The recent uncovering of two minuscule hominin species, namely Homo floresiensis and Homo luzonensis, prompts contemplation regarding the mechanisms behind such extreme corporeal miniaturization observed in certain extinct human lineages inhabiting isolated island environments. Prior investigations conducted at Mata Menge, situated on Flores Island, Indonesia, had indicated that the ancestral populations of Homo floresiensis from the Early Middle Pleistocene epoch possessed mandibles and dentition of even smaller dimensions. Now, paleoanthropologists have unearthed further hominin fossil evidence from analogous strata at the Mata Menge locale. An adult humerus, representing the distal portion of the upper arm bone, has been appraised as being 9-16% shorter and more slender than the holotype specimen of Homo floresiensis, which dates back approximately 60,000 years ago; it also surpasses in size any other known Plio-Pleistocene adult hominin humeri. This discovery lends substantial credence to the hypothesis that the lineage leading to Homo floresiensis most likely originated from early Asian populations of Homo erectus and persisted as a remarkably diminutive species on Flores for an extended duration, dating back at least to around 700,000 years ago.
Reconstruction of Homo floresiensis. Image credit: Elisabeth Daynes.
Homo floresiensis is characterized as a diminutive hominin species documented from the Late Pleistocene era at Liang Bua, a cavernous limestone site located in western Flores.
Evidence derived from archaeological findings suggests that this species inhabited Liang Bua as recently as 50,000 years ago, a temporal period when our own species, Homo sapiens, had already established a significant presence in Australia to the south.
A considerable degree of scholarly discussion has revolved around the origins of these enigmatic hominins from Flores.
Initially, it was put forth as a hypothesis that Homo floresiensis represented a miniaturized descendant of early Asian Homo erectus.
An alternative theory proposes that this ‘Hobbit’ is the surviving remnant of a more ancient hominin lineage originating from Africa, predating Homo erectus, and which was inherently small in stature. Under this theoretical framework, potential candidates include species such as Homo habilis or Australopithecus afarensis, the latter being the species to which the renowned ‘Lucy’ fossil belongs.
Beyond the Liang Bua locality, hominin fossil remains have exclusively been discovered at one other site on Flores: the open-air excavation at Mata Menge, situated 75 km eastward of the cave.
This location, nestled within the sparsely populated tropical grasslands of the So’a Basin, had previously yielded several hominin fossils comprising a mandibular fragment and six teeth. These were recovered from a sandstone stratum deposited by a minor fluvial channel approximately 700,000 years ago.
Predating the Liang Bua hominins by an interval of 650,000 years, the fossil assemblage from Mata Menge has been definitively attributed to at least three individuals exhibiting mandibles and teeth that are even slightly smaller than those of Homo floresiensis, thereby strongly implying that substantial reduction in body size emerged early in the evolutionary trajectory of Flores hominins.
However, the absence of postcranial elements—bones originating from regions below the skull—within the fossil record at this particular site precluded definitive confirmation that these So’a Basin hominins were indeed as small as, or even smaller than, Homo floresiensis.
Furthermore, the precise taxonomic classification of the Mata Menge fossils remained uncertain due to a paucity of more diagnostically informative specimens.
Nevertheless, certain recovered teeth displayed morphological characteristics that were considered to be intermediate between those of early Asian Homo erectus and Homo floresiensis.
This is an artist’s reconstruction of Homo erectus. Image credit: Yale University.
In a recent scholarly publication disseminated in the journal Nature Communications, Professor Yousuke Kaifu from the University of Tokyo, in collaboration with his research team, has reported the discovery of three supplementary hominin fossils from Mata Menge, with an estimated age of 700,000 years. These findings represent the culmination of multiple excavation seasons at this site.
Most significantly, the newly acquired collection includes the inaugural postcranial element: a distal fragment of an adult humerus.
The recovery of this fossil limb bone from the Mata Menge excavation site has been eagerly anticipated due to the substantial evidence it is expected to provide regarding the ancestral origins of Homo floresiensis.
Microstructural analysis conducted via digital microscopy suggests that the diminutive humerus belonged to an adult individual.
Based on estimations of the bone’s length, the research team was able to project the individual’s body height to be approximately 100 cm.
This estimation is roughly 6 cm shorter than the projected body height of the 60,000-year-old Homo floresiensis skeleton recovered from Liang Bua (estimated at approximately 106 cm, derived from femoral length).
“This 700,000-year-old adult humerus is not merely shorter than that attributed to Homo floresiensis; it represents the smallest upper arm bone presently known within the global hominin fossil record,” stated Professor Adam Brumm of Griffith University.
“This exceedingly rare specimen corroborates our prevailing hypothesis that the progenitors of Homo floresiensis exhibited exceptionally small body dimensions.”
“However, it is now evident from the minute proportions of this limb bone that the ancestral forms of the ‘Hobbit’ were even more diminutive than our prior estimations had suggested.”
The two additional hominin teeth recovered from Mata Menge also display reduced dimensions, and one exhibits specific morphological traits that align most closely with early Homo erectus specimens from Java.
This morphological congruence does not lend support to the hypothesis positing that Homo floresiensis evolved from an earlier, more primitive hominin type, for which no fossil evidence has ever been found within Indonesia or, indeed, the broader geographical region beyond Africa.
The hominin remains unearthed at Mata Menge, now totaling 10 fossil specimens, originate from a minimum of four individuals, including two juveniles.
All these specimens exhibit pronounced anatomical similarities to the Liang Bua population of Homo floresiensis and can now be classified as an ancestral variant of this hominin species.
Nonetheless, while it may represent a direct ancestor of the ‘Hobbit,’ this earlier form possessed a less specialized dentition (indicating more primitive teeth) compared to its later counterpart found at Liang Bua.
Furthermore, the minute size of the arm bone clearly indicates that extreme reduction in body size transpired early in the evolutionary history of the Flores hominins.
“The evolutionary narrative of the Flores hominins remains largely uncharted territory,” commented Professor Brumm.
“Nevertheless, the newly discovered fossils compellingly suggest that the story of the ‘Hobbit’ indeed commenced when a segment of the early Asian hominins, designated as Homo erectus, became geographically isolated on this remote Indonesian island through some unknown means, perhaps approximately one million years ago, and subsequently underwent a profound process of body size reduction over a prolonged temporal span.”
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Y. Kaifu et al. 2024. Early evolution of small body size in Homo floresiensis. Nat Commun 15, 6381; doi: 10.1038/s41467-024-50649-7
