Scientists Find Arm Bone of Ancient ‘Hobbit’ Human

A new study describes 700,000-year-old teeth and arm bones from one of our most enigmatic relatives: a toddler-size “hobbit” who lived on a small island between the Indian and Pacific Oceans.

The study, published on Tuesday in the journal Nature Communications, suggests that the species, Homo floresiensis, sometimes nicknamed hobbits, could be even smaller than previously thought. But the results still left scientists divided over how such exceptional humans evolved.

The hobbits were first discovered 20 years ago inside the Liang Bua cave on the Indonesian island of Flores. Australian and Indonesian scientists uncovered bones and teeth, along with stone tools that were most likely used to butcher meat.

Based on those bones, the researchers estimated that Homo floresiensis stood 106 centimeters tall — about three and a half feet. More remarkable than its short stature was its minuscule brain, about one-third the size of a modern human’s. Analyzing the cave floor, scientists determined the Homo floresiensis bones were somewhere between 100,000 and 60,000 years old.

The sensational discovery left scientists struggling to fit Homo floresiensis into the family tree of humans and their extinct relatives, a group known as hominins. The oldest known hominins were short, small-brained apes. But by two million years ago, they had largely been replaced by taller hominins with much bigger brains.

Some scientists hypothesized that the bones came from humans with growth disorders. But many researchers rejected that explanation, because the anatomy of people with those growth disorders today doesn’t closely match that of the fossils.

The debate took a turn in 2016, when researchers reported a tantalizing batch of much older fossils from another area of Flores, called Mata Menge. The fossils, dating back about 700,000 years, consisted of six teeth and part of a jaw. The Mata Menge fossils were as small as those found at Liang Bua — or even smaller.

And on Tuesday, the Mata Menge team unveiled two more tiny teeth, as well as a piece of a humerus, the upper bone of an arm.

“We couldn’t tell if this belonged to a child or adult,” Yousuke Kaifu, a paleoanthropologist at the University of Tokyo, said. “That’s a key question.”

Dr. Kaifu and his colleagues compared the humerus with arm bones from modern-day children and adults. The Mata Menge bone had many signs that it had stopped growing, suggesting the individual was an adult.

That was striking because the Mata Menge humerus was tiny — the smallest humerus of any adult hominin yet found. Dr. Kaifu and his colleagues estimated that the arm’s owner had stood just 100 centimeters tall — less than 3 feet 4 inches.

The fossils found so far at Mata Menge appear to have come from at least eight individuals. The newly revealed teeth and humerus make Dr. Kaifu and his colleagues more confident that the Mata Menge hominins were indeed Homo floresiensis.

Matthew Tocheri, a paleoanthropologist at Lakehead University in Canada who was not involved in the study, agreed. “I mean, what else could they be?” he asked. “Any alternative seems incredibly unlikely.”

However, Deborah Argue, a paleoanthropologist at Australian National University, said she was unconvinced. “More skeletal material of this enigmatic hominin will help in determining if this group represents Homo floresiensis,” she said.

Dr. Kaifu and his colleagues argued that Homo floresiensis evolved from a tall species of hominin called Homo erectus. Originating in Africa, Homo erectus reached Java by about 1.3 million years ago and survived there for more than a million years.

The researchers proposed that Homo erectus traveled 450 miles east from Java to Flores, arriving on the island about a million years ago. That’s the age of the oldest stone tools found there. Once isolated on Flores, Homo erectus then shrank, reaching hobbit stature by 700,000 years ago, the scientists said.

Dr. Kaifu speculated that a scarce food supply on the island might have driven the extraordinary evolution of Homo floresiensis. And in this particular place, their small size did not increase their risk of being killed by predators.

“If you go to an isolated island where there are no lions or tigers, you don’t have to be big,” Dr. Kaifu said.

This shrinking also drastically reduced the brain of Homo floresiensis, the theory goes. And yet the presence of stone tools on Flores suggests that the hobbits still retained powerful mental capacity.

“That’s quite shocking to me,” Dr. Kaifu said. “We thought that becoming clever and having larger brains was destiny for humans. But floresiensis tells us that it’s not necessarily so.”

Dr. Argue did not accept that evolutionary scenario, given the dearth of direct fossil evidence that Homo erectus migrated to the island. “We cannot assume that this species ever arrived on Flores,” she said.

Dr. Tocheri also thought the Homo erectus hypothesis was a stretch. “The evidence for this idea is very weak,” he said.

Until more fossils emerge, Dr. Tocheri and Dr. Argue both said that other explanations remain plausible. It’s even possible that Homo floresiensis descended from hominins in Africa who were already tiny when they moved across Asia.