migration in ancient world, especially to India

[Aemilius]

From this link we can see that in India there have lived, and there still live, peoples from other continents that are far away, like negritos from Africa, aboriginals from Australia, dravidians from Mediterranean, and lastly aryans whose origin is unkown, or is under dispute.
http://www.prokerala.com/kerala/people.htm
How does this connect with buddhism?, you may ask, the answer is that the appearance of a Buddha naturally attracts people from all over the world, before Shakyamuni there have been other Buddhas, and therefore this migration to India in a very ancient period of history.

[Indrajala]

I think the Buddhadharma first appeared in India because at the time it was wealthy, liberal and free enough for it to even arise.

Generosity and wealth was widespread enough so that a monastic institution could flourish. Kingdoms and people were liberal enough to tolerate critics who would call their creator god Brahma a mortal being subject to death. Finally, it was free and safe enough a place that monks and nuns could wander around unmolested for the most part. In the 6th century the Indian subcontinent was relatively quite nice. China was in constant warfare. To the west things were no better.

[Aemilius]

Tathagata Shakyamuni discusses this theme somewhere in the Pali Scriptures, he says that society must have achieved certain development before a Tathagata can appear, and only then his appearance will have beneficial consequences. The issue appears also in the abhidharma, according to Herbert Guenther there have to be four factors present in the society at large, in summary: 1. belief in karma and rebirth, 2.belief in the lower and higher states of existence, 3. belief in the existence of persons liberated from the round of birth and death, 4. belief in the path of practice. This list is in a short abhidharma text that Guenther has translated in the book Mind in Buddhist Psychology. The teaching is present somewhere in the tripitaka.
I think the system of shramanas, or free ascetics outside the brahmin caste, can be, to some extent, compared to the system of university education of the modern society. In truth Indian society did not support a system of monasticism in its later developed form, rather it allowed the existence of various schools of free ascetics, and supported them through the practice of alms giving, and by building halls for public debate. To start with the shramanas were homeless.