Homo erectus origin likely for Flores ‘hobbits’

Dental study rejects modern human or earlier hominin connection with Homo floresiensis

The origin of the diminutive ‘hobbits’ of Flores, Indonesia have been controversial since they were announced as a new human species, Homo floresiensis, in 2003. The most widely accepted view is that they are descended from a group of Homo erectus that reached Flores at least a million years ago and underwent a phenomenon known as insular dwarfism whereby a combination of low risk of predators and a relative scarcity of food means that smaller individuals are favoured from an evolutionary point of view and thus individuals within a population will ‘downsize’ over the course of many generations.

However, there are two alternative viewpoints. The first is that the Homo floresiensis remains simply represent modern humans affected by a condition such as microcephaly or cretinism. The second view accepts that the Flores hominins are indeed descended from an archaic species, but posit that it is something more primitive than Homo erectus – possibly Homo habilis or an australopithecine. It is argued that the absence of fossil evidence for such early hominins leaving Africa is not evidence of absence from Eurasia.

In a newly-published study, researchers carried out extensive comparisons using linear metric analyses, crown contour analyses, and other trait-by-trait morphological comparisons of the molar, premolar and canine teeth of Homo floresiensis against an extensive sample of teeth from present-day modern, prehistoric modern, and archaic humans. Three methods were used: metric analyses based on crown length and breadth data; comparisons of crown contour using normalized Elliptic Fourier Analysis (EFA); and non-metric and linear metric comparisons of individual morphological traits not recorded by the first two methods.

The researchers found suggest that the Homo floresiensis teeth do share derived characteristics with those of Early Pleistocene Homo erectus from East Africa and Java, and with the Dmanisi hominins from Georgia; but none of the ‘hobbit’ teeth exhibit the very primitive morphology associated with Homo habilis or australopithecines. Such characteristics include the occasional absence of a P3 buccal groove, a distally positioned P3 lingual cusp, a more circular P4 crown, the presence of a P4 transverse crest, non-parallelogram M2 crown shape, a mesiodistal short M2 crown, a M1 mid-trigonid crest, equivalent M1 and M2 sizes, and a moderately wide upper dental arcade. The findings rule out the claim that Homo floresiensis evolved from a hominin that was more primitive than Homo erectus.

Nor was a good match found with the modern samples. In comparison to Homo floresiensis, the teeth of Homo sapiens are derived for nine out of 26 character states, contradicting the suggestion that the dentition of Homo floresiensis is wholly modern.

Overall, the results suggest that Homo erectus is the ancestral species; however the dentition of Homo floresiensis did continue to evolve and possesses some unique features not seen in any other hominin species. These include the large (relative) size and the unique occlusal morphology of the P3 that otherwise exhibits primitive morphologies; and the extremely short first molars. In view of the general trend of molar shortening during the evolution of Homo over time, this condition in Homo floresiensis is actually more derived than in Homo sapiens.

It must be assumed that these evolutionary changes reflected the unique habitat of Flores, but regardless they demonstrate the distinctiveness of Homo floresiensis as a species.

Reference:

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Kaifu, Y. et al., Unique Dental Morphology of Homo floresiensis and Its Evolutionary Implications. PLoS One 10 (11) (2015).x