the east coast of Madagascar collects all the flotsam of the Indian ocean, for the trade winds blow toward it perpetually. Beginning about 500 AD, the flotsam began to include occasional Indo-Pacific sailing vessels, some of which carried enough people to form a viable colony.
Madagascar lies some 6500 km downwind and downstream of Java, so navigation presents no problem. Prior to 1500 AD, when oceans contained somewhere between 10 and 30 times as many accessible edible fish as they do today — and these larger than today's net-selected genetic dwarfs — one could eat and drink indefinitely from the ocean.
By 1000 AD, Arab traders were sailing to and from the northwest coast, whence their dhows could return when the monsoon reversed. Some Indo-Pacific people missed the northern tip of Madagascar and landed in East Africa instead. But they stayed only long enough to pick up some Bantu words and genes before heading southeast for Madagascar.
As a result of these various arrivals, there are now 18 officially recognized tribes. The fact that they have a common language and culture implies that despite their varying physiological types, they come from a common source area. (Compare this with California, where Digger Indian tribes — whose ancestors all came over the Bering Land Bridge — spoke some 200 mutually unintelligible languages!)
The Malagasy tribal territories are often based more on historical tenure and "kingdoms" than on ethnic differences. Having discovered that Europeans use racial differences to sow discord among them, Malagasies play down the genetic and physical differences which are evident between tribes, and attempt to treat them as merely administrative groups.

Alternative form for tribes : tribe.