Ancient giant orangutans evolved smaller bodies surprisingly slowly

Giant orangutans that once dwelled in mainland Southeast Asian forests belonged to a single species that gradually shrank in size over nearly 2 million years, a study suggests.

Today, orangutans are found only on the islands of Sumatra and Borneo. But their ancient, super-size kin roamed forests in what’s now southern China and northern Vietnam. Fragmentary Asian fossils of uncertain age have long indicated that these massive, now-extinct orangutans shrank over time. And biological anthropologist Terry Harrison of New York University suspected — based on a small number of fossils from widely different time periods — that the apes rapidly evolved from a larger-bodied species to a different, smaller-bodied species roughly 400,000 years ago as the climate cooled.

But an analysis of 600 ancient orangutan teeth unearthed in 10 caves in southern China supports a different evolutionary scenario, Harrison and colleagues report in the December Journal of Human Evolution.

From around 2 million to 111,000 years ago, the shapes of the teeth remained largely the same, suggesting all were from a single orangutan species. But tooth sizes progressively declined. Using tooth measurements, Harrison, paleoanthropologist Yingqi Zhang of the Chinese Academy of Sciences in Beijing and colleagues estimate that the ancient orangutans’ average body mass started out around 96 kilograms, close to double that of orangutans today.

By around 111,000 years ago, the ancient apes’ average body mass was almost 80 kilograms, which still exceeded that of modern orangutans by nearly 25 kilograms.

Fossils of other ancient Asian animals, including rhinos and monkeys, also show declines in body size over the same period. Cooler, drier conditions that reduced available food starting around 400,000 years ago may have spurred a trend toward smaller bodies, Harrison says.

It’s not clear when orangutans on mainland Asia died out. But climate change and possibly the arrival of humans in the region more than 60,000 years ago contributed to their demise, Harrison speculates.