Life wasn't easy for the first humans to settle in the islands of Southeast Asia. The rain forest was a completely new environment for people in the Late Pleistocene, so the learning curve was probably steep. A 28,000-year-old jawbone from Niah Cave in Northeast Borneo reveals that Pleistocene people who first arrived there ate a tough diet.

The mandible belonged to a person who lived and died in Borneo nearly 10,000 years before the end of the last Ice Age, about 28,000 to 30,000 years ago, according to uranium-series dating (there wasn't enough collagen left in the bone for radiocarbon dating). It's small, but it's also unusually thick. Even without any other bones, the jaw tells us two important things about the Southeast Asians of the Pleistocene.

First, they were small. The jawbone is part of an adult mandible, but its height points to a person of short stature and small body size. That's something the ancient Niah Cave person has in common with modern indigenous people of the highlands of Borneo and the Philippines. It's also a very practical adaptation to life in the rain forest, according to University of New South Wales archaeologist Darren Curnoe—another way human diversity has been shaped by our long relationship with our environments.

Despite its small size, the Niah Cave mandible is very robust, especially at points where chewing muscles attach and at the base of the molars, which means this person did a lot of tough chewing.

"It is a strikingly ruggedly built little jaw," Curnoe told Ars. "The jaw is massive from side to side, and it has distinct markings and well-sculpted grooves where very large muscles were attached to move and support it during chewing."

That's not an inherited trait; like strong muscles and bones elsewhere in the body, a robust jaw comes with exercise. Especially during childhood development, the bone grows thicker to handle the strain. "It seems more likely the Niah Cave people were using their molar teeth at the back of their jaws to chew heavily on tough foods, as this is where their jaw bones are thickest."

Studies of Arctic hunter-gatherers found that a steady diet of stored or dried meat led to similar thickening of the jaw. But what were people in Pleistocene Borneo chewing on?

Niah Cave diet

Unfortunately, the ancient jawbone contained no teeth, which archaeologists could have examined for microscopic wear marks or chemical isotopes that could reveal details about ancient diets. The archaeological record in Niah Cave offers some information, however. Animal bones are abundant in the cave, suggesting that people here ate large amounts of meat.

And granules of starch found in the sediment in the west mouth of Niah Cave, near where the jawbone was discovered, suggest that various species of palm were an important part of the Niah Cave diet, providing palm hearts, fruit and nuts, and even leaves. There's also evidence that yams were staples.

Except for the yams, which had to be cooked in order to be safe to eat, people would have eaten most of their food raw; there's little evidence of hearth fires or cooking in Niah Cave, and most modern hunter-gatherers eat their food raw—and really fresh.

"In fact, most of the diet of hunter-gatherers is raw food and was often eaten as it was being collected, making a simple, nutritious, and convenient meal," Curnoe told Ars.

Raw, or perhaps dried, meat and raw plants may be nutritious and quick, but they make for some tough chewing, which explains the robustness of the jawbone.

Bones with stories to tell

The jawbone and other fossils were unearthed by archaeologists in 1957, but they sat in archives until recently, and some of them were even filed among faunal remains. That's a little surprising, since Pleistocene human remains are rare in the islands of Southeast Asia, where the climate doesn't lend itself to preserving bones for thousands of years.

One of the few examples from the region is a partial skull called the Deep Skull, a 30,000- to 39,000-year-old specimen from Niah Cave, and it has been the focus of 60 years of research on the ancient people of Borneo; these other, more fragmentary bones were ignored in archives.

"There are many fragments of human jawbone, teeth, and parts of the limbs that had sat in the museum collection untouched since they had been found in the 1950s and 1960s. They, too, needed to tell their story about the ancient inhabitants of Borneo," said Curnoe, whose team obtained permission from the Malaysian government for its research.

PLOS One, 2018. DOI: 10.1371/journal.pone.0196633 (About DOIs).