Smallest arm bone in human fossil record sheds light on the dawn of Homo floresiensis

A paper out today in Nature Communications reports the discovery of extremely rare early human fossils from the Indonesian island of Flores, including an astonishingly small adult limb bone.

Dated to about 700,000 years old, the new findings shed light on the evolution of Homo floresiensis, the so-called 'Hobbits' of Flores whose remains were uncovered in 2003 at Liang Bua cave in the island's west by a team co-led by Australian-New Zealand archaeologist Professor Mike Morwood (1950-2013).

Archaeological evidence suggests these diminutive, small-brained humans inhabited Liang Bua as recently as 50,000 years ago, a time when our own species (Homo sapiens) was already long established in Australia to the south.

There has been much debate about the origin of the mysterious humans from Flores. It was first hypothesised that Homo floresiensis was a dwarfed descendant of early Asian Homo erectus.

Another theory is that the 'Hobbit' is a late-surviving remnant of a more ancient hominin from Africa that pre-dates Homo erectus and was small in stature to begin with, in which case possible candidates include Homo habilis or the famous 'Lucy' (Australopithecus afarensis).

Other than Liang Bua, hominin fossils have only ever been found at a single location on Flores: the open-air site of Mata Menge 75km to the east of the cave. Located in the sparsely populated tropical grasslands of the So'a Basin, this site has previously yielded several hominin fossils (a jaw fragment and six teeth) excavated from a layer of sandstone laid down by a small stream around 700,000 years ago.

Pre-dating the Liang Bua hominins by 650,000 years, the Mata Menge fossil remains have been shown to belong to at least three individuals with even slightly smaller jaws and teeth than Homo floresiensis, implying that small body size evolved early in the history of Flores hominins.

However, as postcranial elements (bones from below the head) had not been found in the fossil record at this site it could not be confirmed that these So'a Basin hominins were at least as small as, if not slightly smaller than, Homo floresiensis.

It was also unclear what species the Mata Menge fossils belonged to, owing to the lack of more diagnostic specimens. However, some teeth were deemed to be intermediate in form between those of early Asian Homo erectus and Homo floresiensis.

The new study published in Nature Communications was led by Professor Yousuke Kaifu of the University of Tokyo, Iwan Kurniawan of the Center for Geological Survey in Indonesia, and Associate Professor Gerrit van den Bergh from the University of Wollongong.

It reports the discovery of three additional hominin fossils from Mata Menge dating to 700,000 years ago, the outcome of several field seasons of excavations at this site. Most importantly, the new assemblage includes the first postcranial element, a distal shaft of an adult humerus (lower half of the upper arm bone).

The recovery of a fossil limb bone from the Mata Menge excavation site has been long-awaited because of the wealth of evidence it provides regarding the ancestral origin of Homo floresiensis.

Digital microscopy of the microstructure indicates that the small humerus is from an adult individual. Based on the estimated length of the bone, the team was able to calculate the body height of this hominin to be about 100cm tall. This is around 6cm shorter than the estimated body height of the 60,000-year-old Homo floresiensis skeleton from Liang Bua (~106cm, based on the 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 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 two additional hominin teeth from Mata Menge are also small in size and one bears shape characteristics that are most consistent with early Homo erectus of Java. This similarity does not support the hypothesis that Homo floresiensis evolved from an earlier and more primitive type of hominin, the likes of which have never been recovered from Indonesia, or indeed the wider region outside Africa.

The Mata Menge human remains, which now total 10 fossil specimens, are from at least four individuals (including two children). All of them are very similar anatomically to the Liang Bua Homo floresiensis and can now be regarded as an older variant of this hominin. However, while a direct ancestor of the 'Hobbit', this earlier form had a less specialised dentition (more primitive teeth) than its descendant at Liang Bua.

Further, it is evident from the tiny arm bone that extreme body size reduction occurred early in the history of the Flores hominins.

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Revised dating of the Liujiang skeleton renews understanding of human occupation of China

The emergence of Homo sapiens in Eastern Asia has long been a subject of intense research interest, with the scarcity of well-preserved and dated human fossils posing significant challenges.

Tongtianyan cave, located in the Liujiang District of Liuzhou City, Southern China, has been a focal point of this research, housing one of the most significant fossil finds of Homo sapiens. However, the age of the fossils found within has been a matter of debate -- until now.

In a new international study in Nature Communications, with contributions by Griffith University, researchers have provided new age estimates and revised provenance information for the Liujiang human fossils, shedding light on the presence of Homo sapiens in the region.

Using advanced dating techniques including U-series dating on human fossils, and radiocarbon and optically stimulated luminescence dating on fossil-bearing sediments, the study revealed new ages ranging from approximately 33,000 to 23,000 years ago. Previously, studies had reported ages of up to 227,000 years of age for the skeleton.

"These revised age estimates align with dates from other human fossils in northern China, suggesting a geographically widespread presence of H. sapiens across Eastern Asia after 40,000 years ago," said Professor Michael Petraglia, study co-author and Director of Griffith's Australian Research Centre for Human Evolution.

Dr Junyi Ge, of the Chinese Academy of Sciences, and lead author of the study, said: "This finding holds significant implications for understanding human dispersals and adaptations in the region. It challenges previous interpretations and provides insights into the occupation history of China."

The Liujiang skeletal remains, discovered in 1958, have long been considered among the most significant human fossils from Eastern Asia.

With their excellent preservation, the cranial, dental, and postcranial remains have been the subjects of extensive biological and morphological comparisons across Eurasia.

Dr Qingfeng Shao, of the Nanjing Normal University added: "The findings of this study overturn earlier age estimates and palaeoanthropological interpretations, emphasising the need for robust dating methods and proper provenance documentation in the study of human evolution."

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Sahelanthropus, the oldest representative of humanity, was indeed bipedal...but that's not all!

The acquisition of bipedalism is considered to be a decisive step in human evolution. Nevertheless, there is no consensus on its modalities and age, notably due to the lack of fossil remains. A research team, involving researchers from the CNRS, the University of Poitiers1 and their Chadian partners, examined three limb bones from the oldest human representative currently identified, Sahelanthropus tchadensis. Published in Nature on August 24, 2022, this study reinforces the idea of bipedalism being acquired very early in our history, at a time still associated with the ability to move on four limbs in trees.

At 7 million years old, Sahelanthropus tchadensis is considered the oldest representative species of humanity. Its description dates back to 2001 when the Franco-Chadian Paleoanthropological Mission (MPFT) discovered the remains of several individuals at Toros-Menalla in the Djurab Desert (Chad), including a very well-preserved cranium. This cranium, and in particular the orientation and anterior position of the occipital foramen where the vertebral column is inserted, indicates a mode of locomotion on two legs, suggesting that it was capable of bipedalism2.

In addition to the cranium, nicknamed Toumaï, and fragments of jaws and teeth that have already been published, the locality of Toros-Menalla 266 (TM 266) yielded two ulnae (forearm bone) and a femur (thigh bone). These bones were also attributed to Sahelanthropus because no other large primate was found at the site; however, it is impossible to know if they belong to the same individual as the cranium. Palaeontologists from the University of Poitiers, the CNRS, the University of N'Djamena and the National Centre of Research for Development (CNRD, Chad) published their complete analysis in Nature on August 24, 2022.

The femur and ulnae were subjected to a battery of measurements and analyses, concerning both their external morphology, and their internal structures using microtomography imaging: biometric measurements, geometric morphometrics, biomechanical indicators, etc. These data were compared to those of a relatively large sample of extant and fossil apes: chimpanzees, gorillas, orangutans, Miocene apes, and members of the human group (Orrorin, Ardipithecus, australopithecines, ancient Homo, Homo sapiens).

The structure of the femur indicates that Sahelanthropus was usually bipedal on the ground, but probably also in trees. According to results from the ulnae, this bipedalism coexisted in arboreal environments with a form of quadrupedalism, that is arboreal clambering enabled by firm hand grips, clearly differing from that of gorillas and chimpanzees who lean on the back of their phalanges.

The conclusions of this study, including the identification of habitual bipedalism, are based on the observation and comparison of more than twenty characteristics of the femur and ulnae. They are, by far, the most parsimonious interpretation of the combination of these traits. All these data reinforce the concept of a very early bipedal locomotion in human history, even if at this stage other modes of locomotion were also practiced.

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