This past Friday, our journal club took on “The affinities of Homo floresiensis based on phylogenetic analyses of cranial, dental, and postcranial characters” (Argue et al. 2017). Essentially, Argue and colleagues attempted to figure out what other hominin species H. floresiensis (often called the Hobbit) was most closely related to, using statistical tree-building methods.
Since it was published in 2004 by Brown et al., H. floresiensis has been a bit of a mystery. Much like Homo naledi, there’s been a lot of discussion about where it belongs in the human family tree because its anatomy was A) weird and B) totally unexpected for its age (somewhere between 100-60 thousand years old) and its geographic location (on Flores, a small Indonesian island). The Hobbit was very short in stature, with a very small brain (in the range of orangutans, chimpanzees, and the much-older australopithecines), large teeth for its size, primitive-looking wrist bones, and disproportionately large feet relative to its height and leg length (hence its nickname of the Hobbit). Its discovery on Flores was a surprise because the other hominins that have been found in Indonesia (like Homo erectus from Java) were older and had larger brains (and we generally think brain size in the human lineage has increased over time – but last week’s chat about H. naledi also brought this up as a problematic assumption).
In their new article, Argue et al. set out to test two hypotheses: either the Hobbit is a late survivor from an earlier primitive hominin lineage, or it is a dwarfed descendent of H. erectus. They also commented on another controversial claim that’s been made about the Hobbit – that it is simply a modern human with a genetic/developmental pathology. They tested their hypotheses by applying two tree-building methods to a large sample of characters (particular features or measurements of the skeleton) from the skull, teeth, and postcranial (below the head) skeleton. One method (parsimony) attempts to build the shortest possible tree (one that requires the fewest changes in traits to get from species to species), while the other method uses probability to figure out which trees are most likely to occur (given a particular model of evolution).
When you set out to do a project like this, you’re forced to make some choices as a paleoanthropologist. If you have isolated postcranial bones from a hominin site where you’ve previously found fossils of more than one hominin species from the same time, how do you decide which body belongs with which head? You also confront the issue that not all researchers agree on which specimens belong in which species. And, as always, the fossil record is incomplete; you don’t have all of the characters for all of the species. To account for these potential problems, Argue et al. tested their two hypotheses with several different data sets – for example, they did one test where they considered all of the potential postcranial skeletal material that’s been called Homo habilis to actually be H. habilis and another where they excluded the questionable material.
What Argue et al. found was that their two different hypothesis testing methods and various different data sets produced broadly similar results in support of the first hypothesis: the Hobbit either shared a common ancestor with Homo habilis or is the sister group to the grouping of Homo habilis/Homo erectus/Homo ergaster/Homo sapiens. They are able to reject the hypothesis that the Hobbit is a dwarfed H. erectus (and reject a number of other species as possible close relatives). What this suggests is that (as was proposed for Homo naledi in last week’s papers) there is a long ghost lineage (unknown ancestors) for the Hobbit dating back more than 1.75 million years that is still waiting to be found. Ghost lineages – so hot right now.
Read on for some BS&M discussion bits!
For the morphology nerds out there:
What features were shared by Homo floresiensis and Homo habilis, but not by any other hominins?
Argue et al. found three of these:
1. The greatest axis of the auditory meatus from the inferior margin to the superior margin slopes posteriorly. Essentially, the ear hole is a tilted-backwards oval.
2. The mandible has a prominent inferior transverse torus. There’s more bone on the front, inside surface of the jaw in the Hobbit than there is in an anatomically modern human.
3. The humerofemoral index is medium. This is the length of the humerus divided by the length of the femur multiplied by 100.
What I want to know is – why these three traits? Why would these particular features be shared uniquely between H. habilis and the Hobbit? During our discussion, we suggested three (fairly standard) explanations: neutral evolution (genetic drift of particular alleles – a function of chance, rather than natural selection), function, and scaling. Neutral evolution was proposed for the auditory meatus, as it doesn’t seem likely to have a functional explanation. The torus was suggested to buttress against wishboning of the mandible (a classic explanation), but we called in a BS&M alum whose specialty is the mandible and she said this has been demonstrated not to be the case (as in, there is currently not a good functional/dietary explanation for mandibular robusticity). So, perhaps neutral evolution is an explanation for this feature, as well. Another option is scaling, both for the torus and for the humerofemoral index. This, at least, would be potentially testable (if someone hasn’t done it already).
A couple of questions were also brought up about the Dmanisi Homo erectus/Homo georgicus material. Is it too variable to be considered one group in this analysis? Is it a problem that it comes out as being deeply rooted in Homo? Are we catching something at that site that is in transition from one species to another (whatever that might mean)? Let us know what you think in the comments!
Stay tuned for our next BS&M update! We might be taking a brief hiatus for the summer field season, but you can always stay up to date on our goings-on through our Twitter feed (@CHES_Rutgers) and Facebook page (https://www.facebook.com/CHES.Rutgers/).
References
Argue, D., Groves, C. P., Lee, M. S., & Jungers, W. L. (2017). The affinities of Homo floresiensis based on phylogenetic analyses of cranial, dental, and postcranial characters. Journal of Human Evolution.
Brown, P., & Maeda, T. (2009). Liang Bua Homo floresiensis mandibles and mandibular teeth: a contribution to the comparative morphology of a new hominin species. Journal of Human Evolution, 57(5), 571-596.
Brown, P., Sutikna, T., Morwood, M. J., Soejono, R. P., Saptomo, E. W., & Due, R. A. (2004). A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia. Nature, 431(7012), 1055-1061.
Bones, Stones, and Monkeys 