Hobbit species may not have been human
AFTER five years of arguments over the so-called hobbits, the University of New England paleoanthropologist who formally described the tiny new hominin species from the Indonesian island of Flores is facing another wave of controversy.
This time, Peter Brown could raise the ire of some of the scientists who supported him in an academic debate that degenerated into an international scandal.
Brown, who initially placed the species in the human genus Homo and named it Homo floresiensis, is considering stripping the hobbits of their human status.
More remains have been found, and the species is now represented by six to nine individuals, depending on how the partial skeletons are put together. The skeletons range in age from 17,000 to 95,000 years.
And a big body of research, including Brown's own, since the publication of the first papers on the find has forced a rethink of his initial classification.
In a paper accepted for publication in an upcoming special Homo floresiensis edition of the Journal of Human Evolution, Brown and colleague Tomoko Maeda, also of UNE, say the hobbits' lineage left Africa "possibly before the evolution of the genus Homo". (The root of the human family tree stretches back about two million years to Homo habilis, or Handy Man, in Africa.)
Brown says assigning the Flores hominin to a different genus would worry some scholars. "They will think it somehow marginalises Homo floresiensis; that it's a clear statement that it is not a member of our genus, and it's extinct, so we don't have to worry about it any more," he says. "That's nonsense, because it's part of the broader evolutionary story of our species."
The hobbit drama started when an Australian and Indonesian team led by archeologist Mike Morwood, now of the University of Wollongong, and Radien Soejono of the Indonesian Centre for Archeology published details of the discovery of the 18,000-year-old partial skeleton, LB1, in the British journal Nature in 2004.
The 1m-tall hominin, discovered at the Liang Bua cave site in 2003, had a low, receding forehead and no chin. Its brain capacity was well under the 500cc of the typical chimp, yet it had stone tools, had possibly tamed fire and had hunted the dwarfed elephant-like stegodon and giant rats.
Brown initially argued that the hominins had evolved on Flores from Homo erectus, then thought to be the species that had left 840,000-year-old stone tools at another Flores site. Like many island species, the hobbits had dwarfed under evolutionary pressure in an environment with limited food and few competitors.
The classification struck at the heart of the debate over two competing models of human evolution, the "out of Africa" and multiregionalist theories.
The out of Africa model states that modern humans, Homo sapiens, evolved in Africa up to 200,000 years ago. Our species then dispersed across the globe, replacing the descendants of earlier migrants, classified variously as Homo erectus and Homo heidelbergensis, who left Africa more than a million years ago. The modern human wave washed through southeast Asia, reaching Australia about 50,000 years ago.
Multiregionalists contend that our species evolved in various regions from the earlier African migrants. Interbreeding pushed the species in the same evolutionary direction.
In this scheme, the Africanists' different Homo species are lumped together. Multiregionalism cannot accommodate two different human species in the same region concurrently. The Flores hobbit had overlapped with modern humans, at least in the region, so Brown's assignment of it to Homo challenged the multiregionalist model.
Rival researchers led by veteran Indonesian paleoanthropologist Teuku Jacob, a multiregionalist, "borrowed" the Liang Bua excavators' LB1 specimen in 2004, causing a storm in the international scientific community. Jacob later returned the remains but, to Brown's horror, the precious type specimen - the material on which the formal species definition is based - had been damaged.
A row erupted in the media. Multiregionalists, including Alan Thorne of the Australian National University and Maciej Henneberg of the University of Adelaide, argued that LB1 was not a new human species but a modern human with a brain disorder (called microcephaly) that gave it a tiny skull.
Much research since has tested claims that the hobbits were sick modern humans, healthy pygmies or sick pygmies. Several papers made or countered claims of disorders such as cretinism or congenital hypothyroidism.
Various international teams have exhaustively compared hobbit anatomy with that of ancient hominins and with sick and healthy modern humans.
Evidence has been mounting that the hobbits' direct ancestor preceded Indonesian Homo erectus, says Brown, an Africanist.
Some teams, including one led by the ANU's Debbie Argue, dispute the disease theories but leave Homo floresiensis in the human genus, albeit close to the base of the family tree. Others say hobbits are more closely related to our more distant ancestors the australopithecines, epitomised by Lucy, a 3.2 million-year-old Australopithecus afarensis from Ethiopia.
In their latest research, Brown and Maeda compared two Homo floresiensis lower jawbones and the attached teeth, along with an isolated hobbit premolar, with those of more than 2000 modern humans as well as ancient hominins. They took in data from the 1.8 million-year-old Dmanisi skeletons from the Republic of Georgia, thought to be from a species intermediate between Homo habilis and Homo erectus or its close relative Homo ergaster.
Until now, the Dmanisi material was thought to represent the first hominin species out of Africa.
Mandibles and teeth are powerful diagnostic features because they are preserved preferentially in the fossil record, Brown says. "Because we have lots of this type of information, we have developed methods for interpreting it and comparing it between species."
The two drew some of the data from the scientific literature but made many measurements themselves. They scrutinised about 40 characteristics, including the thickness of tooth enamel, measured in fractions of a millimetre, the shape of tooth crowns and their roots, and diagnostic parts of the chin region. Deep statistical analysis put Homo floresiensis way outside the range of modern humans, including microcephalics.
Combined with other anatomical evidence, the results ruled out Asian Homo erectus as the progenitor. Both jawbones shared characteristics with Australopithecus and early Homo, and were closer to them than the Dmanisi skeletons were. The ancestral hobbits must have left Africa before the hominins who reached Dmanisi, Brown and Maeda reasoned.
"What will come from this is either the redefining of the genus Homo or the argument that this species has so many unique characteristics and so many features shared with australopithecines that it probably belongs in its own genus," Brown tells HES.
If the hobbits wind up outside Homo, it will mean they were never a threat to multiregionalism anyway.
That such a primitive species got to an island so far from the African homeland and survived there until a volcanic eruption wiped it out 17,000 years ago would make the Flores find even more spectacular than previously thought.
Far from being marginalised, the species Brown describes as "out of time and out of place" might just have played a big role in hominin evolution: "Surely it's possible that in some part of Asia, an isolated group of australopithecine-like hominins evolved in a particular direction - towards Homo floresiensis - and spread backwards towards Africa."
http://www.theaustralian.news.com.au/story/0,24897,26143116-12332,00.html |
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