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Big Questions about Little Hominins

The skull of Homo floresiensis (right) is much smaller than ours (left).

The skull of Homo floresiensis (right) is much smaller than ours (left), but other evidence supports that it is a new hominid species and not a modern human that suffered from a genetic or pathological condition. Credit: Debbie Argue

By Debbie Argue

The discovery of diminutive human fossils in Indonesia has challenged paradigms in human evolution – and has therefore been highly controversial. How strong is the evidence that Homo floresiensis is a separate species and not a stunted modern human?

A tiny skeleton was revealed to an unsuspecting world in October 2004 – the bones of a new kind of human called Homo floresiensis (less scientifically nicknamed “the hobbit”). The bones were discovered by Indonesian archaeologists during an excavation in Liang Bua cave on the island of Flores in Indonesia. The excavation team of Indonesian and Australian researchers, led by the late Prof Mike Morwood and Dr Tony Djubiantono under the auspices of the Indonesian National Research Centre for Archaeology, 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 in strata dated to 13,400–10,200 years ago and about 100,000 years ago.

The most spectacular finding, at a depth of 6 metres, was an 18,000-year-old skeleton of a person just over 1 metre tall. The skeleton was named LB1, an archaeological 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. 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, she was the equivalent of a 30-year-old. 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; rather, after death she had sunk into mud in a shallow pool of water where she was slowly covered by silt.

LB1 was not alone. There were more than 85 other bones from a number of adult and juvenile individuals found throughout the 13-metre 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.

When the bones of the diminutive human were first discovered, no one was sure what species they belonged to. It was necessary to compare them with bones of Homo sapiens and archaic hominin species. Dr Peter Brown of the University of New England and his co-workers compared the Liang Bua remains with those of Homo erectus, Homo ergaster, Homo georgicus, Homo sapiens and Australopithecus africanus, and concluded that LB1 showed a mix of archaic and modern characteristics. In this, LB1 was unlike any other species of human. It was declared to be a new species of Homo and given the name Homo floresiensis.

LB1 and her kind were tiny – only 1 metre tall – lacked a chin and had a short forehead that sloped backwards. A mound of bone framed the upper and side regions of the orbits. Their wrists and shoulder joints were quite unlike those of Homo sapiens. Dr Matt Tocheri of the Smithsonian Institution found that the wrists were more like those of African apes, and the hands could not open as expansively as ours.

As well, Dr Susan Larson and colleagues from Stony Brook University discovered that the shoulders faced somewhat forward and the arms could not rotate quite as much as ours. In this aspect, Homo floresiensis was more like Homo ergaster, which lived 1.5 million years ago in Africa. We now know that H. georgicus, the 1.8 million-year-old hominins excavated at Dmanisi in the Republic of Georgia, had a similar shoulder girdle morphology.

Although they were small and had many features unlike modern humans, Homo floresiensis walked upright. The opening where the spine enters the skull – the foramen magnum – is positioned in the same place as Homo sapiens. If Homo floresiensis had walked on all fours the foramen magnum would have been positioned towards the back of the skull.

It is unlikely, however, that this diminutive human walked in the way that we do. LB1’s feet are 70% the length of her shins; in Homo sapiens the ratio is 55%. Dr William Jungers and his colleagues of Stony Brook University concluded that when LB1 walked she had to bend her knees more than modern humans do – just to get ground clearance.

What about the likely cognitive abilities of Homo floresiensis? The brain was tiny at 426 cc – the size of a grapefruit. It was similar to the Australopithecines that lived ~4 -2 million years ago in Africa.

But absolute brain size does not directly measure the cerebral capabilities of an individual. Rather, it’s the complexity and organisation of the brain that is important.

Brains are not usually preserved in ancient fossil remains, but sometimes marks on the inside of the skull reveal the presence and form of arteries and convolutions of the brain. This was the case for LB1. Dr Dean Falk of Florida State University studied these marks and found that the skull of LB1 had housed 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. The implication, therefore, is that although Homo floresiensis was tiny and had a small brain, it probably had some of the same mental abilities as us.

So we have a seemingly archaic member of our genus, with a very serviceable brain, that lived very recently. Where did it fit on our family tree? Prof Colin Groves and I analysed and compared the characters of Homo floresiensis with those of other Homo species – including Homo sapiens – and some Australopithecines. The analysis showed that Homo floresiensis probably branched off our family tree at a very early stage of the evolution of the genus Homo. Remarkably it lived as recently as 13,000 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 of a very early hominin on this remote Indonesian island.

The claim that another hominin, let alone one of such primitive form, lived at the same time as modern humans presents an enormous challenge to predominant ideas about human evolution. Contemporary understandings are based on a conceptual framework of a “branching tree” in which species of Australopithecines living between about 2–4 million years ago were followed by Homo habilis about 2 million years ago and subsequently, on different branches, by H. ergaster, H. erectus, H. heidelbergensis, H. 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 H. erectus and H. sapiens and of H. neanderthalensis and H. sapiens. But until the discovery of Homo floresiensis it was thought that our species, H. sapiens, had been the sole remaining species of Homo for more than 30,000 years.

It is hardly surprising, therefore, that serious disputes erupted immediately after the discovery of Homo floresiensis was announced. The challenge to the status of the new form, as a new species of Homo, centred on its very small stature and its tiny brain. It was suggested that LB1 was a modern human that suffered from a genetic or pathological condition. Different researchers suggested different pathologies – microcephaly, Laron syndrome, hypothyroid cretinism and, more recently, Down syndrome.

Microcephaly is characterised primarily by a marked reduction of brain growth, although microcephalic people may exhibit other abnormalities such as short stature, joint defects and cognitive impairment. Laron syndrome is a rare condition expressed in offspring from members of the same kinship that produces short stature, small hands and feet, and other symptoms. 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. Down Syndrome has a genetic cause.

The problem with the “pathology” hypotheses is that they do not account for all the facts. First, they are based upon limited aspects of the one skeleton, LB1, but she is not the “odd (wo)man out” here – the skeletal parts of all other individuals found have the same small stature.

Microcephaly and Laron syndrome, and to a lesser extent cretinism and even Down syndrome, are rare conditions so 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, non-pathological 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 period of about 90,000 years. 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.

Finally, any pathology-based hypothesis must account for all, or most, of the features of these people. That is, it must 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. The microcephalic/Laron/Down syndrome hypotheses do not address these matters and do not account for them. Scientific practice compels us to favour the hypotheses that account for the greatest number of the facts presented.

To me there is no doubt that we are dealing with a new species of Homo. Yet Homo floresiensis remains a puzzle. At present we know so little about how these beings lived. Where did they come from? How did they get to Flores given that this island has, it is supposed, never been attached to a mainland? Does their presence on Flores hint at a remarkably early capacity to voyage on the ocean? Could they have seen, or interacted with, Homo sapiens? Did they even survive into more recent times?

These are just some of the questions that have yet to be answered. This is a very exciting time for human evolutionary studies. Homo floresiensis requires that we rethink so much about human evolution. That is quite a feat for such a tiny species.

Debbie Argue is a Visiting Fellow in the Australian National University’s School of Archaeology and Anthropology.