Elsevier

Journal of Human Evolution

Volume 57, Issue 5, November 2009, Pages 623-639
Journal of Human Evolution

Homo floresiensis: a cladistic analysis

https://doi.org/10.1016/j.jhevol.2009.05.002Get rights and content

Abstract

The announcement of a new species, Homo floresiensis, a primitive hominin that survived until relatively recent times is an enormous challenge to paradigms of human evolution. Until this announcement, the dominant paradigm stipulated that: 1) only more derived hominins had emerged from Africa, and 2) H. sapiens was the only hominin since the demise of Homo erectus and Homo neanderthalensis. Resistance to H. floresiensis has been intense, and debate centers on two sets of competing hypotheses: 1) that it is a primitive hominin, and 2) that it is a modern human, either a pygmoid form or a pathological individual. Despite a range of analytical techniques having been applied to the question, no resolution has been reached. Here, we use cladistic analysis, a tool that has not, until now, been applied to the problem, to establish the phylogenetic position of the species. Our results produce two equally parsimonious phylogenetic trees. The first suggests that H. floresiensis is an early hominin that emerged after Homo rudolfensis (1.86 Ma) but before H. habilis (1.66 Ma, or after 1.9 Ma if the earlier chronology for H. habilis is retained). The second tree indicates H. floresiensis branched after Homo habilis.

Introduction

In 2004, a team of Indonesian and Australian researchers discovered human bones representing a number of individuals during an archaeological excavation in Liang Bua cave on the island of Flores in Indonesia (Brown et al., 2004). The hominin bones were in stratigraphic levels dated to between 13.4–10.2 ka and ∼100 ka (Roberts et al., 2009); that is, they represent a population that existed for a period of approximately 76,000 years. A critically important component of the assemblage is a partially articulated skeleton, Liang Bua 1 (LB1), found at a depth of 6 m and bracketed by calibrated radiocarbon ages of 19–17.1 ka (Roberts et al., 2009).

Brown et al. (2004) announced the Liang Bua discoveries and attributed all the hominins to a new species, Homo floresiensis, based upon their assessment that its morphology comprises a number of primitive and derived features. The species is characterized by a small endocranial volume (417 cc: Falk et al., 2005) and short stature (106 cm: Brown et al., 2004) similar to Australopithecus afarensis, and robust limb bones similar to australopithecines in general. Unlike Australopithecus afarensis, however, the Liang Bua remains show more derived states such as reduced prognathism and facial height, along with smaller postcanine teeth. Indices of cranial shape, including maximum cranial breadth at the supramastoid region and a broad vault relative to height, reflect those for H. erectus (Brown et al., 2004).

These discoveries generated a robust body of papers, setting the stage for opposing views. Alternative interpretations include the possibility that the Liang Bua fossils represent a new hominin species, H. floresiensis (Brown et al., 2004, Morwood et al., 2004, Morwood et al., 2005, Falk et al., 2005, Argue et al., 2006, Larson et al., 2007, Tocheri et al., 2007, Baab and McNulty, 2009), and that the holotype specimen, LB1, was a modern human, possibly afflicted with a pathological condition (Henneberg and Thorne, 2004, Jacob et al., 2006, Richards, 2006, Hershkovitz et al., 2007, Obendorf et al., 2008). These conflicting hypotheses are based on comparative analyses of the morphology of the bones with both archaic and modern Homo, typically using statistical methods to compare the Liang Bua bones with those of other hominins.

The morphological and morphometric analyses have contributed much to the debate about H. floresiensis, but have not conclusively resolved the controversy about the position of the species in human evolution. We, therefore, use a different tool, cladistic analysis, which has not yet been applied to resolving this problem. Cladistic analysis focuses on evolutionary relationships of species rather than using metric or morphological assessment of similarities and differences between species to resolve phylogenetic relationships. We test a range of phylogenetic hypotheses about the status of the H. floresiensis remains, recognizing these could show special affinities with a wide range of hominin taxa. Ultimately, we present two equally parsimonious hypotheses for the phylogenetic position of H. floresiensis. Most importantly, both hypotheses indicate that H. floresiensis was a very early species of Homo that survived on Flores until at least 17,000 years ago.

Section snippets

Background

Numerous studies have addressed the taxonomic status of H. floresiensis, beginning with Brown et al.'s (2004) announcement of the discovery, which designated LB1 the type specimen of a new species, Homo floresiensis. Originally, it was hypothesized that H. floresiensis was the end product of a long period of isolation for H. erectus, or possibly early Homo, involving a process known as insular dwarfism. Alternatively, the remains could represent the descendant of an unknown small-bodied and

Materials and methods

The comparative sample comprises character states from H. floresiensis (LB1 cranium and postcranium; LB6/4, a right clavicle), H. erectus (Sangiran 2, Sangiran 17, Trinil), H. ergaster (KNM-ER 3733 and KNM-ER 3883), a sample from Dmanisi (D2282, D2280, D2700), H. rhodesiensis (Kabwe 1), H. habilis (KNM-ER 1813, OH 24), H. rudolfensis (KNM-ER 1470), A. africanus (Stw 505, Sts 71, Sts 5), A. afarensis (AL444-2), H. sapiens, Pan troglodytes, and Gorilla (Gorilla gorilla, Gorilla beringei). Pan

Assumptions in cladistic analysis

We present the following assumptions that are inherent in cladistic analysis and follow with a short explanation about how we control for them.

First, the OTUs are real. We have largely followed current convention in delineating hominin taxa. We restrict our H. habilis sample to KNM- ER 1813 and OH 24, retaining KNM-ER 1470 as a separate taxon, H. rudolfensis, as there is debate about its attribution to H. habilis; this is to avoid the possibility of confounding our taxa by conflating what might

Results

The Heuristic algorithm found the two shortest trees comprised of 247 steps (Fig. 1). In one of the trees (Tree 1) H. floresiensis branches after H. rudolfensis and before H. habilis, in the other tree (Tree 2) it branches after H. habilis. There are differences between these trees within the configuration of the later Homo OTUs, and their internal phylogenetic structure is unresolved. In Tree 2, Dmanisi forms a clade with H. ergaster, and H. sapiens forms a clade with H. rhodesiensis to which

Discussion

The objective of this analysis was to test a wide range of hypotheses about the phylogenetic status of the Liang Bua fossil remains. Specifically, we tested whether or not H. floresiensis shared an ancestor with a founder population of archaic Homo, or descended from a species intermediate between Australopithecus and early Homo (Falk et al., 2005, Argue et al., 2006, Larson et al., 2007, Tocheri et al., 2007). In addition, we tried to determine if H. floresiensis is phylogenetically related

Conclusions

Based on rigorous cladistic analyses, we propose that H. floresiensis evolved in the Late Pliocene or Early Pleistocene. The first of our two equally parsimonious trees suggests that H. floresiensis branched after H. rudolfensis (represented by KNM-ER 1470) but prior to the divergence of H. habilis (represented by KNM-ER 1813 and OH 24). Alternatively, our results are equally supportive of H. floresiensis branching after the emergence of H. habilis. Our results sustain H. floresiensis as a new

Acknowledgments

We would like to thank Dr Tony Djubiantano, Rokhus Due Awe, and the Indonesia Centre for Archaeology for kindly providing access to the Liang Bua cranium. We are very appreciative of Colin Groves, John Trueman, Bill Jungers, and two anonymous reviewers for their constructive comments that greatly improved the original manuscript. DA extends special thanks to MM for facilitating this study, Bert Roberts for helpful comments on the dating of the Liang Bua strata, and Doreen Bowdery for assistance

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    • The affinities of Homo floresiensis based on phylogenetic analyses of cranial, dental, and postcranial characters

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      When new skeletal remains comprising a second mandible, a radius, a tibia, and the right humerus and ulna of LB1 were described, however, Morwood et al. (2005) concluded from the new evidence that the ancestors of H. floresiensis could not be attributed to H. erectus. Argue et al. (2006) later independently supported this conclusion (and see Larson et al., 2007; Tocheri et al., 2007; Argue et al., 2009; Baab and McNulty, 2009; Brown and Maeda, 2009; Brown, 2012; Jungers et al., 2009b). On the other hand, Kaifu et al. (2011) described and compared the cranium of H. floresiensis with a large sample of Homo taxa, explored shape and form differences between them, and concluded that the cranio-facial morphology of LB1 was consistent with a hypothesis that H. floresiensis was derived from Javanese H. erectus.

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