If it is true that genetically and linguistically, Pacific Islanders are descended from people who migrated out of insular South-East Asia several millennia ago, why do they look so different? Do Polynesians have substantial Melanesian ancestry, or have Polynesians genuinely diverged genetically from their Asian cousins? Or is it the people who stayed in South-East Asia that have mixed with other non-Austronesian peoples?
According to this study, they moved slowly through Melanesia and there they picked up some Melanesian genes. Although in my opinion Polynesians don't look too different from other Austronesians. In any case, interbreeding with Australoids happened also in the whole SE Asia - Melanesian populations onco inhabited the whole area and there is even some speculation there were Negritos in today's China. These primitive populations were replaced by more developed Austronesians in most of the area, but there was some gene flow between the populations. In the Philippines there are Melanesian negritos to this day and the Moluccas are inhabited my mixed Austronesian and Melanesian populations. The admixture in the Philippines is several percent at most, according to genetic studies.