Final answer:
The Indonesians from Southeast Asia are believed to have first settled Madagascar 1,500 years ago.
Step-by-step explanation:
The group of seafarers believed to have first settled Madagascar 1,500 years ago were Indonesians. These early human inhabitants originated from the regions of Malaysia and Indonesia in Southeast Asia. Along with them, people from the African mainland also migrated to Madagascar, contributing to its diverse cultural heritage. The island's distinct environmental conditions and historical cultural developments are unique, including its later colonial period under France and its independence achieved in 1960.