Human ancestors called ‘Hobbits’ of Flores were smaller than we imagined

The discovery of rare early human fossils on the Indonesian island of Flores, including an extraordinarily small adult limb bone, is providing crucial insights into the evolution of Homo floresiensis, the so-called “Hobbits of Flores.”

The findings, which date back to approximately 700,000 years ago, are detailed in a new study published in the journal Nature Communications.

The remains of Hobbits were first uncovered in 2003 at Liang Bua cave by a team co-led by the late Professor Mike Morwood, an Australian-New Zealand archaeologist.

Unknown origins of Homo floresiensis

Archaeological evidence indicates that these diminutive, small-brained humans inhabited Liang Bua as recently as 50,000 years ago, a period when Homo sapiens were already well-established in Australia to the south. 

The origins of the mysterious Homo floresiensis have been widely debated, with initial hypotheses suggesting they were a dwarfed descendant of early Asian Homo erectus. 

Another theory posits that the Hobbit could be a late-surviving remnant of a more ancient, small-statured hominin from Africa, such as Homo habilis or Australopithecus afarensis (the species of the famous ‘Lucy’).

Hominin fossil sites on Flores

Besides Liang Bua, hominin fossils on Flores have only been found at Mata Menge, an open-air site located 75 km east of the cave in the So’a Basin’s tropical grasslands. 

Previously, this site yielded several hominin fossils, including a jaw fragment and six teeth, from a sandstone layer dating back around 700,000 years.

These fossils pre-date the Liang Bua hominins by 650,000 years and suggest that small body size evolved early in the history of Flores hominins. 

However, without postcranial elements (bones from below the head), it was unclear if these So’a Basin hominins were as small, or even smaller, than Homo floresiensis.

Rare Homo floresiensis fossils

The new study, led by Professor Yousuke Kaifu of the University of Tokyo, Iwan Kurniawan of the Center for Geological Survey in Indonesia, and Professor Gerrit van den Bergh from the University of Wollongong, reports the discovery of three additional hominin fossils from Mata Menge. 

This discovery resulted from several seasons of field excavations and includes the first postcranial element – a distal shaft of an adult humerus (the lower half of the upper arm bone, known as LB1).

The recovery of this fossil limb bone is significant as it provides extensive evidence about the ancestral origins of Homo floresiensis. Digital microscopy of the microstructure confirmed that the small humerus belonged to an adult individual. 

Smallest upper arm bone in the hominin record

The estimated length of the bone allowed the team to calculate the body height of this hominin to be approximately 100cm, which is around six cm shorter than the estimated height of the 60,000-year-old Homo floresiensis skeleton from Liang Bua (~106cm, based on femoral length).

“This 700,000-year-old adult humerus is not just shorter than that of Homo floresiensis, it is the smallest upper arm bone known from the hominin fossil record worldwide,” said co-author Adam Brumm from the Australian Research Center for Human Evolution at Griffith University. 

“This very rare specimen confirms our hypothesis that the ancestors of Homo floresiensis were extremely small in body size; however, it is now apparent from the tiny proportions of this limb bone that the early progenitors of the ‘Hobbit’ were even smaller than we had previously thought.”

Size reduction among Flores Hobbits

Additionally, the two new hominin teeth from Mata Menge are small in size and one has shape characteristics most consistent with early Homo erectus from Java. 

This similarity does not support the hypothesis that Homo floresiensis evolved from a more primitive type of hominin, such as those never recovered from Indonesia or the broader region outside Africa.

The Mata Menge human remains, now totaling 10 fossil specimens from at least four individuals (including two children), are anatomically similar to the Liang Bua Homo floresiensis and can be regarded as an older variant of this hominin. 

However, this earlier form had less specialized dentition (more primitive teeth) than its descendants at Liang Bua. The diminutive arm bone indicates that significant body size reduction occurred early in the history of the Flores hominins.

History of Homo floresiensis

“The evolutionary history of the Flores hominins is still largely unknown,” said Brumm. “However, the new fossils strongly suggest that the ‘Hobbit’ story did indeed begin when a group of the early Asian hominins known as Homo erectus somehow became isolated on this remote Indonesian island, perhaps one million years ago, and underwent a dramatic body size reduction over time.”

The researchers noted that the earliest Flores hominins appeared on this Wallacean island ~1.0–1.27 Ma, probably unintentionally (i.e., through accidental “rafting,” perhaps on tsunami debris), and possibly as part of the initial colonization of the Sunda Shelf region by early H. erectus. 

“The Flores hominins experienced substantial body size reduction soon after this event (within ~300,000 years), despite the presence of large-bodied predators such as ~3 meter-long Komodo monitors and crocodiles from the earliest paleontological record ( ~ 1.4 Ma) onwards,” wrote the study authors. 

“This implies that giant reptilians did not represent a serious predation threat for early H. floresiensis or its progenitors.”

Stable hominin body size 

According to the researchers, this early evolutionary event was followed by long-term stability in hominin body size, possibly also in cultural adaptations (e.g., stone technology), and minor morphological specialization in the dentition.

“How the small brain size reported for the ~60,000 years old LB1 evolved still remains unknown,” the experts wrote. “At present, however, the available fossil data imply that small body size had been a functional adaptation for these insular hominins during and slightly beyond the Middle Pleistocene and indeed potentially up until the arrival of H. sapiens on Flores around 50,000 years ago; an event that, we suspect, precipitated the demise of H. floresiensis.”

This study provides crucial insights into the evolutionary history of Homo floresiensis, suggesting that these early hominins underwent significant evolutionary changes after becoming isolated on Flores. 

The findings highlight the complexity and diversity of human evolution, particularly in isolated environments, and underscore the importance of continued research in the region to further unravel the mysteries of our ancient ancestors.

The full study was published in the journal Nature Communications.

Image credit: Cicero Moraes (CC BY 4.0)

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