Until around 50,000 years ago, mysteriously small, archaic-looking early humans lived on the Indonesian island of Flores. The origin and origin of this Homo floresiensis is still a mystery. Now, further fossil finds on Flores, around 700,000 years old, are providing new clues. A tiny humerus – the smallest ever discovered in a fully grown human fossil – proves for the first time that the forerunners of the “Hobbit people” were already extremely small. Their body size was even slightly smaller than that of Homo floresiensis, as the research team determined. Comparative analyses of the arm bone and two newly discovered molars also suggest that these “Hobbit ancestors” did not develop from Australopithecus or Homo habilis, but from Homo erectus.
In 2003, archaeologists discovered very unusual early human fossils in the Liang Bua cave on the Indonesian island of Flores. According to the approximately 50,000-year-old relics, this Homo floresiensis was only a little over a meter tall and had an unusually small brain – it was barely bigger than that of a chimpanzee. Although the hominid lived at the same time as Homo sapiens, it also appeared surprisingly archaic in other anatomical features. Since the discovery of this “hobbit” human in Ling Bua, scientists have been puzzling over how to classify the find in the human family tree. Some initially suspected that these fossils came from pathologically malformed representatives of anatomically modern humans. However, most anthropologists now assume that Homo floresiensis evolved from one of the early humans found in this region. It may have gradually become dwarfed by its life on the isolated island. However, it is still disputed which hominid the hobbit people descended from: some characteristics pointed to Homo erectus as the precursor, others seemed to point to the similarly small Homo habilis or even the even older pre-human Australopithecus.
The smallest arm bone in human history
In 2016, the discovery of a lower jaw fragment and six individual teeth at the Mata Menge site on Flores, around 70 kilometers from Ling Bua, shed some more light on the matter. These finds were similar in size and type to those of Homo floresiensis, but were 700,000 years old and thus much older than the previously known relics of the “Hobbit people”. The teeth and jaws from Mata Menge were even a little smaller than those from Ling Bua. “This suggests that the drastic reduction in the size of the teeth and jaws on Flores already took place in the early Middle Pleistocene – more than 600,000 years before the earliest known fossils of Homo floresiensis,” explain Yousuke Kaifu from the University of Tokyo and his colleagues. These presumed ancestors of the “Hobbit people” were therefore similarly dwarfed to them. However, because further skeletal parts were missing, the exact classification and relationship of these relics also remained controversial.
Now further finds in Mata Menge are shedding more light on the matter. Kaifu and his team have now discovered the lower part of an upper arm bone (humerus) and two further teeth in the same layer as the tooth and jaw relics. The piece of bone is around 8.8 centimetres long and has a circumference of 4.6 centimetres. Using anatomical models, the team determined that the upper arm bone must have been between 20.6 and 22.6 centimetres long before it broke. Despite this small size, the tissue morphology of the bone clearly shows that it is the bone of an adult, the researchers emphasise. “This 700,000-year-old, fully grown upper arm bone is not only shorter than that of Homo floresiensis, it is also the smallest upper arm bone ever found in a hominin worldwide,” says co-author Adam Brumm from Griffith University in Australia. Even small hominin forms such as Homo naledi or Australopithecus had longer and thicker upper arms.
Homo erectus as an ancestor?
“These rare finds confirm our assumption that the ancestors of Homo floresiensis were extremely small,” says Brumm. According to the team’s estimates, the early human from whom the bone came was between 93 and 96 centimeters or between 103 and 108 centimeters tall, depending on the extrapolation model. The hobbit people from Ling Bua, on the other hand, were 102 and 121 centimeters tall, respectively, according to the same models. “It is now obvious that early forerunners of the ‘hobbits’ were even smaller than we initially thought,” says Brumm. Comparative analyses also showed that the earlier and current finds from Mata Menge come from four different individuals – an older adult, a teenager or young adult, and two children. “The fact that all four individuals were extremely small supports the argument that small body size was not an idiosyncratic, merely individual trait, but a population trait of these Flores hominins,” explain Kaifu and his colleagues.
The new fossil finds also provide further clues about the origins of Homo floresiensis and its ancestors. The morphology of the newly discovered molars, as well as the humerus, shows some similarities to that of Homo erectus from Java. “The hypothesis that Homo floresiensis is a direct descendant of Homo habilis, however, is not confirmed,” write the researchers. The cross-section of the humerus is also more similar to that of small Homo species such as Homo naledi and Homo floresiensis than to that of Australopithecus. According to Kaifu and his team, the new finds therefore support the hypothesis that the dwarf people of Flores developed from representatives of Homo erectus. “The evolutionary history of Homo floresiensis is still largely unclear,” says Brumm. “But the new fossils strongly suggest that the story of these ‘hobbits’ actually began when a group of Homo erectus was transported to this remote Indonesian island perhaps a million years ago, and then over time developed a dramatic reduction in their body size.” This dwarfism may have made it easier for this form of human to survive despite the limited resources on the island.
Source: Yousuke Kaifu (University of Tokyo) et al., Nature Communications, doi: 10.1038/s41467-024-50649-7