The shape of the Neandertal femur is primarily the consequence of a hyperpolar body form

Abstract

Neandertal femora are distinct from contemporaneous near-modern human femora. Traditionally, these contrasts in femoral shape have been explained as the result of the elevated activity levels and limited cultural abilities of Neandertals. More recently, however, researchers have realized that many of these femoral differences may be explained by the cold-adapted bodies of Neandertals vs. the warm-adapted bodies of near-modern humans. This study explicitly tests this proposed link between climate-induced body proportions and femoral shape by considering the entire hip as a unit by using geometric morphometric methods adapted to deal with articulated structures. Based on recent human patterns of variation, most contrasts in shape between the femora of Neandertals and near-modern humans seem to be secondary consequences of differences in climate-induced body proportions. These results, considered in light of hip mechanics during growth, highlight the importance of developmental and functional integration in determining skeletal form.

Fig. 1.

Distinctive features of the Neandertal femur. (Left) The Neandertal 1 (Feldhofer Cave Neandertal) femur. (Right) The Skhul IV near-modern human femur. Relative to near-modern humans, the Neandertal femur has larger articulations (head and distal end), a thicker and rounder shaft, and a lower neck-shaft angle. Adapted from McCown and Keith (9).

Fig. 2.

Differences in hip shape between recent human groups from warm vs. cold climates. (a) UPGMA clustering tree based on the combined hemi-hip shape variables. The major split in the tree is between individuals from warm vs. cold climates. (b) Scores along a hip shape discriminant axis that was calculated to separate groups by climate of origin. A dot signifies a group mean, and the horizontal lines are standard deviations. The horizontal, broken line is the overall sample mean. Groups from warm climates have negative scores; groups from cold climates have positive scores.

Fig. 3.

Changes along the hip discriminant function that was calculated to separate recent human groups by climate of origin. The left stick figure shows –3 (warm climate) and the right stick figure shows +3 (cold climate) SD from the sample mean along the discriminant function axis. Darker shading indicates greater depth in the stick figures. For clarity, the circles representing femoral heads have been scaled to 50% head diameter, and hemi-hip changes have been mirrored. Notice the large articulations, thicker and rounder shaft, and lower neck-shaft angle of the cold-climate femur relative to the warm-climate femur. There are pelvic differences as well.

Fig. 4.

Fossil hominid femur shape. (a) UPGMA clustering tree based on femur shape variables. The Neandertals cluster with cold-climate recent humans, and the near-modern human clusters with warm-climate groups. (b) Scores along a femur discriminant function that was calculated to separate recent humans by climate of origin. For the recent humans, dots signify group means, and horizontal lines are SD. For the fossil specimens, each “+” represents an individual specimen. The horizontal, broken line is the overall sample mean. The Neandertals plot near Aleutian islanders (Spy 2 above Neandertal 1), and the near-modern human plots alongside warm-climate groups.