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Madagascar Population History Suggests Human Expansion Ties to Large Vertebrate Loss

NEW YORK – The giant lemurs, elephant birds, and other large vertebrates that once inhabited Madagascar appeared to start disappearing roughly 1,000 years ago as human populations began expanding on the East African island, according to a new study published in Current Biology on Friday.

"Our study supports the theory that it was not directly the arrival of humans on the island that caused the disappearance of the megafauna, but rather a change in lifestyle that caused both a human population expansion and a reduction in biodiversity in Madagascar," senior and corresponding author Denis Pierron, a researcher with the French National Centre for Scientific Research (CNRS) in Toulouse, said in a statement.

Using array-based genotyping data on samples collected from individuals in hundreds of Malagasy villages over a decade as part of a multidisciplinary Madagascar Genetic and Ethnolinguistic (MAGE) project launched in 2007, together with simulated sequence data, the researchers characterized shared ancestry tracts marked by identity-by-descent haplotype (IBD) sharing in the individuals.

In addition to tracing the present-day populations back to groups with Asian and African ancestry, they brought in available archaeological clues to relate these human population patterns to demographic shifts on the island, including a decline in large-bodied vertebrate representation.

"Specifically, the demographic model based on IBD distribution and local ancestry can accurately detect periods of isolation or population expansion due to external migrations," the authors noted. "The comparison of these models with environmental and cultural data seems to be particularly insightful."

The researchers explained that while the Asian ancestral group seemed to become isolated some 2,000 years ago — remaining a small and isolated population for at least 1,000 years —the arrival of an African group with ancestry from Bantu-speaking populations about 1,000 years ago appeared to prompt speedy population growth and spread into regions where large-bodied vertebrates were previously found.

"This human demographic expansion was simultaneous with a cultural and ecological transition on the island," Pierron said. "Around the same period, cities appeared in Madagascar and all the vertebrates of more than 10 kilograms disappeared."

Though they cautioned that there were potential limitations to the study, including variable mutation rates or generation lengths, the researchers proposed "that for the study of the recent past of populations, the strategy used here, based on the IBD-sharing distribution and simulated genetic data, should be useful for other investigations of human demography and their associated ecosystem impact."