KeywordsDown Syndrome Modern Human Skeletal Remains Archaeological Excavation Partial Skeleton
Homo floresiensis is a new species of hominin discovered in 2003 during archaeological excavations by an Indonesian-Australian team in Liang Bua cave on the island of Flores, Indonesia. The species is dated to ~90,000 years ago; the presence of an archaic-looking species living at such a recent period is an extraordinary occurrence. This entry focuses on the discovery, morphology, phylogeny, and controversies surrounding this enigmatic hominin.
A series of bones, including a partial skeleton, was discovered in 2003 during an archaeological excavation in Liang Bua cave, on the island of Flores in Indonesia (Brown et al. 2004). The excavation team of Indonesian and Australian researchers was led by the late Professor Mike Morwood and Dr. Tony Djubiantono under the auspices of the Indonesian National Research Centre for Archaeology. The excavation aimed to find insights into the origins of the first Australians but instead the team discovered a number of very different and very small, individuals. The strata in which the remains were found have recently been redated to between 58.3 ± 0.05 and 80.8 ± 0.8 thousand years ago (kya) (Sutikna et al. 2016). This is considerably earlier than the previously reported dates of 13.4–10.2 kya to ~100 kya.
Discovery, Description and Evolutionary Scenaries
The most spectacular find was at a depth of 6 m. It comprised a partial skeleton, labeled LB1, an archaeological reference to the cave in which it was found. These remains include the skull, leg bones, the pelvis, hands, feet, and some other fragments. It was so small that, at first, it was thought that the remains were those of a Homo erectus child. But the jaw had a full complement of adult teeth which suggested that, relative to modern humans, this hominin was the equivalent of about a 30-year-old. Although it is not known how death occurred, archaeological evidence shows that the body had not been deliberately buried but, rather, after death, had sunk into mud in a shallow pool of water where it was slowly covered by silt.
There were more than 85 other hominin bones from a number of adult and juvenile individuals found throughout the 13-m deep excavation. They were all from small individuals. At these levels of the archaeological site there were no bones from individuals with the stature of modern humans.
Brown et al. (2004) compared the Liang Bua remains with those of Homo erectus, Homo ergaster, Homo georgicus, Homo sapiens, and Australopithecus africanus and found that they showed a mix of archaic and modern characteristics, unlike any other hominin species and was attributed to a new species, Homo floresiensis.
Wrists and shoulder joints are quite unlike those of modern humans, and in fact Tocheri et al. (2007) found that the wrists are more like those of African apes and the hands could not open as expansively as ours. As well, Larson et al. (2007) discovered the shoulders face somewhat forward and the arms could not rotate quite as much as ours and, in this, Homo floresiensis was more like Homo ergaster, which lived 1.5 million years ago in Africa and H. georgicus, the 1.8 mya hominins excavated at Dmanisi, Republic of Georgia. Stature is estimated to be just over 1 m, and it was a bipedal hominin – it walked upright. Its feet are 70% the length of its shins; in Homo sapiens the ratio is 55%. This configuration meant that when walking, these beings had to bend their knees more than modern humans do, just to get ground clearance (Jungers et al. 2009).
It had a small brain in its small skull – 426 cc, similar to the Australopithecines that lived ~4 -2 million years ago in Africa. Absolute brain size does not directly measure the cerebral capabilities of an individual; rather, it is the complexity and organization of the brain that is important. Although brains are not usually preserved in ancient fossil remains, sometimes marks on the inside of the skull reveal the presence and form of arteries and of convolutions of the brain. This was the case for LB1. Falk et al. (2005) studied these marks and found that the skull of LB1 had housed a relatively large frontal lobe. In H. sapiens this part of the brain is associated with capabilities for planning, for learning from mistakes, and for passing on knowledge from generation to generation. The implication, therefore, is that though H. floresiensis was tiny and had a small brain, it probably had some of the same mental abilities as us.
Brown et al. (2004) proposed two possible hypotheses for H. floresiensis: (1) that they belong to a new species of small-bodied hominin, H. floresiensis that derived from an early lineage of Homo and (2) that they represent a new species, H. floresiensis, but in this scenario the species derives from an as yet undiscovered early population of immigrant H. erectus on Flores that subsequently dwarfed in response to the Island Rule (below). When new skeletal remains comprising a second mandible, a radius, a tibia, and the right humerus and ulna of LB1 were described, however, Morwood et al. (2005) concluded from the new evidence that the ancestors of H. floresiensis could not be attributed to H. erectus. Nevertheless, these two hypotheses continue to be rigorously debated.
Many researchers support the hypothesis that H. floresiensis represents a remnant population of a very early member of Homo. For example, Falk et al.’s (2005) study of the brain highlighted its archaic characters; Argue et al. (2006) used metric analyses of hominin crania and showed that H. floresiensis sits in with early Homo from 1.5 mya in Africa and does not cluster with modern humans; Tocheri et al. (2007) discovered the ape-like characteristics of its wrist bones; Larson et al. (2007) showed that its shoulder bone configuration was last seen on 1.5 mya hominins; Jungers et al. (2009) draw attention to the extraordinary long feet of H. floresiensis; and Peter Brown and colleagues undertook a detailed analyses of the two H. floresiensis mandibles, in which it is clear that it possesses very archaic characters that do not appear on H. erectus (or, indeed, H. sapiens). In 2009, Argue and colleagues analyzed and compared the characters of Homo floresiensis with those of other Homo species – including H. sapiens – and some Australopithecines. The analysis showed that H. floresiensis probably branched off the human evolutionary tree at a very early stage of the evolution of the genus, Homo 1–2 million years after the disappearance of similar hominins from the African continent. They conclude, therefore, H. floresiensis is likely to represent a remnant population of a very early hominin on this remote Indonesian island.
The alternative hypothesis is based upon the island rule for insular dwarfing that stipulates that body size of mammals alters when a founder population reaches an island, becomes reproductively separated from its mainland group, and faces an environment different from those of its mainland sister species. For example, a smaller body size would be expected as a response to a limited food supply, a larger size in the absence of predation. In this case, it is assumed that H. erectus, known from further west in Indonesia, at Sangiran, arrived on Flores and then, as an evolutionary response to the conditions encountered on Flores, dwarfed. This hypothesis may be tested by analyzing the similarities between the two species. Kaifu et al. (2011) undertook a detailed morphometric and morphological comparison of the LB1 cranium with H. habilis, H. erectus, and H. georgicus. They assessed 67 cranial characters and found that only a few traits support a relationship between H. floresiensis and H. habilis while 17 characters supported, or are compatible with, the hypothesis that H. floresiensis is an island dwarfed form of H. erectus from Java. More recently, Kaifu and colleagues reiterated this hypothesis based upon a comparative study of dentition. There are several other studies in which this hypothesis is supported.
A third hypothesis was proposed almost as soon as the original publication that described the new species came out. Rather than H. floresiensis representing an archaic hominin, Henneberg and Thorne (2004) claimed that H. floresiensis represents a modern human population with microcephaly. Successive publications have proposed a range of metabolic or genetic disorders in modern humans to explain the morphology of H. floresiensis, such as Laron’s syndrome, hypothyroid cretinism, and, more recently, Down syndrome. Each of the hypotheses has been fully tested and none has been corroborated. Whether the new dates for H. floresiensis, in which an overlap with modern humans on the island is very unlikely, will impact on these hypotheses is too early to say. In any case, the problem with the “pathology” hypotheses is that they do not account for all the facts. Firstly, they are based upon limited aspects of the one skeleton, LB1. But LB1 is not the only individual represented in the Liang Bua cave excavations. The skeletal parts of all other individuals represented are of the same small stature as LB1. Microcephaly and Laron syndrome, and, to a lesser extent, cretinism and even Down Syndrome are rare conditions and one would expect that, even if an archaeological excavation did reveal such a rare skeleton showing evidence of any of these syndromes, most of the skeletal remains would be of a normal, nonpathological population. The absence of modern-looking, modern-stature human skeletal material in the deposit is not explained by the proponents of the various pathology-based hypotheses.
Further, the skeletal remains span a long period. The pathology hypotheses fail to explain how a rare condition such as microcephaly or Laron Syndrome could be sustained in all known members of a single population for such a long time.
Nor do the pathology hypotheses account for the non–H. sapiens mandibular structure, the archaic head shape, archaic facial features, archaic shoulder, and ape-like wrist structure, the relatively short legs in relation to arms, and very long feet on a very short body.
While H. floresiensis is known from excavations at only one site, Liang Bua cave, recently a partial mandible and isolated teeth were excavated from Mate Menge in the Soa Basin on Flores. These fossils are dated to ~700,000 years ago – and are therefore more than 600,000 years older than H. floresiensis. The new fossils are said to be H. floresiensis-like but the researchers await the discovery of more material before they can determine if the remains are indeed H. floresiensis.
Homo floresiensis remains a puzzle, and its place on the human evolutionary tree is unresolved. It is not known where these hominins came from; when they first arrived on Flores; or how they got there, given that this island has, it is supposed, never been attached to a mainland. The recent discovery of H. floresiensis-like hominins at ~700,000 years ago hints at a long existence of this species and opens up the tantalizing probability that more fossil remains will be discovered on this island.
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