Rise And Fall Of Rainforests Drove Early Human And Megafauna Extinctions In Asia

Homo erectus flourished on the savannahs of Southeast Asia 1 million years ago, but couldn't adapt as rainforests recovered towards the end of the Pleistocene Era. Peter Schouten

Across Southeast Asia, rainforests have turned to savannah and back again over the last 2.6 million years. These swings led to the extinction of many large animals, probably including Homo erectus, arguably the most successful member of the human family, according to a new paper.

Over 100,000 years ago every continent aside from Antarctica was home to many large animals, known as megafauna. Whether their demise was human or climate driven is among the most heated debates in science. However, curiously little attention has been paid to the lost species of Southeast Asia, noted Dr Julien Louys of Griffith University, despite the presence of at least five members of the Homo genus in the region.

Louys and Dr Patrick Roberts of the Max Planck Institute for the Science Of Human History have addressed this with a paper in Nature reconstructing the diets of various extinct Southeast Asian animals from the isotopes in their teeth. "These types of analyses provide us with unique and unparalleled snapshots into the diets of these species and the environments in which they roamed," Roberts said in a statement. The isotopes can even distinguish animals that browsed the forest floor from canopy specialists.

These isotopes show rainforests were dominant in the early Pleistocene Era but were largely replaced by grasslands 1 million years ago. Grazing animals, such as the elephant's relative Stegodon thrived in the new environment. Many forest dwellers, such as Gigantopithecus blacki, the largest ape to ever live, died out.

Homo erectus, the first member of the human family to leave Africa, and the one that survived the longest, flourished in this environment. Other early human species, including H. floresiensis and H. luzonensis established niches on the islands from which they got their names. For much of this time, the landmass was much larger than it is today, with the now-submerged region known as Sundaland joining Borneo, Java, and Sumatra to the Asian mainland.

In the early Pleistocene Indochina was dominated by rainforests and Sundaland by grasslands, but they later aligned first as Indochina shifted to savannahs, and then as rainforests came to dominate both. Louys and Roberts/Nature

However, a combination of global temperatures and regional geologic changes led to an increase in rainfall, Louys and Roberts report, causing rainforests to expand again, eventually becoming dominant until recent industrial-scale deforestation. In the process, many savannah species died out, including some of our nearest relatives.

"It is only our species, Homo sapiens, that appears to have had the required skills to successfully exploit and thrive in rainforest environments," said Roberts. "All other hominin species were apparently unable to adapt to these dynamic, extreme environments."

Modern humans arrived in the area somewhere between 72,000 and 45,000 years ago, conveniently as rainforests were expanding. Either changing conditions or a new competitor proved too much for the other members of the human family.

Now, however, deforestation, particularly for palm oil plantations, means it is the region's rainforest species that are most at risk, struggling to cope with changes many times faster than those they previously survived.

"Rather than benefiting from the expansion of rainforests over the last few thousand years, Southeast Asian mammals are under unprecedented threat from the actions of humans," said Louys. "By taking over vast tracts of rainforest through urban expansion, deforestation and overhunting, we're at risk of losing some of the last megafaunas still walking the Earth."

With the other humans gone, the three orangutan species are among our nearest relatives, and knowledge of the past may help save them. “Our long-term perspective thus provides critical insights that are relevant to current conservation priorities,” Louys and Roberts write.

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