Hobbit hominins from Indonesia may have had even smaller ancestors

Hominins living on an Indonesian island 700,000 years ago were even smaller than Homo floresiensis, the so-called hobbits that lived on the same island much more recently. Newly analysed fossils may represent the hobbits’ ancestors – but the evolutionary story of these small-bodied hominins is still shrouded in mystery.

Fossils of H. floresiensis were first discovered in 2003 in Liang Bua cave on the island of Flores. The hobbit bones date from between 90,000 and 50,000 years ago.

In 2016, Yousuke Kaifu at the University of Tokyo and his colleagues uncovered hominin remains from Mata Menge, an open-air site further east on Flores that was once a riverbed. The remains are about 700,000 years old and include part of a skull, a piece of jawbone and six teeth, all unusually small for a hominin.

The obvious interpretation was that the Mata Menge hominins were the ancestors of the hobbits. But because the remains were so fragmentary, it wasn’t possible to be confident.

Kaifu and his colleagues have now described three new remains from Mata Menge: two teeth and, crucially, a piece of an upper arm bone, or humerus. With this limb bone, “we could finally determine the body size,” says Kaifu.

Unfortunately, the humerus isn’t complete: the shaft is snapped. To determine exactly how far along the break occurred, the team looked for key markers, including a groove that supports a nerve and the attachment point for a muscle. Using these clues, they determined that the bone had broken about halfway along – enabling them to estimate its total length as between 20.6 and 22.6 centimetres.

There are telltale features of the microstructure of the bone that confirm it is from an adult. Extrapolating from the humerus to the entire body, the team estimates the Mata Menge hominin was between 93 and 121 cm tall, with a best estimate of 100 cm. That is a little shorter than the H. floresiensis specimens from Liang Bua, which Kaifu says were at least 6 cm taller – and would make it the smallest adult hominin ever found.

The findings point to a likely explanation for the evolution of H. floresiensis, says Kaifu. It has long been suspected that the species was descended from large-bodied hominins called Homo erectus, which are the first hominin species known to have lived outside Africa – including on Java in Indonesia about a million years ago. “I’m almost sure that they are derived from those populations,” says Kaifu. This is because of similarities between the teeth from Mata Menge and those of H. erectus from Java, and the close proximity of the dates and locations.

The suggestion is that a small population of H. erectus reached Flores, possibly by accident, and lived there in isolation. They must have then evolved a smaller body size within 300,000 years, says Kaifu. “They were small early and then they remained small for a long, long time,” he says.

It’s common for island-dwelling animals to shrink through evolution, because food resources are limited and the lack of large predators means there’s no advantage to being bulky. In line with this, Flores was home to dwarf elephants and other species that had shrunk over many generations.

However, there are alternative explanations, according to Debbie Argue at the Australian National University in Canberra, author of Little Species, Big Mystery: The story of Homo floresiensis.

Argue points out that the Mata Menge teeth don’t look especially similar to the H. floresiensis teeth from Liang Bua. For instance, a molar from Mata Menge has five pointed “cusps”, while H. floresiensis molars have four. “There’s no clear indication of anyone evolving into anyone else,” she says, and it’s not clear why the later H. floresiensis would have evolved slightly larger bodies than their Mata Menge ancestors. Furthermore, “there’s no evidence for Homo erectus from the island.”

For these reasons, Argue says we shouldn’t assume that the Mata Menge hominins are the ancestors of the hobbits. “I would be considering another hypothesis, that the Mata Menge hominins are a new unknown species.” If island life could cause one hominin population to evolve smaller bodies, it could do so twice, she suggests.

In 2017, Argue and her colleagues compared H. floresiensis with other hominins and concluded that their closest known relatives weren’t H. erectus, but instead an older species called Homo habilis, which is only known from Africa. On this basis, they proposed that H. floresiensis actually evolved in Africa, from the same ancestral population that gave rise to H. habilis. Later, some of them migrated east, ending up on Flores. Argue says we probably need more fossils to resolve the question of the hobbits’ origins.