Hobbit tales: Homo floresiensis ten years on

28 October 2014

By Dr Debbie Argue
School of Archaeology and Anthropology,
Australian National University

Ten years ago today, on 28 October 2004, a tiny skeleton was revealed to an unsuspecting world - the bones of a new kind of human that, because it was so small, was nick-named "the hobbit." The bones were discovered by an Indonesian team during an archaeological excavation in Liang Bua cave, on the island of Flores in Indonesia. The team of Indonesian and Australian researchers was led by 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. No-one could have imagined that the finds would throw the scientific world into a frenzy of excitement and controversy.

The most spectacular find was at a depth of six meters. It was an 18,000-year-old skeleton of a person just over one meter tall. The skeleton was first named LB1, in reference to the cave in which it was found. The remains included the skull, leg bones, parts of the pelvis, hands, feet, and some other fragments. Judging by the pelvis, LB1 was probably female. Although it is not known how she died, archaeological evidence shows that she had not been deliberately buried but, rather, after death, had sunk into mud in a shallow pool of water where she was slowly covered by silt.

There were many other bones from a number of individuals found throughout the 13 meters of excavation. These remains were in stratigraphic levels that have been dated to 10.2 and 100 thousand years before present. They were all from individuals estimated to be only one meter tall, that walked upright, with relatively short legs, which resulted in their arms extending much lower than ours; and they had long feet. Their wrists and shoulder joints were quite unlike those of Homo sapiens: the wrists were more like those of African apes, and the shoulders faced forwards somewhat and couldn't rotate like ours, quite similar to an early member of our species, H. ergaster that lived 1.5 million years ago in Africa. They lacked a chin and had a short forehead that sloped backwards. A mound of bone framed the upper and side regions of the orbits. Its brain was small, even for such a short being, but it had a relatively large frontal lobe. In Homo 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.

When the bones of the diminutive human were first discovered, it was unclear to what species they belonged, so they were initially studied in relation to those of Homo erectus, Homo ergaster, Homo georgicus, Homo sapiens, and Australopithecus africanus. The investigators showed that LB1 comprised a mix of archaic and modern characteristics. A it was unlike any other species of human it was declared to be a new species of Homo and given the name Homo floresiensis; and two competing hypotheses were presented - that Homo floresiensis is an early hominin similar to the earliest in our genus, Homo, but until now known only from East Africa, that somehow got to Flores Island, Indonesia and survived there until recently; or that it is a dwarfed descendant of Homo erectus, the only known non-sapiens hominin to once have existed in south East Asia. In this case Homo floresiensis is said to be the result of a biological response to specific ecological conditions faced by species genetically isolated on islands.

Homo floresiensis challenged existing paradigms of human evolution. It had been thought that Homo erectus was the earliest hominin to live in the archipelago that is now Indonesia and had been present there about one million years ago. It seemed now that an archaic species, whether a very early archaic species of Homo or a dwarfed Homo erectus lived on Flores during the period that modern humans were present on nearby New Guinea and Australia, though, as yet, we do not know whether the two species were ever present on the same island or land mass at the same time.

The claim that another hominin, let alone one of such primitive form, lived in the Indonesian region at the same time as modern humans presents an enormous challenge to predominant ideas about human evolution. Contemporary understandings are based in a conceptual framework of a 'branching tree' in which species of Australopithecines living between about four million and a little over two million years ago and Homo habilis - at about two million years ago - and, subsequently, on different branches, a range of hominin species - Homo ergaster, Homo erectus, Homo heidelbergensis, Homo neanderthalensis and, finally, H. sapiens. There are debates about the relationships between all these species, and there are debates about possible temporal overlaps in the existence of Homo erectus and Homo sapiens and of Homo neanderthalensis and Homo sapiens. But until the discovery of Homo floresiensis it was thought that our species, Homo sapiens, had been the sole remaining species of Homo for over 30,000 years.

It is hardly surprising, therefore, that serious disputes erupted immediately after the discovery of Homo floresiensis was announced. The challenges to the status of the new form, as a new species of Homo, centered on its very small stature and its tiny brain. It was suggested that LB1 was a modern human that suffered from a pathological condition. Different workers suggested different pathologies - microcephaly, Laron's syndrome, hypothyroid cretinism, and, in August this year, Down Syndrome was proposed as an explanation for the morphology of LB1.

The first - microcephaly - is characterized, primarily, by a marked reduction of brain growth though microcephalic people may exhibit other abnormalities, such as short stature, joint defects and cognitive impairment. The second - Laron syndrome - is a rare condition expressed in consanguineous families that produces short stature, underdeveloped musculature, shallow orbits, small hands and feet, and other symptoms. And the third - hypothyroid cretinism - is caused by an iodine deficiency that results in thyroid malfunction in some individuals in a population and, like the other pathologies, leads to very short stature and some abnormalities in the limbs.
There are, however, several reasons why these interpretations may be rejected.
First, most prioritize limited features of just one specimen - the small stature and brain size of LB1 - but the excavation at Liang Bua produced skeletal remains of other similar individuals - LB1 is not the 'odd (wo)man out' in the set. Bearing in mind that all the pathologies mentioned are rare conditions, it is unlikely that all individuals in an excavation would show the same abnormalities. The absence of human remains of modern form is itself evidence that pathology is an unlikely explanation. The Down Syndrome hypothesis does not explain why many characteristics of the LB1 cranium, mandible and face never occur on modern humans, nor why Down Syndrome people do not have the distinctive archaic characteristics we see on LB1.

Secondly, the skeletal remains from Liang Bua span a period of about 90 thousand years. The pathology-based hypotheses fail to explain how such rare conditions as microcephaly or Laron's syndrome, or even cretinism, could have been sustained in a single population for such a long time. Rather, available evidence suggests a flourishing population of Homo floresiensis.
Other facts also show that Homo floresiensis is not a modern human with pathology. The archaic head shape, facial features, mandibular structure, shoulder and wrist structure, and the very long arms and feet on a very short body, are not accounted for in these interpretations. The arm to leg ratio, the form of the palate, and the existence of mounds of bone above the orbits, are just a few of the characters that reveal that the remains found at Liang Bua are not those of Homo sapiens. And, further, Homo floresiensis has a particular buttressing structure on the inside of the front of the jaw that is never found on modern humans. Our buttressing is on the front of our jaw - in the form of a chin.

So we have a seemingly archaic member of our genus, with good cognitive skills, that lived very recently. Where did it fit, then, on our family tree? Recent studies compared the characters of Homo floresiensis with those of other Homo species - including Homo sapiens and Homo erectus- and some Australopithecines. The results showed that Homo floresiensis was no-where near Homo erectus on the tree, and the hypothesis that it was closely related to that species could not be supported, and, therefore, it is highly unlikely Homo floresiensis is a dwarfed form of that species. Instead, Homo floresiensis is located close to the earliest of our genus on the family tree, Homo habilis, branching at a very early stage in the evolution of the genus, Homo. This suggests that hominins dispersed from Africa at an earlier time that we had previously considered.

But, remarkably, it lived as recently as 13,000-10,000 years ago - 1-2 million years after the disappearance of similar hominins from the African continent. It is, therefore, likely to represent a remnant population on this remote Indonesian island of a very early hominin.

In ten years we have come a long way in in our understanding of H. floresiensis but many questions remain. How and did it get to Flores, an island that has always been separated from other islands and land masses? Did it colonise other islands? What was its diet? When did H. sapiens arrive on Flores and was there any overlap between the two species? When did H. floresiensis become extinct? Most of these questions may be answered by further archaeological work on Flores and other islands; questions relating to diet can be investigated by isotopic analyses of teeth; but just how the species arrived on Flores will probably remain speculative.

Courtesy of McGraw-Hill Education: D. Argue, "Homo floresiensis: further insights," AccessScience, McGraw-Hill Education, 2014; http://www.accessscience.com/content/homo-floresiensis-further-insights/YB140319