Why pygmies?

March 29, 2009 • 6:07 am

Some human populations in South America, Africa, and Southeast Asia are of extraordinarily small stature: they are called pygmies.   All of them inhabit rainforests that are warm and humid.   Although they bear a common name, genetic work shows that each group has evolved independently, so it is better to speak of “the pygmy phenotype” (“phenotype” refers to any aspect of an organism that can be observed or measured).  A population is said to show the pygmy phenotype when the height of adult males averages about 160 cm (5′ 3″) or smaller. (The smallest pygmy population comprises the Efe hunter-gatherers of the Congo, where adult males and females are 143 and 136 cm tall respectively [4′ 8″ and 4′ 6″]).

A new paper in Trends in Ecology and Evolution summarizes what is known about the distribution, genetics, and evolutionary basis of the pygmy stature.  The first thing we learn is that we’re not absolutely sure if the height difference is due to genetic differences between pygmies and populations of “normal” stature.  Could it instead be due to differences in nourishment alone? The evidence is against this because pygmy populations show no clinical signs of malnutrition.  Still, other nongenetic environmental factors could be responsible.  The best way to show that the height difference is based on genes is, of course, to rear pygmy and non-pygmy infants in a common environment and show that the height difference remains; surprisingly, this has not been done (though surely there are some pygmy children brought up in different environments— the authors don’t discuss this).

Why are pygmies so short?  If the this phenotype is indeed genetically based, the obvious hypothesis is that natural selection in warm tropical environments causes humans to evolve smaller size. But why?  The authors give four hypotheses:

1.  There is a scarcity of food in the rainforest, and this selects for smaller individuals who are able to maintain their bodies with fewer calories.  The evidence for this hypothesis is mixed: pygmy populations don’t especially suffer a dearth of calories, although this may be due to their recent trading for food with other populations outside the rainforest.

2.  Living in a hot, humid environment selects for smaller bodies because smaller individuals have a higher ratio of body surface are to body volume. This allows them to lose, through sweating and heat transfer, relatively more heat than larger individuals.   The weakness of this idea is that among pygmy populations there is no correlation between body size and ambient humidity.

3.  It is easier to move through dense, tangled forest if you are small.  Bending down repeatedly while walking apparently uses quite a bit of energy.  There is only anecdotal evidence for this idea, but it may be true.

4.  If there is high mortality, then it may pay you to mature and reproduce early because otherwise you could die and leave no genes.  The authors note that infant mortality up to age 5 of  African rainforest poulations are 27-40%, about twice that of nearby populations that live in other habitats.  This idea predicts that among populations, there will be a positive correlation between life expectancy and average adult height.  This is indeed observed, providing some support for the idea.

Of course, all or some of these factors could work together.  At present, we have some fruitful ideas about why the pygmy phenotype evolved, but nothing definitive yet.  It is surprising that of the many differences in appearance between human ethnic groups– differences that involve skin color, hair configuration, facial configuration, height, body configuration, and physiology –the only one whose evolution we understand fairly well is pigmentation (see WEIT for the explanation, based on sun exposure).  The rest is mystery.  In my book I broach the idea that sexual selection may account for some of these, though it’s hard to explain the short pygmy phenotype this way (why would the target of sexual selection be correlated with humid, forested habitats?).

Note: Another population that was abnormally short were the “hobbits”: Homo floresiensis, a population (described as a species) of humans that lived on the Indonesian island of Flores. Again described in WEIT, this species is based on a single skull from one individual and an arm bone from another.  Their apparent height was about 106 cm (3′ 6″), they weighed about 50 pounds, and were the size of a modern 5-year-old child. They lived about 18,000 years ago, when H. sapiens of modern stature already lived throughout the world.  Although some think that the single tiny individual was really diseased and not a “normal” individual, it does appear from the arm bone that they really were this small.  But they had one feature not present in modern pygmy populations: very small brains, about half the size of modern human brains (the brains of pygmies scale roughly the same as short humans elsewhere).  H. floresiensis did not represent the pygmy phenotype, and were more likely an ancient population of a different species of Homo that, isolated on its island, was bypassed by the evolution of other populations into the modern human phenotype.

Here is a figure from Perry and Dominy’s paper showing the distribution of pygmy populations throughout the world (red dots) and some pictures of pygmy individuals:

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Figure 1. Association of the human pygmy phenotype with tropical rainforest habitats. (a) Approximate locations of small-bodied hunter-gatherer populations discussed in this article, with mean adult male stature estimates [6], [10], [65], [78] and [79]. The smallest modern human statures (mean adult male height < 155 cm) are always associated with tropical rainforests (red circles). Some hunter-gatherer populations occupying savanna-woodlands (black circles) are also relatively small, such as the Hiwi of the Venezuelan llanos, the Hadza of Tanzania and the !Kung San of Botswana and Namibia. Precipitation data are from the Tropical Rainfall Measuring Mission (Goddard Space Flight Center, National Aeronautics and Space Administration; http://trmm.gsfc.nasa.gov). (b) Yanomamö male, Venezuela (photograph by Raymond Hames, with permission). (c) Efe male, Democratic Republic of Congo (photograph by William Wheeler, with permission from the National Anthropological Archives, Smithsonian Institution). (d) Batek male, Malaysia, with white-handed gibbon (Hylobates lar) hunted by blowdart (photograph by Kirk Endicott, with permission).

Photo and caption from Perry, G. H., and N. J. Dominy. 2009. Evolution of the human pygmy phenotype. Trends in Ecology & Evolution 24:218-225.

Research hot off the lab bench:  My friend Graham Coop at The University of California at Davis sends me this note that they may be zeroing in on the genes responsible for the pygmy phenotype:

We’ve just published a large scan for selection across various human populations: two of the of the top hundred SNPs [single nucleotide polymorphisms] whose allele frequency most differentiate Bantu populations from pygmy populations fall next to genes in the insulin growth factor signaling system (discussed on the bottom of page 6). Obviously these will require much more followup, but these seem like reasonable candidates for genomic regions habouring variation affecting height in pygmies.

Dick Lewontin is 80 today

March 29, 2009 • 5:15 am

My Ph.D. advisor, mentor, and friend Richard C. “Dick” Lewontin turns 80 today.  Still active at Harvard’s Museum of Comparative Zoology, churning out pieces for The New York Review of Books (here’s a good exemplar), and chopping wood on his Vermont farm, let no one say that Dick has reached his dotage.  Distressing to the rest of us who are ageing fast, Dick has only a few gray hairs and wrinkles, and looks pretty much as he did when I came to his lab in 1973.   On his 70th birthday, several hundred of his former students, postdocs, and collaborators assembled at Harvard for “Dickfest”, and there will be a “Dickfest II” this summer.   This is not the place for me to wax effusive about Dick’s many talents as researcher, writer, and teacher; just let me say that I dedicated WEIT to him.  Hoist one for the “old” guy today.

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If you’d like to see him in action, eloquent as always, below is an hour-long interview (“Conversation with History”) he did with Harry Kreisler at Berkeley:

Caturday felid

March 28, 2009 • 1:44 pm

Presenting Verismo Leonetti Reserve Red, the Guinness World Record holder for “longest cat.”   This behemoth Maine Coon cat is 48 inches long from the tip of his nose to the tip of his tail, and weighs 35 pounds. He is NOT fat!  Leo lives in the suburbs of Chicago, and I’ve not yet seen him, but would love to.  A cat like this can keep you warm in those Chicago winters. . . leoatvet_t

Guinness compares Verismo Leonetti Reserve Red to the same size as an 8 year old child and his paw just fits into a size 2 child’s shoe.  But if he were a human to Guinness’s scale, Leo would stretch over 8 feet tall and weigh 220 pounds.

Freida [his owner] says “We have to keep our eye on him when we’re cooking, because he can stand up and put his paws on the kitchen counter.”  Leo’s dietary favorites are listed by Guinness.

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TOMORROW:  Why are there pygmies?

Caturday mustelid

March 28, 2009 • 10:02 am

Not to worry, a felid is coming, but for those of us who love animal schmalz, this video of “otters holding hands” is a must-see.  I dedicate it to the inamorata (you know who you are!):

Oh, and on matters otterine, one of the best nature-related books I’ve ever read is Ring of Bright Water, by Gavin Maxwell.  Maxwell had a fantastic life as a world traveller, naturalist, and writer, and I think this is his best book.  It simply recounts his years at Sandaig, a remote cottage on the west coast of Scotland, his relations with the local wildlife, and especially his friendship with several pet otters.  The writing is ineffably beautiful: some of the finest prose I’ve seen in any book about humans and their relationship to nature. Highly recommended.

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This biography, by Douglas Botting, is also superb

Outcome in Texas: Mixed but not that great. Lunacy spreads to Florida

March 28, 2009 • 6:07 am

The school board hearings have ended in Texas, and the outcome is mixed. In other words, we’ll all have to keep watching and fighting the benighted hordes who keep trying to insert scripture into the school curriculum. According to Gordy Slack’s report over at Salon, the motion to include teaching about the “strengths and weaknesses” of science (read: evolution) was rejected by an 8-7 vote, but Don McLeroy (the creationist head of the board) and his minions retreated, dug in, and won some stuff by proposing a variety of amendments:

Eugenie Scott, director of the National Center for Science Education, an Oakland, Calif.-based organization dedicated to protecting the integrity of science education in the public schools, says that once McLeroy and his allies failed to pass the “strengths and weakness” language, “they had a fallback position, which was to continue amending the standards to achieve through the back door what they couldn’t achieve upfront.”

And they succeeded. Casey Luskin, a Discovery Institute lawyer, and its guy on the Austin scene, was psyched by the outcome. “These are the strongest standards in the country now,” he says. “The language adapted requires students to have critical thinking about all of science, including evolution, and it urges them to look at all sides of the issue.”

Here is what they voted in:

1). A requirement that students “analyze and evaluate scientific explanations concerning any data on sudden appearance and stasis and the sequential groups in the fossil record.” In other words, McLeroy succeeded in getting his “stasis is God” position officially adopted (see yesterday’s post).

2). A requirement that teachers and textbooks compel students to “analyze and evaluate scientific explanation concerning the complexity of the cell.” This is, as we all know, part of the intelligent design claim that cells are too complex to have evolved.

Now how in the world are public school students going to meet these requirements without having to be taught intelligent-design/creationist positions? I can see it now: textbooks will have to say, “Some scientists think that cells are too complex to have evolved by natural selection,” and “Some scientists think that the Cambrian Explosion and the existence of living fossils implies that A Great Designer created the world in an instant, calling all species into being.”

What really worries me is that textbook publishers are going to have to include nonsense like this to satisfy the Texas standards. I have been criticized for using the Holocaust analogy, but I think it’s an apt one: imagine a history class (which, after all, depends on assertions of empirical fact) being subject to the same standards. After reading about the Holocaust, students are then given the disclaimer, “Some people think that the Holocaust never happened, and that this story was fabricated by the Jews out of self-pity.” That’s what the Texas shenanigans really amount to. Remember, the Wedge Document of intelligent design is aimed not just at science, but at expelling ALL materialistic ways of investigating nature from the schools.

We’re in trouble.

And down in Florida, Tampa Bay Online reports that creationists have filed another bill in the state senate requiring “A thorough presentation and critical analysis of the scientific theory of evolution.” (Thanks to PZ over at Pharyngula for this link.)

NOTE:  Florida Citizens for Science says that this bill is dead for the year.  But given what has happened down there, I suspect that it– or something like it– will be back soon.

Clearly, this is the next-generation strategy of creationists. It has the merit of not looking explicitly religious, although of course its motivation is precisely that. It’s a clever strategy. Let’s see how the courts deal with it.

More on Texas: Good guys winning, but it’s dicey

March 27, 2009 • 5:41 am

According to the Dallas News and the NCSE report (and noted by Genie Scott yesterday), the Texas Board of Education voted down (by voting a tie) for the “strengths and weaknesses” clause discussed here yesterday:

Against the proposal were three other Republicans and four Democrats.” A final vote is expected on March 27, 2009, but the outcome is not likely to change. It remains to be seen whether the board will vote to rescind the flawed amendments undermining the teaching of evolution proposed at the board’s January 2009 meeting.

But the fight isn’t over yet: Board member Barbara Cargill is trying to slip in an amendment requiring children to learn that “there are different estimates for the age of the universe”!!! We all know what that means: she doesn’t mean 13.7 plus or minus .2 billion years, she means 6,000 to 13.7 billion years. In other words, this gives wiggle-room for the ridiculous Biblically based estimate of 6000 to 10000 years. Here’s the report from the live blog on the meeting by Steve Schafersman:

I can’t get a copy of these amendments right now. However, the first one she wants is to strike the current standard for the Big Bang and remove the 14 billion year old age from it. She says she wants teachers to tell students that there are different estimates for the age of the universe. What would these be? 13.7 billion years and 10,000 years? She is promoting a Young Earth Creationist view, of course. Many times in the past the SBOE has changed standards and textbook content that mention millions and billions of years to simply “a long time ago.”

Cargill wants to substitute a standard from Astronomy that simply adds, “and current theories of the evolution of the universe including estimates for the age of the universe” to the Big Bang standard 4A. This Astronomy standard is poor in several ways: it is vague, it is non-specific, there is only one current theory for the origin of the universe, and there is currently a well-established consensus that the universe is 13.7 billion years old, so there are not multiple “estimates.” It is sad that the astronomy teachers came up with such an incompetent standard, and now it is being inflicted on ESS. Cargill’s amendment that strips a very ancient number of years and replaces it with vague “estimates” that are equivocal about the age of the universe.

While speaking for her amendment, Cargill says she “has no intention of opening the door to teaching Creationist ideas about the age of the universe.” Yeah, right. Next, she made a Freudian slip and her secret intentions were revealed. She said “universal common design” when she meant to say “universal common descent.” Her unfortunate amendment passes by a vote of 11-3, with only Knight, Miller, and Nunez voting no. So the SBOE holds true to its wonderful tradition of stripping any date older than 10,000 years from science standards!

Stay tuned. In the meantime, some video clips from the controversy. First, the good guys (gals): Genie Scott testifying about the S&W clause the day before yesterday:

Second, the benighted dentist and chairman of the Texas Board of Education (it makes me cringe to write that), Don McLeroy, reading from Stephen Jay Gould in an attempt to prove that “stasis is God”.  I only wish Steve were still alive to respond to this — he would crush McLeroy like a bug.