A hobbit's contemporaries: Biogeography and insular evolution on Flores

Jul 15 2010 Published by under [Biology&Environment]

Painters create networks. The subject of the piece, even if it’s a simple splotch of color, garners the most attention, but without a descriptive background or other kinds of supporting elements to contextualize the portion of the painting where the artist wants you to look, the intended focus is lost. The subject loses a certain clarity of interpretation in the absence of those elements.

Hanneke Meijer and her colleagues are trying to provide that sort of context to Homo floresiensis in this paper from the Journal of Biogeography, noting the importance of the natural history and evolutionary development of other vertebrates on the island of Flores and how this might inform and contextualize the life history of this tiny mystery.

Digging on Flores began in the 1950’s with an archeologist named Theodoor Verhoeven, a Dutch Catholic priest and missionary on the island. He found stone artifacts with the remains of a Stegodon and a “large murid” in those early digs, leading him to conclude that Homo erectus had arrived in the southeast islands of the Sunda archipelago by the early Pleistocene, across Wallace’s Line, which marks a transitional zone between Asia and Australia where organisms from both continents are thought to have mixed and created an area with high levels of endemism. Verhoeven, in his published report of Liang Bua (pictured above), a different site where he began digging in 1965 and found some neolithic remains and artifacts, said, “This cave is extra important.” He was right. It was in this limestone cave that the curious story of Homo floresiensis, the so-called hobbit, began many years later.

As excavations of Liang Bua and other sites progressed, a more complete picture of Flores faunal evolution, ecology and biogeography started to emerge. Though these data span almost a million years of history, there are commonalities, beginning with some of the more typical of other insular environments, a relatively poor diversity of taxa. The oldest records date to over 900,000 years ago, and thus far reveal a very few vertebrates: a tortoise of the Geochalone genus, now extinct, the Komodo dragon and the smallest known species of pygmy Stegodon, S. sondaari, which weighed in around 300 kg (just the tusks of a full grown African elephant can weigh up to 80 kg). Somewhere during the transition of the early to middle Pleistocene the tortoise went extinct, as did S. sondaari, but as the Pleistocene progressed, the proboscidean was replaced by another of the same genus, the larger S. florensis florensis, another immigrant to the island that probably evolved into the contemporary of H. floresiensis, S. florensis insularis. The giant rat Hooijeromys nusatenggera appears in the fossil record of this time as well as another monitor besides the Komodo, Varanus hooijeri, both of which the authors speculate were probably present in earlier time periods, but have smaller more delicate bones resistant to preservation. Alongside these remains stone artifacts from hominins have been found and of course, the very skeletal remains of H. floresiensis.

Based on this history, particularly evidence from the middle to late Pleistocene, the authors are trying to illustrate a Flores where the inhabitants are part of a highly endemic, phylogenetic continuum of species that underlines just how normal this kind of situation is for an isolated island community. S. florensis florensis was a rare vertebrate immigrant, but was filling a vacant niche and evolved into S. florensis insularis. This is supported by the general trend of island proboscideans displaying high levels of endemism and the fact that an entirely different species lived on Timor at the time. H. nusatenggera is very closely related to the extant giant rats on the island, and is probably ancestral to most of them.

As an island system, Flores certainly conforms to the so-called island rule (if such a thing can be said) home to giants rats and dwarf ungulates, adaptive responses to resource limitations, unique niche availability, absence of predators and the dynamics of decreased competition. Changes in body size are observed with other phenomena, such as the reduction of length and fusion of bones in the distal limbs for stability, increased hypsodonty in teeth for a changing diet or an often extreme reduction in brain size (the energetic requirements of nervous tissue is elevated).

But the first founding structure was determined by the geography of the island. Wilson and MacArthur’s Theory of Island Biogeography links the extremity of isolation with the relative richness of species, and the “impoverished” state of Flores historically fits this notion. The oceanic barrier to dispersal is crossed by only the most capable: birds, bats, tortoises, proboscideans, humans, lizards and rodents, which accurately describes what the fossil record shows. Until recently, with the habits of Homo sapiens acting as a mass transit system of dispersal, immigration to the island was a rarity. The composition of insular communities like Flores are described as “disharmonious” because of the structural differences between island and mainland ecosystems, which maintain larger populations and top level, typically mammalian predators. It's important to note (and integral to the authors' argument) that while it's interesting to highlight the oddities of island ecology, Flores is a very normal island community considering its biogeography.

All of this leads up to the subject, the focus of the entire picture: that H. floresiensis, as a part of this community, was the result of the “insular dwarfing of Homo erectus”:

Homo erectus is the prime candidate for the role of ancestor of H. floresiensis, as it was the only hominin present in Southeast Asia in the Pleistocene. Having reached Java by the Early Pleistocene (Swisher et al., 1994; van den Bergh et al., 1996), H. erectus was almost certainly present on Flores by the Middle Pleistocene (Sondaar et al., 1994; Morwood et al., 1998), as evidenced by the artefacts from the site of Mata Menge. Bromham & Cardillo (2007) showed that insular primates conform to the island rule and undergo shifts towards smaller body size in an insular environment. We therefore suggest that after its arrival on Flores, H. erectus followed the evolutionary path towards dwarfism and decreased in body size as a response to the absence of mammalian carnivores and the limited energy resources, as might any large-bodied mammal in an insular setting. Moreover, the observed size decrease in H. floresiensis when compared to H. erectus (c. 52% of H. erectus) falls within the range of other insular primates (Bromham & Cardillo, 2007).

This is largely in response to the cladistic analysis performed by Argue et al. in 2009, who...

...refuted the insular dwarfing hypothesis. However, the loss of derived features in insular evolution introduces a number of homoplasies, which in a cladistic analysis can easily be misconstrued for synplesiomorphies. This would lead to a more primitive position of the island form, exactly as was found by Argue et al. (2009). Given the small difference in tree length between the most parsimonious solution and the one in which H. floresiensis is derived from H. erectus, we find the cladistic analysis unconvincing. A careful evaluation of the characters, to see whether any homoplasies can indeed be attributed to typical insular developments, is needed to resolve this.

In case you’re not up on your plasies, what Meijer et al. are saying is that some of the characteristics of H. floresiensis acquired through insular evolution (homoplasies), such as the “robustness” of their limbs, could be mistaken for more primitive features associated with earlier hominins (synplesiomorphies) and need to be better accounted for in future studies.

In Liang Bua, there are very distinct layers of volcanic ash between the Holocene and Pleistocene layers of sediment, and across that boundary, very different assemblages of organisms. Stegodon and H. floresiensis are thought to have gone extinct around the time of that eruption ~19,000 years ago and as all islands in the area, were eventually replaced by Homo sapiens and their selected companion/invader organisms.

Image by Rosino

Meijer, H., Van Den Hoek Ostende, L., Van Den Bergh, G., & De Vos, J. (2010). The fellowship of the hobbit: the fauna surrounding Homo floresiensis Journal of Biogeography, 37 (6), 995-1006 DOI: 10.1111/j.1365-2699.2010.02308.x

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