Evolution of Human Limb Proportions: Part Two

As I mentioned in Part One The OH62 femur plays an important role in any discussion of the evolution of human limb proportions.

OH-62 Femur

Looking at the above picture, several things should be noticed. First, the head and a small portion of the neck are missing. Second, the shaft extends to a little past the nutrient foramen (although you can’t really see this in the picture – for those unfamiliar with osteology the nutrient foramen is an opening for the passage of blood vessels and bone). Third, there is a significant amount of bone exfoliation. In order to determine femur length an estimation needs to be made.

Length estimations are legitimate procedures in paleoanthropology, but some are better than others based on the assumptions made. In this case, the assumption is that OH62s’ femur is the same size as Lucys’ or shorter (see Richmond et al, 2002 “Early Hominin Limb Proportions”, J. Hum. Evol. 43:529-548, for example). One is naturally tempted to ask how good these estimates are…and the answer is not very. An interesting articla published in Current Anthropology (Reno et al 2005, 46(4):575-588) examined the question of hominid limb proportions in some detail. One of the more interesting points of the artical is that the proximal femur is a poor indicator of maximum femur length. Reno et al compared proximal femur length with maximum femur length and distal femur length with maximum femur length in a sample composed of Homo, Pan and Gorilla. Below is a plot of their results.

Femur Length 1

Notice in figure A that there is little relationship between proximal femur length and maximum femur length. Consequently any estimate based on proximal femur length is likely to be inaccurate (this also has implications for stature estimations and things like dimorphism and locomotion). On the other hand, distal femur length does better as a predictor of maximum femur length. Reno et al attribute this to the fact that most of the growth in the hindlimb occurs at the distal femur and proximal tibia. They also argue that this implies that the distal end of the femur will be one of the principal sites of hindlimb elongation or shortening. This is somewhat simplistic, though basically correct, as in the femur longitudinal growth is produced by endochondral formation of bone combined with proliferation by the epiphyseal plate and involves a great amount of remodeling, relocation, deposition and absorption at both ends.
Additionally, they performed a least squares regression on femur length using: the distance from the lesser trochanter to nurient foramen and the anteroposterior (taken just below th lesser trochanter) and mediolateral (taken at the same level as the anteropostior but perpendicular to it) subtrochanteric diameters. The OH62 femur was regressed based on human and ape samples and all showed an extremely large errors. For example, the regression based on Homo gave 350 mm with a 95% PI of 304-396 mm. Why is this range important? As the Richmond et al article, mentioned earlier, points out a change of only .25 mm in any of the limb length estimates for OH62 or AL288-1 makes the difference in limb proportions between the two become nonsignificant (they included estimates for the OH62 humerus in the analysis).
In line with this, Haeusler and McHenry’s 2004 article “Body Proportions of Homo habilis Reviewed (J. Hum. Evol. 46:433-465) argued that the best way to estimate the length of the OH62 femur was to compare it to the more complete OH34 femur which yields more human like hindlimb proportions (an interesting bit of serendipity – they remark in the abstract to that paper that “ “long legs may imply long distance terrestrial travel. Perhaps this adaptation evolved early in the genus Homo, with H. habilis providing an early representative of this important change.”)

There are two morals. One is that the use of estimates without examining the underlying rational (in this case proximal femur length as a proxy for maximum femur length) can give misleading results. The second is, of course, that we need more fossil postcranial bones…
Added Later: Here is a picture from Haeusler and McHenry 2004 that gives a good comparison of the three femurs:

OH-62 Comparison

The lines on OH34 represent different ideas about reconstruction.

Updated 03/31/2013 to fix pictures.

One Response

  1. I stopped by for a second, and ended up reading all the White Shark posts, the Cheetah post, and reviewing all the human and hominid limb articles. What time is it, I should have been in bed an hour ago. Great information! If only I could paint faster.

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