Haggling over the hobbits

From six million-year-old Orrorin tugenensis to Australia's 40,000 year-old Mungo Man, it seems that controversy swirls around almost every new skeleton that falls out of the human family closet.

If Debbie Argue's cladistic analysis of the morphology of diminutive Homo floresiensis is on the mark, the two million-year-old australopithecine roots of the human family tree threw up a little sucker that persisted into very recent times.

Some critics have argued that the "hobbits" are pathological, microcephalic modern Homo sapiens, but the consistent size and morphology of the partial fossils during the 6000-year period from 18,000 years to 12,000 years ago argues otherwise.

The cave floor sediments yielded small stone tools dating from 94,000 to 13,000 years ago, a period encompassing the time of its occupation by "hobbits", and supporting claims that they represent a new and distinct species of Homo.

In their folklore, the nearby Nage people of Flores describe encounters with tiny pot-bellied, hairy humans around a metre tall, with prominent ears, called ebu gogo, who conversed with each other in murmurs, and parroted words spoken to them. Ebu gogo means "grandmother who eats anything" - they were apparently gluttons.

If the tales are true, ebu gogo were extant when Portuguese colonists arrived 400 years ago, and may have persisted until the later Dutch occupation until about 100 years ago - a possibility that fuelled the media sensation surrounding the hobbits' discovery.

While the hobbits have been formally accorded the status of a distinct species, not everyone is convinced.

How likely is it that another species of Homo persisted for at least 15,000 years after the last Neanderthals went extinct in Europe, despite living in the direct migratory path of waves of island-hopping modern H. sapiens who for at least 42,000 years had been using Flores as a stepping stone to Timor, en route to colonising Australia, New Guinea and the Pacific?

Dr Peter Obendorf and Dr Ben Kefford, of RMIT University's School of Applied Science, and Professor Emeritus Charles Oxnard, of the Centre for Forensic Science at the University of Western Australia, have issued another challenge to the hobbits' newfound species status.

In a paper in Proceedings of the Royal Society BL Biological Science, they revisit the hypothesis that hobbits are simply pathological modern H. sapiens. They offer an environmental explanation for the recurrence, over 6000 years, of a set of physical traits including diminutive stature, microcephaly and apparently primitive skeletal features.

The paper provoked a storm of controversy when it was published earlier this year. Professor Colin Groves from the School of Archaeology and Anthropology at the ANU said he was sorry to see serious scientists involved in such a "travesty" and that many of the claims were sheer speculation and some flew in the face of the evidence.

Professor Peter Brown from the University of New England, who was part of the team that discovered H. floresiensis, said the conclusions in the paper were not supported by the facts. "The authors have not examined the original fossil, have little or no experience with fossil hominids and depend upon data obtained by others," he said.

But others, such as Professor Maciej Henneberg from the University of Adelaide, who first proposed that the hobbits were not a new species but a pathological modern human, said the paper was a welcome addition to the debate.

---PB--- Hypothyroidism hypothesis

So what's the argument? Obendorf, Kefford and Oxnard believe the hobbit's physical features are consistent with human endemic cretins, the product of a hypothyroidism due to a combination of iodine deficiency and thiocyanate toxicity - thiocyanates are cyanogenic compounds present in certain food plants.

Their hypothyroidism hypothesis can account for folkloric descriptions of the appearance of the ebu gogo, and therefore their gluttony, as well as reports that they still existed as little as a century ago.

Their explanation also invokes a pattern of succession evident throughout human history, where a technologically superior culture invades territory occupied by an aboriginal culture, and displaces it from the resource-rich coastal region into marginal hinterlands.

Obendorf says endemic cretinism was the first hypothesis that came to mind after he heard that Mike Morwood and Peter Brown's team had discovered a tiny, small-skulled human fossil in a limestone cave in central Flores in 2004.

He was surprised that no-one considered endemic cretinism as an explanation, given that cretinism, associated with small stature and smaller brains, was not uncommon in limestone regions of Europe, the US, south-east Asia and Australia until iodised salt was recognised as a simple treatment for hypothyroidism and goitre early last century. Endemic cretinism due to maternal iodine deficiency was still occurring in Tasmania in the 1960s.

Occam's razor requires that the first thing is to try to explain the fossil as a modern human. Only if that fails do you consider the possibility that it's a Martian, or something else.

At a conference in Sydney in 2005, the post-cranial remains of Liang Bua 1 (LB1), the first specimen discovered, were discussed.

"I was interested to learn that the humerus (the long bone between shoulder and elbow) was twisted in LB1, to a lesser extent than seen in humans or even apes," Obendorf says.

He quickly found that humeral torsion is a feature of endemic cretinism in humans - iodine deficiency restricts elongation at the growth plates at either end, but lateral expansion is unaffected, resulting in thickening and twisting.

"I thought I was on the right track, but I needed some help from a physical anthropologist who knew the intricacies of bones better than I did," he says.

"I approached Charles Oxnard at the University of WA, who said he had had his own thoughts on the fossils. He was happy to discuss it but said, 'Don't expect me to agree with you.'

"Charles looked at some crania from [myxoedematous endemic, or ME] cretins in the Royal College of Surgeons in Edinburgh, and found a lot of missing features. In ME cretinism, some of the cranial bones remain cartilaginous and are lost during fossilisation - the same bones were missing in LB1.

"That's what got Charles interested. In addition we found that there were lots of experiments in the 1930s in which people experimentally introduced cretinism in growing animals, and showed that the long bones became relatively thick and stunted, consistent with the LB1 long bones."

Obendorf and Oxnard found the cranium displayed many features of congenital hypothyroidism, including an enlarged pituitary fossa - the depression in the wing-like sphenoid bone that houses the pituitary gland at the base of the skull.

Peter Brown disputes this. "Their argument hinges on LB1 having large pituitary fossa," he says. "If they had looked at the original, which I have, they would have seen that it does not."

---PB--- Endemic cretinism

The Obendorf team conclude that the hobbits could have been ME human cretins, members of an inland population of mostly unaffected modern humans - possibly the descendents of Flores' original negrito inhabitants.

Short-statured Negrito populations are found on the Andaman Islands in the Bay of Bengal, and in remote inland regions of the Malay Peninsula, Thailand, and the Philippines. They are believed to be descendants of one of the first wave of modern humans to migrate out of Africa some 60-70,000 years ago.

In their PRS:B paper, Oxnard, Obendorf and Kefford - an aquatic toxicologist who performed the multivariate analyses - note that ME cretins are born without a functioning thyroid, resulting in congenital hypothyroidism.

In neurological cretinism, the skeleton grows normally, but the function of both the brain and peripheral nervous system are severely impaired. Affected individuals suffer mental retardation and impaired motor coordination.

ME cretinism is characterised by restricted bone growth and severe dwarfing, and swelling of soft tissues, resulting in a pot-bellied appearance. Despite their reduced brain size, individuals show less mental retardation and motor impairment than neurological cretins. They learn little language.

Obendorf says certain characters in LB1 that have been interpreted as primitive, including the morphology of the wrist, and a double-rooted lower premolar tooth may be explained as consequences of endemic cretinism.

Endemic cretinism was once common in limestone areas of Europe that formed from uplifted sea floor.

Over time, weathering depletes limestone landscapes of micronutrients, especially iodine and selenium. Obendorf says that in the Carpathian mountains, iodine deficiency was historically exacerbated by high levels of cabbage in the diet - eating cabbage generates high levels of thiocyanates in the serum, that affect the thyroid's ability to synthesise thyroxin.

In central Africa, consumption of cassava had the same effect on populations in limestone areas.

"All these factors are present in Indonesia, in regions like Aceh province on Sumatra, and in Borneo, where there was a high rate of ME cretinism," Obendorf says.

"People did not practice agriculture on Flores, or anywhere else, at that time, so we investigated what wild plants they might have used as foods, and found a number of candidates including a bitter yam, Dioscorea, bamboos, which are high in cyanogenic glucosides, and possibly Acacia seeds."

The age of the Floresian hobbit fossils spans the period between the height of the last glacial period (18kya) and the end of the last ice age, when reduced precipitation would have reduced plant growth and animal numbers in the elevated limestone hinterland of Flores.

Like Australia, Flores and the other islands of Sundaland are also affected by El Nino droughts, which would force people to eat plant foods they might not eat normally - possibly accounting for the episodic appearance of ME cretinism.

"We believed the area would be limestone and selenium deficient, like central Java. ME cretinism would have been a problem until the hunter-gatherers assimilated into farming communities which were spreading out from the coast.

"The farmers would also have had close links with fishermen on the coast, and may have traded some of their food for fish, which contain iodine from sea salt."

---PB--- DNA deliberations

Other features of the Flores fossils are indicative of ME cretinism, according to Obendorf, but this is another area that Brown disputes.

In normal humans the adult mandible has an angle close to 90 degrees, but in children it is shallower - as it is in adult cretins, whose skeletons exhibit juvenile states.

The double-rooted pre-molar of LB1 could be explained by the fact that ME cretins sometimes retain some deciduous teeth into their 30s. Obendorf has images taken from a 3-D scan of the jaw shown in a BBC TV documentary, which shows that the roots and the crown of this tooth shows the characteristic features of the deciduous or 'milk' molar that would have first appeared in that position.

Brown says the fossil only has adult premolars. He also disputes their reference to an unfused area on the frontal bone, which he says is excavation damage.

The aberrant trapezoid bone of the wrist, identified as a primitive feature shared with australopithecine hominins, is also characteristic of ME cretins. Obendorf says delayed ossification in cretins may result in carpal bones having two or more separate pieces that may remain unfused even in adults.

In the original Science paper describing LB1's skeleton, the trapezoid looked small, but Obendorf says one of the authors told him the image had been scaled up by 25 per cent to facilitate comparison with the other bones of the hand, and so it was even smaller than expected, and consistent with it not being a complete bone.

Obendorf says a DNA sample, which is most likely to be obtained from a tooth, should resolve the issue of whether the "hobbits" are a different human species, or pathologically deformed ME human cretins.

He is surprised that DNA has not yet been recovered. "I heard rumours a year or two ago that they had recovered DNA, but got a result that suggested it was contaminated by modern DNA.

"If we're right, the DNA should be not very different from the DNA of the indigenous, modern Floresians, some of whom worked at the site."