Neandertals: something on their chests

New in the Journal of Human Evolution: Asier Gómez-Olivencia and colleagues present a new description of the ribcage of Kebara 2. When it comes to postcrania, we have more remains of Neandertals than any earlier or contemporary human group. Only recent people are better known. So you’d think we would already really know about the shape of their bodies.

But even in this group with many fossils, relatively complete ribcages are very rare. And ribs are generally broken into little pieces. We have to see the curvature of ribs to understand the shape of the thorax, but little broken pieces don’t give many opportunities to look at overall curvature. So this is one aspect of Neandertal anatomy that we know less about than most others.

Neandertal ribs can be a surprisingly hot topic. We know something about the pelvis, but less about ribs. Early humans had broad pelves – from Gona and Jinniushan to Atapuerca and Neandertals, their pelvic form was wider compared to their statures than is the case for most recent people. But the ribs determine more of the overall shape of the trunk than the pelvis. The lower edge of the ribcage may be linked to pelvis size, because both structures support the abdominal muscles and enclose the abdomen’s contents. But the shape of the ribcage reflects the musculature of the shoulders and arms as well as the size of the inflated lungs.

The Gona pelvis (discussed here last year) has re-opened the examination of pelvis size and shape in early humans. Neandertals are generally like earlier humans in these characteristics, with some anatomical distinction, especially the long pubis. But current trends in reconstruction of the ribcage give the Neandertals a thorax shape different from recent people. Sawyer and Maley (2005) gave their reconstruction of the Kebara ribcage a “bell-shaped” profile with flaring lower ribs. The present paper reconstructs it differently, in the more “barrel-shaped” profile, where the upper ribs are big and curved and the lower ribs more similar to those of living people.

Whether it was large from front to back, side to side, or both, the Neandertals had big ribcages. Big ribcages ought to have had big lungs inside them. Why big lungs? A traditional answer is cold adaptation – the idea being either that (a) cold climates take more energy, which takes more oxygen ventilation, which needs bigger lungs; or (b) cold climates take more heating and moistening of inspired air, which may be buffered by a large residual lung volume; or (c) cold climates increase the risk from (if not the incidence of) respiratory diseases, which are ameliorated by reserve lung capacity.

The authors of this paper adopt a different hypothesis: Neandertal lung size was determined by high energy expenditure, which they shared with other early humans:

A comparison of the raw variables for stature and body mass used by Churchill (2006) to calculate TEE [total energy expenditure] indicates that middle Pleistocene individuals would have estimated BMR (and TEE) values equal to or greater than those estimated for Neandertals. We can also postulate that a large chest would be necessary for the increased oxygen consumption derived from higher energetic demands of: (1) a larger body compared to modern humans, regardless of their economy, and (2) higher activity levels compared to modern industrial society standards due to a hunter-gatherer lifestyle. If true, Neandertals would have inherited from their middle Pleistocene ancestors (and maybe even earlier) relatively large chests that would have been advantageous in cold climate environments due to a higher heat production tied to high caloric intake (see Churchill, 2006). However, this would have been the result of an exaptation, rather than cold-adaptation per se. Two additional points are important to make in this regard. The ?rst is that it follows from the above discussion that Neandertal thoracic shape is not autapomorphic in Neandertals as has been explicitly argued by some (Sawyer and Maley, 2005; Tattersall and Schwartz, 2006). Secondly, the idea of relatively large ventilatory capacities in Neandertals does not imply for us evidence for hyper-adaptation in the sense that their vital capacities would be unusually high (i.e., signi?cantly exceeding the upper ranges of variation found in either earlier hominins or even extremes found in living populations, see Weinstein, 2008), even if it could be demonstrated that Neandertals had higher ventilatory capacities compared to industrialized populations (87-88).

In other words, Neandertals weren’t special. Their ventilation capacity was the same as Middle Pleistocene humans (like those from Atapuerca) and earlier. You can’t determine that presently from ribs alone, so we should treat this as a hypothesis. But given our increasing knowledge of trunk width from the pelvis, it seems like it ought to be the null hypothesis. The high-energy lifestyle was a prerequisite for living in a relatively cold climate; not an adaptation that developed within the cold.

The paper evaluates the argument, earlier presented by Franciscus and Churchill (2002), that European and Levantine Neandertals had differently shaped ribcages. The comparison here is very difficult because the problems of fragmentary ribs are compounded by the different preservation of specimens. To make a long story short, the current paper does not support the idea – meaning that this particular idea for geographic patterning didn’t work out.

And of course, the Neandertal thorax was an ideal atlatl target for early modern humans. Er, uh…Ok, forget that.

References:

Gómez-Olivencia A, Eaves-Johnson KL, Franciscus RG, Carretero JM, Arsuaga JL. 2009. Kebara 2: new insights regarding the most complete Neandertal thorax. J Hum Evol 57:75-90. doi:10.1016/j.jhevol.2009.02.009

Sawyer GJ, Maley B. 2005. Neanderthal reconstructed. Anat Rec 283B:23-31. doi:10.1002.a.b.20057

Franciscus RG, Churchill SE. 2002. The costal skeleton of Shanidar 3 and a reappraisal of Neandertal thoracic morphology. J Hum Evol 42:303-356.