Stature estimates for Sima de los Huesos

Jos-Miguel Carretero and colleagues Carretero:stature:2012 report on the lengths of long bones from Sima de los Huesos, Spain. I’ve long been hoping this research would come out, because we’ve gotten interested in the pattern of body size as an aspect of evolution in early Homo.

Sima de los Huesos is the single largest sample of fossil Homo, and Carretero and colleagues include 27 mostly complete long bones in their sample. That’s around a dozen fewer than the entire sample of Neandertal long bones. This one site has more long bones than the rest of the Early and Middle Pleistocene combined.

Here are the tibiae, for example:

Tibiae from Sima de los Huesos

Complete tibiae from Sima de los Huesos, from Carretero et al. Carretero:stature:2012, figure 2.

The paper shows that the Sima hominins averaged a bit taller than Neandertals for most of the long bones.

That conclusion isn’t quite as simple as it might look, because the sample of male Neandertal femora actually average 3 mm longer than the Sima de los Huesos femora. Both samples have more than double the number of males as females, so the male comparison draws on a much larger sample size. The Neandertal male femoral sample is biased a bit high by the inclusion of both left and right femora from Amud, the tallest of the Neandertal skeletons. The tibia sample gives a substantially shorter stature for Neandertal males, both because Amud isn’t there, and because the limb proportions of Neandertals have short tibiae relative to their femora.

That’s the problem of using stature estimates instead of simple bone lengths: Nothing’s simple. Fossil samples impose some limits on the kind of analyses we can undertake. Carretero and colleagues address stature both because of its biological relevance and because estimating stature is the most reasonable way we can incorporate different long bones into a single size comparison. But considering stature introduces some problems of estimation. We can’t be sure how many individuals are represented by the long bones. We can determine a minimum: Six right tibiae came from a minimum of six bodies, for example. But if two arm bones and a leg bone all came from the same skeleton, that individual will be represented three times within this sample, and we don’t have a way to exclude that possibility. Worse, estimating stature requires a regression drawn from some population, but that population may have different proportions than the fossils. In this case, Neandertals and the Sima de los Huesos samples probably have different crural indices, the ratio of the length of the tibia to the length of the femur. So statures estimated from these bone lengths based on some recent human population will have systematic biases due to the different proportions in the fossil populations.

Carretero and colleagues note that most of the bones (humerus, radius, tibia) have shorter average statures in the Neandertal sample compared to the Sima de los Huesos sample. The femora and ulnae are longer in the male Neandertals. All the bones that can be compared are shorter in the female Neandertals than the female Sima de los Huesos individuals. It’s probably a good bet that the Sima people were a bit taller than Neandertals. Still, the tall West Asian Amud skeleton points to the possibility of variation among Neandertals from different regions.

The differences between Neandertals and the Sima de los Huesos sample are quite small compared to the much taller statures attributed to modern humans from West Asia (Skhul and Qafzeh). These skeletons are more ancient than most of the Neandertal sample, but at 100,000 years old, much later than the other skeletal samples included in the paper including Sima de los Huesos. The authors make a strong point of this, suggesting that tall stature is a fundamentally new feature of the evolution of modern humans (which they equate with “early H. sapiens”):

As we have shown here, medium height and above-medium height people seem to characterize the primitive Homo biotype, while a very tall body characterizes the derived biotype. The heights proposed for all fossil human species, except early H. sapiens, seem to average around 165170 cm, although tall individuals exist within all samples (e.g., Amud 1, Kabwe and Jinniushan). It is only the first H. sapiens that are consistently and dramatically taller. Therefore, the evolution of stature (and perhaps also body size and shape) in humans seems to have been characterized by a long period of stasis during which the primitive body plan shared by the different Homo species varied rather little in stature throughout the Pleistocene, until the rapid appearance 200 ka of a new species with a new biotype, the light H. sapiens.

The paper’s broad assertion is that Early and Middle Pleistocene humans everywhere in the world shared the same basic body plan, with stature around 165-170 cm (for males) and relatively broad pelves. The reference to modern humans as “light” concerns the relatively narrower pelvis of recent humans.

I have no disagreement about the issue of pelvic breadth, although it deserves a separate review. But the stature of the Skhul-Qafzeh sample is neither very extreme nor is it typical of other Late Pleistocene or Holocene modern human samples. I will reprint a quote from my 2007 post about the statures of the Dmanisi hominins (“News flash: Dmanisi hominids were not short”):

Pretty and colleagues (1998) studied an archaeological sample of Aboriginal Australians from the Murray River region. Using stature estimation methods for the tibia, femur and humerus, they found that males in their sample (n=55) had an average stature of 166 cm and females (n=40) an average of around 153 cm. Wells (1952) reported a mean for !Khu (Northern Bushmen) males of 158 cm and females of 148 cm, both with standard deviations around 5 cm. Ruff (2000) puts the average stature of males at Pecos Pueblo at 161.2 cm with a range from 155 to 168 cm. In the KNM-WT 15000 monograph, Ruff and Walker (1993) report the average stature of African population samples, excluding Pygmies, as 162.3 cm. And although it is common knowledge that the Early Upper Paleolithic people of Europe were tall, the average male stature in the Late Upper Paleolithic was around 166 cm, and the average female stature around 153 cm (Formicola and Giannecchini 1999) -- virtually the same as Australians.

The Skhul and Qafzeh people were indeed tall relative to these other human samples, with male skeletal elements yielding stature estimates from 170-190 cm. The average stature of American men today is 176 cm. Holliday Holliday:2002 showed that early Upper Paleolithic males had an average stature around 170 cm. According to Carretero and colleagues, the average Sima de los Huesos adult male had a stature around 168-170 cm. And as they note, taller individuals with stature estimates of 180 cm or more are present in the Early and Middle Pleistocene sample – most notably the large Kabwe tibia, but we can also mention KNM-ER 1808 and KNM-ER 736 from the Early Pleistocene of Kenya.

I disagree with the paper’s suggestion that modern humans represent a new pattern of tall stature compared to earlier humans. I propose instead as a null hypothesis that human stature has not changed systematically since the Early Pleistocene.

That doesn’t mean human stature hasn’t evolved. Human populations today are variable in stature, and they were in the recent past. We have pygmy populations with statures that average 150 cm or less in males, and peoples with statures that average close to 180 cm. Tall and short-statured populations today live in nearly every region, or did so in early Holocene times. Some of the variation in stature among populations is nutritional, some is additive, and both sources of variation appear to have emerged repeatedly in different contexts in recent human evolution.

I suggest that pattern of variability would also have been present in earlier populations of humans. The differences between Neandertals and early Upper Paleolithic Europeans and the Skhul-Qafzeh sample were substantial but do not exceed the differences among recent human populations. The human stature adaptation is variable within a relatively broad niche, and has been so for nearly 2 million years.