Madagascar was once home to towering elephant birds, giant tortoises, and even giant lemurs. Today no animal heavier than a car tire exists, and researchers have long debated whether humans or climate change were to blame. Now, a study of cave deposits on another Indian Ocean island has helped provide an answer: Unusually dry conditions did make life hard for these giant animals, but humans were the straw that broke the elephant bird’s back.
Sitting 425 kilometers off the southeastern coast of Africa, Madagascar was long thought to be among the last places humans settled. But 2 years ago, researchers discovered butchered elephant bones dating back 10,500 years, suggesting people and giant animals coexisted for millennia there before these megafauna went extinct some 1500 years ago.
To better understand the region’s climatic history, Xi’an Jiaotong University geochemist Hai Cheng and graduate student Hangling Li turned to the caves of Rodrigues—a small, remote island 1600 kilometers east of Madagascar. The island was in “apparently pristine isolation” until recent centuries, says David Burney, a paleoecologist at the National Tropical Botanical Garden on Kauai, who was not involved with the work. That made it the perfect place to analyze speleothems, rocky cave deposits like stalactites and stalagmites that can serve as a record of ancient climates. Human activity, such as fires, can distort that record.
The researchers dated segments of the deposits, which—like tree rings and ice cores—build new layers over time. In some cases, they were able to get decade-by-decade resolution, stretching back 8000 years. Next, they analyzed how heavy oxygen, heavy carbon, and trace elements changed from layer to layer, revealing how wet the climate was at any given point in the past.
The southwestern Indian Ocean underwent four major droughts over the entire period, including extremely dry conditions that coincided with the mass extinctions 1500 years ago, the group concludes today in Science Advances. Because Madagascar’s megafauna had survived previous dry spells—some of them even more severe—Cheng suggests human activity, be it overhunting or habitat destruction, might have been “the last straw.”
The new study gives other researchers “a really clear picture of what was happening” to the overall climate in that region, says Kristina Douglass, an anthropologist at Pennsylvania State University, University Park. She adds that using Rodrigues as a proxy for Madagascar was especially innovative.
But Madagascar is huge, Douglass notes, with a wide range of topography and local climates, and varying degrees of human habitation over time. That means “the path to extinction is going to look different in different places” and requires more local study, she says.
James Hansford, a paleoecologist at the Zoological Society of London who helped discover the butchered elephant bones, thinks the extinctions may have accelerated only after a boom in human populations led to the rise of farming and urban centers—and the destruction of the animals’ natural habitats. But fire, hunting, disease, and invasive species brought by humans may still have played a role, he says.
The new methods are helping “revolutionize” our understanding of how ecosystem changes affect extinctions, says Henry Wright, an archaeologist at the University of Michigan’s Museum of Anthropological Archaeology. And those lessons, he says, carry into the present day.