, but the suffix -svati is an honorific – the name itself is Sara.) Furthermore, the names of some cities in the northeast of India, where the Brahmins entered, are similar to some of the names of Abraham’s sons.
ADDITION: Rav Yaakov Weinberg zt"l makes this assertion, not with the same level of historical analysis but he definitely says that Avraham's children who went to the East became the roots of Eastern religion. He explains that their focus on pure Being and a Oneness of Existence is predicated on the Middah of Chessed, without ever taking it further to Din as Yitzchak did. The fascinating shiur is freely available here: https://www.dropbox.com/s/nbwrlgytina7692/Buddhism%20and%20Religions%20of%20the%20East.mp3?dl=0
The Zohar I:100b records a lengthy, fantastic story about R' Aba who visited a city in the East and learned things from them that sounded just like Torah. R' Aba then tells them (translated into Hebrew):
אָמַרְתִּי לָהֶם, בָּנַי, דָּבָר זֶה קָרוֹב לְדִבְרֵי תוֹרָה, אֲבָל יֵשׁ לָכֶם לְהִתְרַחֵק [מתוך] מֵאוֹתָם הַסְּפָרִים [הללו] כְּדֵי שֶׁלֹּא יִסְטֶה לִבְּכֶם לָעֲבוֹדוֹת הַלָּלוּ וּלְכָל אוֹתָם הַצְּדָדִים שֶׁאָמַר כָּאן שֶׁמָּא חַס וְשָׁלוֹם [לא] תִּסְטוּ מֵאַחַר עֲבוֹדַת הַקָּבָּ''ה, שֶׁהֲרֵי כָּל הַסְּפָרִים הַלָּלוּ מַטְעִים אֶת בְּנֵי הָאָדָם, מִשּׁוּם שֶׁבְּנֵי קֶדֶם הָיוּ חֲכָמִים, וִירֻשַּׁת הַחָכְמָה הַזּוֹ יָרְשׁוּ מֵאַבְרָהָם שֶׁנָּתַן לִבְנֵי הַפִּילַגְשִׁים, שֶׁכָּתוּב (בראשית כה) וְלִבְנֵי הַפִּילַגְשִׁים אֲשֶׁר לְאַבְרָהָם נָתַן אַבְרָהָם מַתָּנֹת וַיְשַׁלְּחֵם מֵעַל יִצְחָק בְּנוֹ בְּעוֹדֶנּוּ חַי קֵדְמָה אֶל אֶרֶץ קֶדֶם. וְאַחַר כָּךְ נִמְשְׁכוּ בְּאוֹתָהּ חָכְמָה לְכַמָּה צְדָדִים.
I said to them: My children, this matter is close to words of Torah, but you should distance yourself from these books so as not to be strayed into those forms of worship, lest you stray from worship of God. For behold, all these books cause man to err, for after all the People of the East are wise men, and their inheritance of wisdom was inherited to them from Avraham who gave it to the children of his concubines. as it says, "And to the children of concubines which were to Avraham, he gave them gifts and sent them from Yitzchak his sons while he was still alive, eastward to the land of the east".
Rav Yaakov Weinberg zt"l makes the same assertion that Avraham's children who went to the East became the roots of Eastern religion. He does not delve into the same areas of historicity as A. Goldstein did, but he explains that their focus on pure Being and a Oneness of Existence is predicated on the Middah of Chessed, without ever taking it further to Din as Yitzchak did. The fascinating shiur is available here: