The Shape of the Neandertal Femur is Primarily the Consequence of a Hyperpolar Body Form

The Neandertal femur, when compared to anatomically modern humans, has distinctively larger articulations relative to absolute femoral length, a thicker rounded shaft, and a more obtuse angle between the femoral shaft and neck. In the early 20th century, when Marcellin Boule first described the Neandertal skeleton in detail, these distinct features of the Neandertal femur where attributed to a simpler social and cultural complexity which resulted in a high activity lifestyle. The author, Timothy D. Weaver (2003), notes that the assumption that elevated activity levels where consequences of Neandertal culture and social organization is unfounded because it is difficult to distinguish Modern Human from Neandertal cultures in the archaeological record. Although humans have the ability to buffer some environmental stresses through cultural adaptations, researchers now theorize that climactic adaptations account for the different dimensions of the Neandertal femur.

The theories now generally accepted by most researchers to explain different body dimensions within the same species were originally proposed by Bergmann and Allen. In summary, their theories state that “Endothermic species exhibit climate-related geographical patterns in overall size and body proportions” (p. 6926). Therefore, it is expected that humans inhabiting cold climates maximize heat retention though a lower surface area to body mass ratio, and humans inhabiting warm climates maximize heat loss through a higher surface area to body mass ratio. Cold-adapted humans generally have a small limb to trunk ratio, wider bodies, and reduced distal limb lengths. The “hyper-polar” or “hyper-arctic” body dimensions of cold-adapted Neandertals inhabiting glacial regions during the Pleistocene are most obvious in the absolute bi-iliac breadth and femur length relative to bi-iliac breath.

To determine if there is a relationship between femoral dimensions and climate induced body proportions, Weaver compares modern human males from different climates to fossils of Neandertal and near-modern humans (see tables 1 and 2 for sample summary). Weaver uses geometric morphometric methods to measure the overall hip dimensions of the 97 modern human males. These dimensions were calculated using 26 unilateral landmarks on the pelvis and 13 on the femur. The hip dimensions of Neandertal 1 and Skhul IV (near-modern human) were calculating using 13 unilateral landmarks on the femur. Because of fhe missing superior greater trochanter on the right femur of Spy 2, only 11 unilateral landmarks were available. Weaver’s results demonstrate that femur dimensions are correlated to climate induced body proportions. He found that:

“[i]ndividuals from cold climates tend to have relatively wide bodies as compared with individuals from warm climates… Cold adapted individuals have femora with large femoral heads and distal ends relative to length, thick and round shafts, and low neck; shaft angles; warm adapted individuals show the reverse pattern. These femoral changes are accompanied by pelvic changes. Individuals from cold climates tend to have wider pelvic apendatures, longer pubic bones, more flared iliac blades, more laterally pointing anterior-superior iliac spines, more anteriorly located iliac
tubercles, and more posteriorly rotated dorsal iliac blades relative to individuals from warm climates” (p.6927-6928).

Unfortunately, none of this is new information. There are also a few problems in Weaver’s testing methods which he does not address and have probably altered the results of his comparison. The most significant problem concerns Weaver’s disproportionate sample sizes. Weaver’s fossil sample, 1.75 (due to fragmentation) Neandertal femurs, and 1 near-modern human femur can not accurately represent populations. Although he uses only male femurs and pelvises in his modern-human skeletal sample, Weaver does not control for gender among his fossil samples, or age among any of the samples. Weaver’s calculations are also somewhat problematic as he uses 39 unilateral landmarks on the femur and pelvis of modern humans; however, he makes these same calculations from the fossil samples using only 13 (11 for the fragmented femur) unilateral landmarks only from femurs (not controlled for side, table 2). It appears that the only real value in Weaver’s test is in his comparison of femur and pelvic dimensions among modern humans inhabiting different climates. The sample of fossil Neandertal femurs used in Weaver’s study is too small to prove that the dimensions of the Neandertal femur are the consequence of a hyper-polar body form.

Reference:
Weaver, Timothy D.
2003 The shape of the Neanderthal femur is primarily the consequence of a hyperpolar body form. The National Academy of Sciences 100(12):6926-6929.