Saturday, September 8, 2007

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


Fig 1 full size
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


Fig 2 full size
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.”

37 comments:

Human macroecology admin said...

Hi.
I plan to edit this post a bit more and maybe add some terms to it, but I wanted to get it posted sooner than later. If there were any terms in either paper that you think should be defined, let me know and perhaps I can add them to the brief list here.
I hope you are all enjoying your weekends.
Oskar

Human macroecology admin said...

One of Paul's comments on Thursday offer a nice transition to this week's papers. Recall that we were discussing the issue of taxonomic units for human biogeography and how to deal with the challenge posed by Terrell - that because of isolation by distance there are only subtle changes with distance rather than discrete boundaries to populations. Paul suggested that we could just take samples from different places (and times) for a certain trait and then look how this trait varied as a function of some ecological variable. We would think of this as clinal variation. These papers give examples of this approach by looking at variation in body size at large scales. Note the interesting changes in size through time and keep in mind that body size is a very important ecological important variable. Next week's readings will related to these themes as well.
Cheers,
O

dodegard said...

H. floresiensis is said to have a smaller mental capacity than a normal sized H. erectus and that it would have been incapable of creating the tools found in the cave. Has any research come out confirming the cognitive abilities of the individual?

Anonymous said...

Yeah, there has been a lot on this but its inconclusive. If you go to that John Hawks blog that we link to in the post and search (control F) on cognition you'll find his take on a lot of it. He's a bit biased toward the anti-Flores perspective but you get quotes from the original papers and all of that to see for yourself. Some people say that the right parts of the Flores brain were enlarged for language and complex tasks, but others say no. It will be a while before it gets resolved. They really need another cranium.

Michael said...

Referencing the Ruff paper and with the large body of research pertaining to recent nutrition and its relationship to stature in mind, it seems a bit counterintuitive to assume that as encephalization and technology (and with it, more efficient subsistence strategies) increase, body size decreases—namely, while anatomically modern humans are still hunting and gathering pre-agricultural revolution circa 50 kya.

Fred Whiteman said...

Ruff states that the average body size of individuals in the genus Homo increased around 500 kya, due to their expansion into cooler climates. However, later in the paper he talks about latitudinal clines in terms of body shape, and says that body size alone is not a reliable measurement. Since it's probably difficult to obtain the relative body shape from fragmented fossil remains, how do we reconcile these findings?

Wenyun said...
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Wenyun said...

Is it possible that two isolate populations can have similar evolutionary trends or endemic change? If it is, will we find genetic similarities or culture/art similarities?

Dan said...

The Ruth article attempts to outline a population sensitive model for assessing over and underweight individuals. He uses the skinfold measurements to assess the effectiveness of the BMI and TFI methods. Why isn't the skinfold method used as the standard?

Steven M. said...

Ruff discusses some of the behavioral reasons that might lead to sexual size dimorphism in H. sapiens and related species.

H. sapiens is not alone in sexual size dimorphism, how similar are the reasons for sexual size dimorphism in H. sapiens to sexual size dimorphism in other animals, say moose?

Steven M. said...

I am still not completely sure what the term clinal variation means.

It seems to be used to indicate that a trait varies with response to some environmental factor, but I am not sure.

Could someone please provide a clear definition?

Deepta said...

How do measurements of teeth lead to systemic overestimates of body mass? Wouldn't there be varying patterns of wear on the teeth depending on what is being consumed? It seems there is a greater risk of underestimated the size.

Deepta said...

Also, does anyone know anything about the research mentioned on gut size (Pilbeam & Gould, 1974)? I tried to find the article and failed.

tlvandeest said...

In Ruff's paper he discusses the changes in ancestral hominin's sexual dimorphism. But the sex of the individuals are assessed on size and not on other morphological traits, as in modern human osteological analysis. So how can judgements be made on the amount of sexual dimorphism in hominins when body size is the determining factor of sex?

Justin Smith said...

How well does the observed size of H. Floresiensis fit that predicted by insular biogeography?

Has the population of H. sapiens on Flores evidenced a significant decreasing trend in body size since arriving on Flores 55-35kya? Should we expect modern humans living on islands to shrink in future generations?

dtinucci said...

Has recent work on tectonic movements suggested a date for the separation of Flores Island from a larger mass? It would seem that directional selection who have been in play for a serious amount of time to effect such dramatic changes.

paul said...

Ruff: Mass dimorphism has dropped since Australopithecines. We do have some body size dimorphism (about 1.2), but our lack of dimorphism in canines is fairly unique for primates. Plavcan and van Schaik (one of Ruff's references) say this might indicate distinct behavioral/social organization.

Canine dimorphism is strongly associated with male-male competition. So was there less competition in our lineage, or was competition not so tooth-based? We really use our hands, eh...

A couple additional things could affect all this. 1st, greater pair-bonding and monogamy will probably reduce male-male competition. 2nd, differences in male and female production behavior (nearly absent in other apes, right?) could drive dimorphism, rather than differences in physical competetiveness. This was presumably increasing as we entered our recent foraging niche.

So a hypothetical question: if males and females had the same production behavior, would the size dimorphism go away? (probably not, I'm guessing.)

Senorita Myra said...

Michael, I think a lot underestimate the amount of energy it takes to support more brain mass. Isn't there a stat that roughly 20% of our total energy goes to run our brains? I would think that the negative correlation between brain size and body mass is predictable and dead on......Oh yeah, dido tlvandeest...I was thinking the same thing when i read about the sex of skeletal remains being determined by size of the bones instead of other traits.

Kenneth Letendre said...

This is a question that might apply broadly to a large number of features that may differ among populations in different world regions, but: to what extent will modern interbreeding between previously widely separated populations lead to homogenization and diminishment of the clinal variation discussed in Ruff? Will local selection be sufficient to counteract this trend toward homogenization?

paul said...

Ruff’s drivers of body size change: terrestrial lifestyle, living out in the open, need to go long distances, diet change, colder climate, new production technology, nutrition, inbreeding, crowding, disease, social inequality.

Different thought: Could a change in body size reflect a change in extrinsic mortality? Extrinsic mortality decreases, so the future is more certain, so it’s worth investing more time and energy in the body, cause you’ll be a stronger producer and reproducer as an adult. Kaplan & Robson’s (2001) model says this would happen, right? (if you call body mass itself the capital stock they talk about).

On the other hand, if investing in a big body is like investing in armor (i.e. you do it as a way of endogenously decreasing your mortality), then perhaps you would ease off in a less dangerous environment. For example, when there’s male-male competition, there’s an arms race dynamic, where your optimal investment in size or weaponry depends on how strong your competitors are. If they’re really wimpy, no reason to go overboard and invest a ton in getting huge. If competition or the strength of competitors decrease, you back off on being bulky. If having a big body protects you from predation (elephants have to worry a lot less about lions than zebras, right?) and then all the predators die off, would you want to get smaller? I don’t know. Seems like there’d be reproductive benefits to staying big. (See the following post.)

Empirically, mortality scales with body size at -1/4 across species. Brown et al. (2004) say this could be because metabolic turnover is slower in bigger (and colder) animals, so senescing damage at the level of the cell comes slower; or maybe because bigger (and colder) animals have lower rates of interactions that lead to death (competition, predation, parasitism, disease).

Seems like bigger organisms are more robust to damage than small ones. If a mouse gets stepped on, it’s dead. If I get stepped on, I might lose a toe-nail, but it’ll grow back. Same absolute amount of damage (e.g. # of cells killed) is very different to the two of us. This is like large populations being more robust to extinction than small ones. I don’t feel like the Brown et al. (2004) hypotheses capture this.

One hypothesis for insular gigantism is that islands can’t support big predators, so little prey can get big, cause they don’t have to run and hide. This is different logic from the big-body-protects idea. It’s a small-body-keeps-you-safe idea. No need to be safe, no need to stay small. The Kaplan and Robson model could predict the same relationship by a different route: remove a source of mortality (predation), and organisms invest more, go for a bigger, slower strategy.

paul said...

Scaling theory (e.g. West et al.) rarely yields optimal body sizes, right? Seem the result is often that elephants and mice end up looking similar in terms of the amount of resources they’re harvesting, but the mice convert it into a lot of little offspring quickly, and the elephants covert it into one or two offspring slowly. Or am I wrong about that? Help me out.

Dan said...
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Dan said...

Paul thanks for the thought provocaing posts. Here are a few thought I had:

I agree that sexual dimorphism would not disappear without a sexual division of labor, I mean we still have sexual selection right?

Couldn't a decrease in extrinsic mortality patterns also select for a smaller body size because more energy could be put toward reproduction than growth?

The first result on a google search on insular gigantism is an article about Tiger snakes (a predetor!) from Australia and didn't they find a komodo dragon on Flores? It seems insular selective pressures are complicated and are confusing at least to me.

Also, I didn't really follow your last post, what is scaling theory? (metabolic?) or when was the West article published? I had trouble finding it.

paul said...

Hey Dan, metabolic scaling theory's got a whole set of papers, mostly since the first big one in 1997. Here's a big-picture sum-up: Brown et al. 2004

(Don't know if that link's gonna work...)

Oskar pointed out that since lifespan scales with body size at 1/4, and fertility at -1/4, any organism (that stays on the line) will have the same # of offspring in a lifetime. So that's an invariant.

For scaling theory to be totally agnostic about optimal body size, all measures of ultimate reproductive success are going to have to be invariant, right? Is that true?

paul said...

damn, it didn't work. this should:
brown paper

Michael said...

Regarding the question posed in class pertaining to the reduced genetic variation (inbreeding) at the beginning of the Neolithic, it seems Ruff is assuming that with the advent of agriculture, human populations became entirely sedentary, preventing genes from flowing freely. Studies analyzing the population genetics of groups like the Yanomamo have confirmed a fission-fusion pattern for population growth and expansion that may also be applicable in these early river valleys. Also, considering the finite availability of cultivatable land, it can be assumed with a high degree of certainty that these early human groups were in a position to participate in various systems that would promote gene flow e.g trade networks, wars or slavery.

Human macroecology admin said...
This comment has been removed by the author.
Human macroecology admin said...

Michael,
Good point. I think you're right. That must be what Ruff is referring to. Thanks.

Paul,
Brown, Marquet, and Taper have the well known BMT optimal body size paper (1993). This is obviously before the metablic theory of ecology came along. The BMT model hasn't been widely applied and many seem to think it has a few problems. Its built on the maximum power principle and its a very cool read regardless. They basically show that the optimal size is the most efficient for converting harvested energy into reproductive power. It has that explicit thermodynamic focus - where 'work' is reproduction. You can find the paper here
href="http://biology.unm.edu/jhbrown/Documents/Publications/BrownMarquet&Taper1993AmNat.pdf">

(hope that works - my last try didn't!).
best,
Oskar

Fred Whiteman said...

Does Flores have special characteristics that encourage insular dwarfism more than other islands? If not, was it simply chance that led a small Homo species to evolve there? Should we look for remains of unusual human populations on other islands?

Perhaps a related question is, isn't it relatively rare to find fossils in such tropical locations?

tlvandeest said...

The last sentence in the Morwood et al. (2004) article states, "Size reduction is a predictable evolutionary trend, but other trends will reflect island-specific adapatations, demographic changes, and the impacts of catastrophic events, such as volcanic eruptions." What trends are they referring to and could any of these other trends provide an explanation of the stature of H. Floresiensis?

Dan said...
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Wenyun said...

For the last 50,000 years body size decreasing, could we explain it by temperature increaing?

dodegard said...

Are there any disadvantages to using TFI instead of BMI? It seems like it is not the newest way to measure body fat and not the most common. What has kept it from being used more readily?

Senorita Myra said...

I feel there is a lot missing with this theory of why body mass has been decreasing over time. What about climatic events affecting resource availability, what about quality of resources affecting nutrition and growth, what about sexual selection pressures on the human body? Are these so minute to be left out? Also, can't the same questions be applied to try and understand if the Florensis is a completely new species? What if some of the above factors, especially availability of resources, just made Florensis smaller, but not exactly a different species?

dtinucci said...

Taking Ruff's article to its further extreme, in terms of body size do you think the populations of today growing, declining or stable?

Dan said...
This comment has been removed by the author.
Dan said...

What constitutes a species? The horse, mule, donkey (or my favorite the zonkey) problem.
Almost no one would argue that a horse and donkey are the same species. However they can rarely produce a fertile female mule (the male mule has never sired offspring). So is the mule, a horse? is it a donkey? Well it is a hybrid of the two. When was the exact point in time when the horse/donkey line split? Was there an exact time when they split? My guess is there was a disruptive secular trend which developed very slowly, meaning there was no particular point of speciation. During this slow process the donkey line and the horse line possibly could have interbred producing viable offspring in higher frequencies then they do now, which decreased through time eventually leading to no possible viable offspring like say a horse and a moose (or do they have morses?). So my point is, that the specimens of H. floresiensis may have been a dwarfed H. erectus of the same species or a completely different species. But if you take isolation and time as a given almost certainly H. floresiensis would have constituted a different species. So, if you crossed Flore with Erectus would you then get a Gimli (He didn't have any Children)?

 
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