Dated to approximately 700,000 years old, these findings provide significant insights into the evolution of Homo floresiensis, the diminutive humans discovered in 2003 at Liang Bua cave by a team co-led by the late Professor Mike Morwood.
Archaeological evidence indicates that these small-brained humans inhabited Liang Bua until about 50,000 years ago, during a period when Homo sapiens were already established in Australia.
The origins of Homo floresiensis have been widely debated. Initial hypotheses suggested they were a dwarfed descendant of early Asian Homo erectus. Another theory proposed they were a late-surviving remnant of a smaller-statured hominin from Africa, such as Homo habilis or Australopithecus afarensis ('Lucy').
Apart from Liang Bua, hominin fossils have only been found at Mata Menge, an open-air site 75km east of the cave in the So'a Basin. This site has previously yielded several hominin fossils, including a jaw fragment and six teeth, dated to around 700,000 years ago.
These fossils predate the Liang Bua hominins by 650,000 years and suggest that small body size evolved early among Flores hominins. However, the lack of postcranial elements at Mata Menge left the exact size of these early hominins uncertain.
The new study, led by Professor Yousuke Kaifu, Iwan Kurniawan, and Associate Professor Gerrit van den Bergh, reports the discovery of three additional hominin fossils from Mata Menge, including the first postcranial element - a distal shaft of an adult humerus.
Digital microscopy confirmed that the small humerus belonged to an adult. Based on the bone's estimated length, the team calculated the body height of this hominin to be about 100cm, slightly shorter than the estimated 106cm of the Homo floresiensis skeleton from Liang Bua.
"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 Professor Adam Brumm from Griffith University's Australian Research Centre for Human Evolution, a co-author of the paper.
"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."
The additional hominin teeth from Mata Menge are also small, with one resembling the early Homo erectus of Java, challenging the hypothesis that Homo floresiensis evolved from a more primitive hominin.
The Mata Menge remains, now totaling 10 fossils from at least four individuals, are anatomically similar to Homo floresiensis but display less specialized dentition.
"The evolutionary history of the Flores hominins is still largely unknown," said Professor 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."
Research Report:Early evolution of extremely small body size in Homo floresiensis
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