Why are the Malagasy people considered unusual compared to other populations living near Africa?

That split origin is the whole puzzle. Every other population living close to Africa draws most of its ancestry from the continent or from nearby regions. The Malagasy do not.

Two founding peoples, one island

Half of the genetic lineages of the Malagasy Motif come from roughly 4,500 miles away in Borneo, while the other half derive from East Africa. Research has been able to assign every genetic lineage in the population to a likely geographic origin, confirming that the Malagasy are a roughly 50/50 mix of two ancestral groups: Indonesians and East Africans.

The Asian half is not vague. Linguistic evidence shows that the Malagasy language branches from the Southeast Barito language family of southern Borneo, with the closest living language spoken today by the Ma'anyan people. The language traveled with the people. The Indonesian colonizers reached Madagascar by a maritime crossing estimated to have taken place around 650 CE. That is a voyage across open ocean, with no GPS and no guarantee of landfall.

Geneticists used a specific maternal lineage marker, sometimes called the Malagasy Motif, to estimate that the founding maternal pool of Madagascar consisted of approximately 30 women. Thirty women. An entire branch of humanity on the world's fourth-largest island traces back to that small a founding group, which is one of the strongest founder-effect signatures, where a small original group shapes the genetics of every descendant, known in human genetics.

What the mix looks like today

The Malagasy trace their ancestry to eastern African Bantu-speaking populations and Austronesian groups most likely from modern Borneo. These ancestral groups likely settled the island at different times and in waves over the course of 1,000 to 2,000 years, with the mixing of the two populations beginning approximately 28 generations in the past.

The proportions are not uniform across the island. The average proportion of African-related ancestry is higher on the West Coast at roughly 66 percent, while the Central Highlands carry a higher proportion of Austronesian-related ancestry, sitting closer to 41 percent African.

The mixing even drove natural selection: strong recent positive selection for African ancestry affects 25 percent of chromosome 1, including the Duffy blood group gene, whose null allele provides resistance to Plasmodium vivax malaria. The island's disease environment, in other words, was actively shaping which ancestral genes survived and spread.

The deeper point is this: the Malagasy are a living record of one of the most audacious ocean crossings in human history. A small group of Southeast Asian mariners crossed the full width of the Indian Ocean, landed on an uninhabited island, met migrants coming west from Africa, and together built an entirely distinct civilization. Their language, their genetics and their culture all point east, not to the continent just 400 kilometers away.