Afarensis

Like Carl Zimmer, I think I have come down with a case of Homo floresiensisitis. The more I hear about it the more puzzled I become. Let's start at the beggining of my current befuddlement with the issue. Some of you may remember This post of Carl's, about Gert van den Bergh's theory that the hobbit was quadrupedal. The argument was based on the amount of humeral torsion present on the fossil. What is humeral torsion you ask?

Good question. Below is a picture of a humerus.

The important parts (for the purpose of this post) are the head, the medial and lateral medial and lateral epicondyles and the trocklea and capitulum. The head of the humerus articulates with the glenoid fossa of the scapula like this.

In humans, the shoulder is laterally directed while in, say monkeys, the shoulder is directed more towards the front.

The fact that the shoulder is more laterally facing in humans creates a problem. Without humeral torsion, the elbow could not be flexed in a sagittal plane.

Humeral torsion is defined as the angle made by the axis of the head of the humerus with the mediolateral axis of the elbow joint. It varies in monkeys, apes and humans.

In LB1 the humeral torsion is 110 degrees, which places it near Gibbons and Macaques. The similarities in humeral torsion between macaques and LB1 lead Gert van den Bergh to suggest that Homo floresiensis was quadrupedal. This suggestion lead nowhere (justifiably so) since the relationships between humeral torsion, locomotion, aboreality and terrestriality are not clear cut. Nothing puzzling there.

Then Kelly Hale, over at Anthropology.Net wrote a post called Yet more news from the Hobbit front. In the post, she quotes a Science article (at least I think it's this:
Culotta, E. [2006, 19 May]. How the Hobbit Shrugged: Tiny Hominid's Story Takes New Turn. Science, 312[5776], 983-984.):

In modern humans, the top or head of the humerus is twisted with respect to the elbow joint by about 145 to 165 degrees. As a result, when you stand straight, the insides of your elbows face slightly forward, allowing you to bend your elbows and work with your hands in front of your body....

Larson found that the LB1 humeral head was in fact rotated only about 110 degrees. (No rotation would be expressed as 90 degrees.) Curious, she examined LB1's broken collarbone plus a shoulder blade from another individual.

Larson concluded that the upper arm and shoulder were oriented slightly differently in H. floresiensis than in living people. The shoulder blade was shrugged slightly forward, changing its articulation with the humerus and allowing the small humans to bend their elbows and work with their hands as we do. This slightly hunched posture would not have hampered the little people, except when it came to making long overhand throws: They would have been bad baseball pitchers, says Larson.

Totally missing the part about the clavicle (D'OH) I thought humeral torsion would make a great post in my series on "What can we learn from bone fragments". Then Hot Cup of Joe did a post on it:

Susan Larson of Stoney Brook University gave a presentation in Puerto Rico at a Paleoanthropology Society meeting in which she described the shoulder joint of H. floresiensis as being more representative of H. erectus than of H. sapiens (Culotta 2006). She noted that the specimen may not even be female as was originally suggested and that the humeral articulation at the shoulder reinforce the assertion that H. floresiensis is a new species descended from H. erectus.

Which made me even more curious so I tracked down the Abstracts for the 2006 meetings of the Paleoanthropological Society and found this:

The unusually short stature and small brain size of the newly described Homo floresiensis have made this one of the most controversial fossil discoveries in recent years. No less remarkable is the reported minimal degree of torsion of a nearly complete humerus (LB 1/50) (Morwood et al. 2005). Humeral torsion is considered to be a hallmark of hominoids, including humans, related to the dorsal repositioning of the scapula onto an elliptical rib cage, and the concomitant lateral reorientation of the glenoid. Although the amount of humeral torsion varies among extant hominoids, it is most extreme in humans and African apes. Since no complete early hominin humeri exist, it is unclear whether this similarity in humeral torsion is a shared derived feature or the result of convergence, although estimates of only modest torsion among early hominins based on incomplete humeri suggest the latter may be the case. Nonetheless, the published estimate for H. floresiensis indicates even less humeral torsion in this new species of Homo. If correct, this minimal degree of torsion challenges our functional interpretations of shoulder morphology. Fortunately, additional upper limb material exists for H. floresiensis including a clavicle of the LB 1 skeleton, and a nearly complete scapula (LB 6/4). In most respects the scapula is similar to modern humans including possessing a large bar/glenoid angle, and the LB 1 humerus is an appropriate length for a hominin of its overall size. However, it appears that H. floresiensis had a short clavicle relative to humeral length, even when compared to modern African pygmies of similar body size. Interestingly, the KNM-WT 15000 H. erectus skeleton also had a relatively short clavicle. [emphasis mine - afarensis] A short clavicle may indicate a more protracted scapular position, raising the possibility of a previously unsuspected transitional stage in the course of hominin pectoral girdle evolution.

(Parenthetically, according to Trinkhouse and Vandermeersch's 1995 article [Postcranial Remains of Regourdou 1 Neanderthal: Shoulder and Arm Regions] neanderthals display less humeral torsion than modern humans. Aiello and Dean also mention that Australopithecus afarensis may have less humeral torsion modern humans - but that is based on an earlier work of Larson)

What captured my attention was the part I have bolded. This is an interesting argument that I wanted to look at further. According to Walker (in The Nariokotome Homo Erectus Skeleton) the upper limbs of KNM-WT 15000 are "...unremarkable from a modern human perspective (page 424)." What I realy wanted to do was compare clavicle/humerus in LB1 and KNM-WT 15000 to see if this was correct. Walker and Leakey give 130.5 mm and 130.4 mm as the lengths of the right and left clavicles, repsectively, and 319.0 mm as the estimated length of the humerus for WT-KNM 15000. Moorewood et al (Further evidence for small-bodied hominins from the Late Pleistocene of Flores, Indonesia) gives 243 mm for the LB! humerus but I can't find measurments for the clavicle (the only mention of the clavicle I can find is in Brown's article [ A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia] which says that the clavicle was represented by fragments. So, in Larson's presentation at the Paleoanthropology Society Annual meetings, either the size was estimated based on fragments or they have new material...I would love to see their data on this. Hopefully, more will be coming out on this issue soon. Will I am trying to sort out my puzzlement on the issue, you might check out what Hawks has to say on the subject.