Readings for week 4: variation in human body form

This week we read two papers that show interesting patterns, across space and through time, in the size and shape humans. Here they are:

Morwood M.J., Soejono R.P., Roberts R.G., Sutikna T., Turney C.S.M., Westaway K.E., Rink W.J., Zhao J.x., van den Bergh G.D., Due R.A., Hobbs D.R., Moore M.W., Bird M.I. & Fifield L.K. (2004) Archaeology and age of a new hominin from Flores in eastern Indonesia. Nature, 431, 1087-1091

Ruff, C. 2002. Variation in Human Body Size and Shape. Annual Review of Anthropology 31:211 - 232.

There is some technical language in each paper. For instance, the Ruff paper discusses some of the methodological issues in estimating stature from skeletal measurements. Don't worry about the specifics here but it is interesting to think about how the lifestyle behind the fossil in question and its geographic location affect its size and how we estimate it. Its also good to think about the details of data gathering for these big picture trends, which are not addressed by a lot of the papers we'll read. The Morwood et al paper has some details about specific dating techniques. I don't plan to discuss these at all - just know that they used a variety of techniques to determine the range of dates that bound the geological context of the finds. If you want to understand the specifics of the techniques let me know or try a google search.

Pay special attention to the trends in body size change and the brief mentions of their proposed explanations. Note that even though there is not any life history theory here, we know that major shifts in size must be related to shifts in life history characteristics. What sets body size? Why should an organism get bigger or smaller? But the focus of both papers is the range in variation in body size and the 'clinal' patterning we see in time and space.

Some Flores background:
Most of you have probably heard about the Flores finds - the potentially new species of hominid found on the Island of Flores in the Lesser Sunda Islands a few years ago. Its been hotly debated ever since. A lot of researchers do not like the idea that H. floresiensis is a dwarfed form of H. erectus, but this was the first proposed explanation and still represents an important and current study with implications for human biogeographic patterns of size, colonization, evolution, and so on. It is often referred to as the 'hobbit' because of its very small size.

This is a figure from a news and views piece in Nature by Lahr and Foley that helps place the Flores finds in the context of our evolutionary tree (assuming you accept the finds as that of a new species).

Nature Published online: 27 October 2004; doi:10.1038/4311043a Human evolution writ small Marta Mirazon Lahr & Robert Foley

Figure 1. Homo floresiensis in the context of the evolution and dispersal of the genus Homo. a, The new species as part of the Asian dispersals of the descendants of H. ergaster and H. erectus, with an outline of the descent of other Homo species provided for context. b, The evolutionary history of Homo is becoming increasingly complex as new species are discovered. Homo floresiensis (left) is believed1 to be a long-term, isolated descendant of Javanese H. erectus, but it could be a recent divergence. 1, H. ergaster/African erectus; 2, georgicus; 3, Javanese and Chinese erectus; 4, antecessor; 5, cepranensis; 6, heidelbergensis; 7, helmei; 8, neanderthalensis; 9, sapiens; 10, floresiensis. Solid lines show probable evolutionary relationships; dashed lines, possible alternatives.

Here you see how floresiensis compares to other hominin species with respect to brain and body size (from the same article by Lahr and Foley - yes the same Foley that wrote the paper we read in week 2).

Human evolution writ small Marta Mirazon Lahr & Robert Foley

Figure 2. The relative brain and body size of H. floresiensis. The dimensions of the skull and skeleton (LB1) described by Brown et al. fall well outside the extremes seen in H. sapiens and the 'erectines' (a range of hominin species, of which H. erectus is the most familiar). LB1 is closer in size to, but even smaller than, the australopithecines, of which the best known example is Lucy. On various anatomical grounds, however, Brown et al. believe that LB1 represents a dwarfed H. erectus.

There is a vast amount of discussion of the Flores finds on the John Hawks blog as well.


Terms and concepts:

clinal variation – [in response to Steven’s question] within a given species or related group of species (also called a taxon), gradual variation in a trait along some geographic axis. For example, the gradual decrease in the size of rabbit ears with increasing latitude is a clinal variation. Another example is a gradual change in some plant’s flower color from red to reddish pink to pink to light pink to white over the plant’s range. If instead the plant just had red flowers in one area and white in another, with no “mixtures” in between, you’d say the plant exhibits “discrete variation” in flower color.

directional selection – [in response to Dennis’s comment] selection toward an extreme version of a trait. For example, a population of rabbits in which small-eared individuals are most likely to survive and reproduce is undergoing directional selection for ear size. Think of direction selection shifting the bell curve for some trait toward one end or the other.

Sexual dimorphism: difference between the sexes. For height this is often expressed as average male size divided by average female size (or vice versa). It is often thought that higher levels of dimorphism reflect important behavioral and mating characteristics. If males compete for access to groups of females, they may have to be very large to fight off rival males. When females and males are closer to the same size it is sometimes considered to indicate a prevalence of monogamy. If so, observing an increasing trend toward lower levels of dimorphism in the hominin fossil record has important implications for social structure and life history. Potentially, when male investment becomes more important, pair-bonds become more stable, and male-male competition over females decreases. This might link to some arguments about slow growing and highly dependent offspring that are key characteristics of the human life history. The grandmother hypothesis that we talked about a few weeks ago is at odds with the view that increased similarity in size between males and females indicates that our species was pair-bonding (stable marriage-like pattern of male-female bonding). The grandmother hypothesis argues that males produce meat from hunting in order to gain access to mates apart from their wives. Essentially, males are actually parasites on the productivity of their wives and their wives' mothers and meat is not really a nutritionally important resource. Please think carefully about interpretations of sexual dimorphism.

stature - height

bi-iliac breadth - this is one of a few measures of human body form mentioned by Ruff. It is the widest distance between the pelvic bones. (the bones that you can feel as your 'hips'). it is measured with a special kind of caliper.

Bergmann's Rule - Mammals and birds tend to be larger at colder temperatures and higher latitudes.

Allen's Rule - Warm-blooded animals tend to have shorter limbs in colder climates.

secular trend - a trend that continues over large periods of time. Often meant to mean directional change.

clinal variation - Gradual continuous change in an observable physical (morphological) trait across space and within the range of some group of related organisms. The observed change is usually related to some environmental feature.

stegodon - an extinct species of elephant. Dwarf stegodons were found near the Flores hominids; in the same levels and close by. A common trend is that large mammals that colonize become much smaller. It has been hypothesized that this process, insular dwarfism, may explain the small hominids on Flores.

what do we call ourselves? There is a lot of nomenclature in the readings this week. Keep in mind that the evolutionary tree for primates is getting reorganized all the time and often this requires using new terms or changing the definition of existing terms. For the most part - hominid refers to all great apes. This includes fossil relatives like australopithicines and living species like chimps, orangutans, and gorillas. Note that for the sake of confusion some authors used hominid to refer to only bipedal ancestors. Hominin is used by Ruff to mean humans and their ancestors (homo sapiens and the extinct varieties of homo that preceded us) but slightly different definitions exist as well.


Condensed abstract of Pilbeam & Gould, 1974, in response to blog query:

Pilbeam, D. and Gould, S.J. 1974. Size and scaling in human evolution. Science 186: 892-901.

“Our general conclusion is simply stated: many lineages display phyletic size increase; allometric changes almost always accompany increase in body size. We cannot judge adaptation until we separate such changes into those required by increasing size and those serving as special adaptations to changing environments.
In our view, the three australopithecines are, in a number of features, scaled variants of the "same" animal. In these characters, A. africanus is no more "advanced" than the larger, more robust forms.
The fossil hominids of Africa fall into two major groupings. One probable lineage, the australopithecines, apparently became extinct without issue; the other evolved to modern humans. Both groups displayed steady increase in body size. We consider quantitatively two key characters of the hominid skull: cranial capacity and cheek tooth size. The variables are allometrically related to body size in both lineages.”