What makes a Haplorrhine a Haplorrhine?

Williams, Richard Kay, Christopher Kirk and Callum Ross have published a new paper in the Journal of Human Evolution reassessing the phylogenetic placement of Darwinius masillae, the much-hyped Adapid fossil published last summer.  Brian Switek at Laelaps and Eric Michael Johnson at The Primate Diaries have written some excellent posts summarizing the most recent paper, as well as the whole fiasco from the beginning.

Much of the debate about this little fossil surrounds whether it’s a Haplorrhine- which would group it with tarsiers, monkeys, and apes- or a Strepsirrhine- which would group it with lemurs, lorises, and galagos.  But how do we decide which is which?

This is the rhinarium on a cat, but it's very similar in Strepsirrhines. Note how the rhinarium attaches to the gums. Image from wikimedia commons.

The first thing we look at is the nose.  Strepsirrhines have a wet nose, like a dog.  Haplorrhines, on the other hand, have a dry nose, like humans.  The wet tissue around the nose is called the rhinarium, and Haplorrhines are defined by the absence of a rhinarium.  Pretty straightforward when you have a living animal on your hands- but what if you only have a skeleton or a fossil?  The rhinarium attaches to the gum right in between the two upper central incisors, and the bone underneath supports this attachment.  In the wet-nosed strespirrhines, there is a gap between the two incisor roots to allow for this bony reinforcement.  Since Haplorrhines don’t have a rhinarium, they don’t have this gap between their incisor roots.

Haplorrhines also exhibit a trend toward a reduced reliance on their sense of smell, particularly when searching for a mate.  Loss of the rhinarium is part of that, since the rhinarium can help to pick up pheromones from the environment.  They’ve also reduced the amount of a special tissue located in the nose that helps them smell things more acutely, the olfactory epithelium.  Along with that, they’ve reduced the part of their brain which processes those chemical signals, which is located in paleocortex.  If we want to look at manifestations of this in the skeleton, we see that a lot of the bony structures which support that olfactory epithelium have been reduced or lost in Haplorrhines.  We can also see that the little nook in the skull where the olfactory bulb of the brain sits is smaller in Haplorrhines.

Haplorrhines have reduced their reliance on smell, but have increased their reliance on vision.  The cone photoreceptors are much more densely packed onto their retina than in the mostly-nocturnal Strepsirrhines.  They have also developed a little pit in their retina called a fovea centralis.  The cones cells here are thinner, which allows them to be more densely packed, and they are very well-connected to the brain by retinal ganglion cells.  The areas of the brain which process visual information are relatively much larger in Haplorrhines than in Strepsirrhines, as well.

In the skeleton, this increased reliance on vision manifests itself as a bony cup behind the eye  called the postorbital septum.  The postorbital septum isolates the eyeball from the chewing muscles, and might be the result of the increased size of the eyes, or perhaps the fact that they’ve moved from the side of the head to the front of it.  Strepsirrhines have a bony strut that forms a ring around the eye, but they don’t form a septum like the Haplorrhines do.

Many of the traits that distinguish Haplorrhines from Strepsirrhines represent derived traits in Haplorrhines.  However, there are also a few traits which are derived in Strepsirrhines but absent or primitive in Haplorrhines.  For example, Strepsirrhines have a tooth comb, but Haplorrhines lack this structure.  The tooth comb is formed by the lower two incisors, as well as the lower canines.  It sticks directly out and Strepsirrhines use it to groom themselves and each other.   Strepsirrhines also have a little claw on their second toe called the toilet claw or the grooming claw, which again, is used for grooming.

And now comes the tricky part.  The traits I’ve outlined above (as well as some other more technical traits) are the result of millions of years of evolution.  There was an ancestor to all living Haplorrhines who did not possess the full suite of Haplorrhine traits.  And it can be tricky to figure out which traits are primitive retentions from an insectivore ancestor or novel traits in the lineage.  Take the toilet claw:  Is it a primitive retention from a more basal primate, or is it a novel trait?

You also have to keep in mind that the absence of derived traits is not particularly damning for a fossil.  Australopithecines do not have very large brains (a derived trait) but that doesn’t exclude them from the human lineage.

So what do we do?  We should try to do what the authors of this newest paper have done, and put our new fossil into the context of other discoveries of similar animals from around the same time period.  They come down with Frahm et al in describing Darwinius as an adapid.  Where they differ is whether or not they would place adapids into the Haplorrhine or Strepsirrhine clade.

Everyone who has looked at Darwinius agrees that she is an adapid, and in particular, a cercamonine adapid.  In order to move her over to the Haplorrhine clade, you’d have to take the rest of the cercamonines with her.   And probably the rest of the group above that, the Notharctids, and then also the group above that, the Adapids.  And most of us are simply not convinced that there is compelling evidence to do this.  In fact, the evidence is quite compelling that this should not be done.

Even if this particular adapid shares some convergences with Haplorrhines and not just primitive retentions (and that’s a big if- I’m certainly not convinced), the overall picture of Eocene primate evolution overwhelmingly shows that the adapids are Strepsirrhines.

References:

Williams, B., Kay, R., Christopher Kirk, E., & Ross, C. (2010). Darwinius masillae is a strepsirrhine—a reply to Franzen et al. (2009) Journal of Human Evolution DOI: 10.1016/j.jhevol.2010.01.003

Franzen, J., Gingerich, P., Habersetzer, J., Hurum, J., von Koenigswald, W., & Smith, B. (2009). Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005723

Optimal Foraging in Marginal Environments

Foraging for food requires a lot of energy. Let’s say you’re a monkey and you eat fruit.  There are all kinds of different fruit, but there is one fruit in particular that tastes way better than all of the other fruits in the jungle.  And you don’t know it, but this fruit is rich in calories, sugars, and proteins.  There are two trees which bear this fruit in your home range, and if you could, you would eat nothing but this fruit.  You’d walk in a straight line between the two trees, gorging yourself on it each time you get there until that tree is bare, and then leave for the other one.  But, this particular fruit is kind of rare. The two trees that bear it are about a two day walk away from one another.  And to make matters worse, it’s only ripe for two weeks out of the entire year.  So what do you do?

Sooner or later you’re going to have to eat other things.  Behavioral ecologists study these kinds of diet choices under the umbrella of “Optimal Foraging Theory.”  Optimal foraging says that organisms, be they monkeys, insects, birds, or humans, should eat the foods with the highest amount of calories, but cost the least amount of energy to find.

A model within Optimal Foraging Theory is the “classical prey model,” and it predicts how animals make decisions about what to eat.  When that favorite fruit of yours is ripe, you probably won’t waste precious time or energy digging out that hard, tough fruit on the tree next door.  However, when your favorite fruit isn’t ripe, and it’s the dry season and there’s not much food to be had, you might go after that hard fruit as if it were your favorite fruit!

Himalayan langur, from treknature.com

Ken Sayers and his colleagues recently published a paper in the American Journal of Physical Anthropology which explored Optimal Foraging Theory in Himalayan gray langurs (Semnopithecus entellus).  The Himalayan monkeys were an ideal species to study for this because they eat a wide variety of foods which are seasonal.

According to the classical prey model, there is a threshold value: net energy obtained by eating a food divded by the time spent looking for it.  Animals should only eat foods in which the “profit” (energy obtained/time spent looking for it) is greater than the threshold.  The threshold will change seasonally, since the dry season has less vegetation than the wet season.  And it will also change depending on where you live: monkeys living in the rocky cliffs of the Himalayas have very little food resources available when compared with monkeys who live in a forested river basin.

So let’s step back a bit and think about this “optimal foraging” thing.  It seems to imply that the animals we are studying are rational beings, capable of “deciding” what to eat, displaying an obvious preference for one food over another, and actually preferring the foods which are higher in calories and nutrition.  Studies in humans have shown that we are not particularly rational when it comes to food.  And monkeys are animals who should eat anything as long as it is food, and sometimes even if it’s not!  Are we really going to say that monkeys are rational enough to forage “optimally”?

No, I don’t think we would be saying that.  I think what we are saying is that natural selection will act on the behaviors that promote the survival of the organism.  If you are so picky an eater that you will only eat something that’s only ripe two weeks out of the year, you’ll probably die before you get a chance to mate.  But at the same time, you like food that tastes good, so you’re not going to be so unpicky an eater that you would eat a pile of leaves when there are some delicious berries nearby.  And, of course, every model like this is an abstraction of nature.  Sayers and his colleagues make this point in the paper.  A model will never predict behavior of any animal perfectly, but if you can nail down a general trend or pattern, you’re doing pretty well.

Back to the langurs.  They follow the model pretty well, which, if you think of these animals as stomach-driven brutes who are only in search of food and sex, might be surprising.  They do deviate from it, however.  They eat a lot of foods that are below the threshold for their particular environment.  Are they perceiving their environment to be more marginal than it actually is?  Or is the search cost higher than the equation allows?  These monkeys live in the moutains after all, and climbing a mountain is no small feat.  The authors suggest that the langurs are revisiting a few of these low-quality resources.  They aren’t just wandering around, deciding on the spot whether something is good to eat or not.  Rather, they remember that there was a potato patch (yes!  They eat potatoes!) back that way, and maybe it would be easier to go back there today than it would be to walk over that large cliff.

I’m not a behaviorist, but I liked this paper.  Maybe I just like the thought of monkeys in the Himalayas who eat potatoes.  But I also like when researchers find an animal that makes testing a model or hypothesis easy. Nail down the basics before you try to study it in a complex environment or organism.  Of course, there will always be a good deal of complexity in a biological system, but I like to see how researchers from other fields break that complexity down into a manageable packet.

Sayers, K., Norconk, M., & Conklin-Brittain, N. (2009). Optimal foraging on the roof of the world: Himalayan langurs and the classical prey model American Journal of Physical Anthropology DOI: 10.1002/ajpa.21149

Research Blogging Awards 2010

Hi Everyone!

The Research Blogging Awards are coming up.  If you head over to this page, you can see all of the finalists for 2010 in different categories.  You’ll see some familiar names over there: The Primate Diaries, Not Exactly Rocket Science, A Very Remote Period Indeed, and my own blog.  Hopefully you’ll also see some new blogs to add to your subscription method of choice, too.  I’ve discovored Mauka to Maukai, Southern Fried Science, and the EEB and Flow.

From researchblogging.org:

Voting: Voting for the winners will be conducted by invitation to bloggers registered with ResearchBlogging.org. Invitations will be sent on Thursday, March 4. If you’re registered with us, you may want to check your account to make sure your email address is up-to-date. If you’re not registered (and you blog about peer-reviewed research), you still have time to register so you can vote. Visit this page for more information.

So, go register!  I know a lot of my readers and regular commenters also have really great blogs where you write really well on peer-reviewed research, and the researchblogging.org site will allow you to reach a wider audience.  Plus, then you can vote for me, and I can get $50.  And as a poor graduate student on her way to Albuquerque this spring, I need 50 bucks!

Reading these excellent blogs has given me some ideas about how to tweak the writing that I do here a little bit.  I’m going to continue to work on how to write in a way that’s more accessible to the lay person, but still reports the science accurately.  If you’d like to offer any feedback, I’m open for suggestions!

A few good links

I’ve been reading some interesting things on the internet this week.  Have a look:

Chistopher Brochu and his colleagues have published a paper describing a new species of Miocene crocodile from Olduvai Gorge, Crocodylus anthropophagus. As you may guess from the name, the authors think that this big predator might have been the one leaving all of those tooth marks on our little hominid bones. But,  also interesting is their discussion of crocodylian diversity during the Miocene.  We sometimes think of crocodiles as unchanging “living fossils,” but they have a surprising amount of diversity within their clade, which I hadn’t really thought about until I read this paper.  The paper is open access, so go read it!

Darya Pino is the writer of one of my favorite “healthy foodie” blogs, Summer Tomato. This week she did a post about the Paleodiet. It’s a good post, with some interesting discussion in the comments. And Darya brings up a good point: If you limit your processed foods, you’ll probably be healthier- regardless of whether or not you eat grains or dairy.

John Hawks writes about the “New Synthesis” of archaeology and genetics.  Anthropology departments are funny places, where you have ecologists, geneticists, morphologists, linguists, archaeologists, and cultural anthropologists all living under the same roof (and sometimes within the same person!).  With all of the lip service we pay to being a “holistic” discipline, collaborations between the sub-disciplines are actually pretty rare.

Afarensis is bringing back “Know Your Primate,” with Pseudopotto martini filling the starring role this week!

The Open Dinosaur Project is cooking along, and they’re getting ready to do some yummy PCA!  We will soon know a lot more about the evolution of quadrupedality in multiple lineages of ornithischians thanks to the hard work of lots of “citizen scientists.”

Eating Disorders, Oxytocin, and Vasopressin

February 21-27 is National Eating Disorders Awareness Week.  I have a history of some pretty disordered eating which landed me in the hospital a few years back, but it’s not something that I’m extremely comfortable talking about.  In the spirit of “awareness,” I’ve decided that it’s something I should talk about more.  In my personal life, this will mean that I’ll have to be a little more open with my friends.  Here on this blog, I’d like to share a little bit of the interesting information I’ve found in my search to understand my relationship with food a little more clearly.

I decided one day that I could either be a scientist or an anorexic for a living.  The energy I was spending counting calories and weighing my food and planning my next meal was a full-time job, and at the end of the day, I didn’t have any time left over for studying or reading or writing.  I carefully weighed the two options and decided that I should go with scientist- it has a slightly higher paycheck.  So I thought I’d start my endeavor by trying to understand why food can be such an all-consuming enterprise.  I had just learned about the little monogamous prairie voles, and thought that perhaps the “addiction” that those guys get to each other is similar to the “addiction” that some people have to chocolate, and that maybe in myself, that reward pathway had been broken or usurped by something else.  So I looked to oxytocin and vasopressin for answers.

It’s well-established that hormones and neurotransmitters are intrinsically linked with food intake, satiety, and metabolism.  Leptin, for instance, has been shown to act as a long-term regulator of appetite.  It is released by fat tissue and is able to cross the blood/brain barrier  to interact with receptors in a specific area of the hypothalamus.  Once there, leptin binds to another peptide called Neuropeptide Y and together they reduce the activity of the neurons in this area of the brain.  This tells the rest of the body that you are full.  Some research suggests that people can become “leptin resistant” in the same way that you can develop insulin resistance.

I spoke earlier of my “relationship” with food- and lots of people describe food this way, which is funny.  You can love food, but it’ll never love you back!  So why do we feel this emotional connection to a specific combination of apples, butter, and cinnamon that you have every year at Thanksgiving?  Or to fried green tomatoes like grandma used to make?  Or to those little fried potato sticks that used to be a staple of your elementary school lunches?  (Excuse me as I wipe a wistful tear from my eye…)

There’s no doubt that certain combinations of foods trigger responses in the neuroendocrine system, so it’s not unreasonable to say that those emotional connections to food are caused by neurotransmitters.  However, it is impossible to point to one peptide and say, “this is what is causing your craving for chocolate.”  The neuroendocrine system is highly interrelated, so we have to look not only at a specific peptide, but also to its relationship to other peptides.  As it turns out, oxytocin and vasopressin are no different.

Oxytocin and vasopressin are very similar to each other.  Both are only nine amino acids long, and between the two peptides, only two of the amino acids are different. They both act in both the peripheral and central nervous systems.  In the peripheral nervous system, oxytocin is one of the hormones responsible for inducing uterine contractions during birth,  release of milk into the collecting ducts in new mothers, and is elevated during orgasm in both men and women.  Vasopressin is important for kidney function and blood pressure regulation.  It helps to regulate how much water is released from the body as urine, and can increase blood pressure.  In the Central Nervous System, oxytocin has been shown to have very important functions related to memory, mother-infant bonding, pair bonding, and trust.  Vasopressin also plays a role in memory and partner preference, as well as aggression and circadian rhythm.  In many cases, oxytocin and vasopressin seem to be reciprocal to one another:  Oxytocin disrupts memory, while vasopressin enhances it.

In people with anorexia nervosa, the concentration of oxytocin in the brain is reduced, and peripheral nervous system responses are reduced or impaired.  Vasopressin, on the other hand, is elevated.  In patients with bulimia, oxytocin levels are normal, but vasopressin is elevated.  These reduced and elevated hormonal profiles are most likely to be the result of restricted eating and starvation rather than the cause of them.  But the elevated levels of vasopressin may play a role in recovery.  If vasopressin is important during the formation and retrieval of memories, it could be contributing to the difficulty many patients have in changing their old routines of restriction.

I said earlier that I simply “decided one day” to stop having an eating disorder, but it really didn’t work that way.  I’ve committed to memory the approximate caloric value of most foods that I eat regularly, so it’s a rare day that I don’t have a running tally going on in my head of how much I’ve eaten that day.  Is an elevated level of vasopressin influencing that behavior?  Maybe.  Is it the sole cause of it?  Probably not.   But I think it’s helpful to remember that recovery is not a simple act of willpower.  Willpower and empowerment play an important part, but there are all sorts of reasons why it’s harder than it seems like it should be, and we’ve only just begun to scratch the surface when it comes to this kind of research.

Bailer, U., & Kaye, W. (2003). A Review of Neuropeptide and Neuroendocrine Dysregulation in Anorexia and Bulimia Nervosa Current Drug Targets-CNS & Neurological Disorders, 2 (1), 53-59 DOI: 10.2174/1568007033338689
Frank, G. (2000). CSF oxytocin and vasopressin levels after recovery from bulimia nervosa and anorexia nervosa, bulimic subtype Biological Psychiatry, 48 (4), 315-318 DOI: 10.1016/S0006-3223(00)00243-2
Kaye, W. (1996). Neuropeptide abnormalities in anorexia nervosa Psychiatry Research, 62 (1), 65-74 DOI: 10.1016/0165-1781(96)02985-X

Is Homo floresiensis really that strange?

BMC Biology has recently published a paper (It’s Open Access!) which explores trends in brain size in the Primates.  A trend toward a larger brain is usually considered one of the “hallmarks” of the Primates, but Stephen Montgomery and his colleagues have shown that in many lineages, there is a trend towards secondarily “shrunken” brains.

The authors looked at three different traits- absolute brain mass, absolute body mass, and relative brain mass (a derivitive of brain mass and body mass) in 37 living species, and 23 extinct species.  They reconstructed the ancestral state using three different phylogenetic methods:  Parsimony, maximum likelihood, and a Bayesian Markov-chain Monte Carlo.  They found that there are increases in both absolute and relative brain size in the Primate lineage, but not necessarily in body size.  So primates, in general, have more brain per pound of body than most other mammals.

In a few branches, once an initial increase in brain size occurred, there was a secondary decrease in brain size.  On the surface, it would seem that this doesn’t make sense.  More brain= more smarts= more behavioral flexibility= more food, more mating, more survival.  Right?  Usually.  But in some species, the energy it costs to maintain all of that extra brain costs more than it’s worth.  If you are a bat who hunts for insects, you may not need to remember the location of all of the fruiting trees in the area, and the extra weight incurred by having a big brain weighs you down when you’re trying to fly.  In that case, it might be advantageous to reduce your brain size.

There’s also the issue of a “phyletic dwarf” or “phyletic giant.”  These are species which are very closely related to each other, but one is very big or very small.  Something like the Aye-aye and the Giant Aye-aye.  Brain size generally exhibits negative allometry because it’s such a specialized organ- so, as the body gets larger, the brain doesn’t keep up and is, as a result, proportionately smaller.

It’s extremely important for most of your organs to increase with body size.  For example, a bigger animal needs to pump more blood, so it needs a bigger heart.  A bigger animal eats more food and needs a bigger liver.  There are certain areas of the brain that increase allometrically with body size- usually areas that are in charge of motor skills.  If you’ve got bigger legs, you’ve got bigger muscles, and you need more neural projections in order to control them.  But does a larger animal need to think more?  Will it benefit from an extra few cubic centimeters of neocortex?  Probably not, so it’s not worth the extra time and energy it takes to develop that neocortex.

And that sort of brings us to an important question in evolutionary neurobiology: Does absolute brain size matter, or is it solely brain size relative to body size?  Brains that are absolutely larger have more neurons, which could have important cognitive implications.  But how many of those extra neurons are just being used to control the physiological functions of the body?

Does size even tell us anything at all?  Any way you look at it, brain size is a crude measurement of cognitive ability.  In an ideal world, we would know the proportion of each of the different regions of the brain in each species and go from there.  But, those kinds of measurements are hard to obtain in living species, and impossible in fossils.  Ralph Holloway has been saying since 1967 that there has got to be a better way than just plain ol’ cranial capacity… but other than noting the relative position of different sulci and gyri on endocasts, there isn’t too much else to be done.

Anyway, those considerations aside, Montgomery and friends found that there were initial increases in proportional brain size- one at the node between ancestral primates and strepsirrhines, another between ancestral primates and haplorrhines, and then another between the ancestral haplorrhines and anthropoids.  At the terminal ends of the branch, tarsiers, galagos, aye-ayes, and humans all show large increases in relative brain size.

Cool!  But what about the decreases in brain size?  They found that absolute brain size decreased in about 14% of the branches, in clades like Mangabeys, tamarins and marmosets, and some of the small lemurs.  In every one of those cases, the decrease in brain mass was accompanied by a decrease in body mass.  And body mass decreased a lot more often than brain mass- 46% of the branches showed a decrease in body mass.  So what that means is that brains tend to stay the same size in lineages where the body size is decreasing.

Only 4% of the branches showed a decrease in relative brain size.  Most of these are lineages where the body size increased disproportionately to brain size- the negative allometry discussed above.  Think about gorillas:  They have a small proportion of brain mass to body mass.  But is it because they have tiny brains, or because they have huge bodies?  This study seems to support the idea that it’s their large bodies that are influencing the numbers.

Okay, so we’ve got lots of increases in brain size, and a few decreases.  In the cases where we have decreases, we usually have body size decreases as well.  More often than not, we have body size decreases which result in a disproportionately large brain size, but occasionally we have a body size increase which results in a disproportionately small brain size.  And all of that brings us to the Hobbit.

The authors looked at Homo floresiensis in relation to the Dmanisi hominids, Homo habilis, and a Homo erectus from Ngangdong and found that if we use Dmanisi or habilis as an ancestor, the decrease in brain size and body size isn’t exceptionally weird when compared to other primate groups.  The mouse lemur decreased in both to a greater degree, for example.

But if you use the Ngangdong erectus as the ancestor, it is a really weird decrease.

So, I guess the question is, is it reasonable to use Dmanisi or Homo habilis as the ancestor and not Homo erectus?  And of course, we don’t know that yet!

Montgomery, S., Capellini, I., Barton, R., & Mundy, N. (2010). Reconstructing the ups and downs of primate brain evolution: implications for adaptive hypotheses and Homo floresiensis BMC Biology, 8 (1) DOI: 10.1186/1741-7007-8-9

Sapolsky on TED talks

Robert Sapolsky. ‘Nuff said.

Okay, maybe I will say one more thing: Sapolsky’s book A Primate’s Memoir is one of the most beautiful books I have ever read, and I really don’t think it’s because I am a primate geek. I think it’s one of the rare science books that would be enjoyable for anyone, or maybe everyone, to read.

Four Stone Hearth #84! (Gratuitous Gelada Edition)

Welcome to the 84th edition of the Four Stone Hearth!  The last time I did this, I tried to separate the posts into whichever “subfield” they fit into, but the intersectionality this time was phenomenal.  Grab your favorite beverage and sit down for some good old-fashioned anthropology reading.

It’s not very often that we get to talk about culture from primates other than ourselves (though it’s becoming more and more frequent!), but Eric Michael Johnson discusses Susan Savage-Rumbaugh’s TED talk on Bonobos and the emergence of culture at The Primate Diaries.

Speaking of chimps, Michelle at SpiderMonkeyTales writes about the Fongoli Chimps and the things they do to cope with a more marginal environment.  Nocturnal Chimps?  In addition, the same chimps can understand fire.

Continuing on with the chimps-in-marginal-environments theme, Greg Laden discusses chimps, underground storage organs, volcanoes, and what all of them have to do with human evolution over at his place.

Raymond Ho writes about obedient young Campbell’s monkeys over at The Prancing Papio.

Beast Ape has written about Friendship, Fatherhood, and MHC in baboons.  They’ve done lots of stuff with mate choice and MHC, but this is the first time I’ve seen it used in a parent-offspring project- with surprising results!  Really, really cool.

Speaking of the bleeding heart baboons, I revealed that I have a major crush on geladas here.  And why am I so sure that knuckle-walking evolved twice?  And is it tacky to link to your own posts when you’re hosting a blog carnival?

Anna has written a beautiful piece about Childhood- its evolutionary origins, its philosophical implications, and cross-cultural variability- in The Ape That Wouldn’t Grow Up.  Happy birthday, Anna!

Zacharoo from Lawn Chair Anthropology has written a fascinating post about signatures of hybridization in gorillas.  Neat stuff!

On the more human side of things, Tim Jones at anthropology.net draws our attention to a really cool skeletal pathology found at Atapueurca- a craniosyntosis!- and what it means (or what it might not mean) about the sociobiology of Pleistocene hominids.

And, while he was at it, he reviewed some of recent news stories about Neandertals.  Did they throw spears or javelins?  Does it even matter?  Why would anyone want to hurt Shanidar III?  And do their teeth have anything to say about their love lives?

Julien from A Very Remote Period Indeed discusses the recent paper about Neandertal pigments in context with other Neandertal pigment and shell ornament papers.  A fragmented horse metatarsal?  Very clever.

Michelle wrote us another great post- this one about a possible Homo erectus hearth in Israel.

What if you could go back in time and sit around that ancient hearth?  Martin from Aardvarchaeology discusses what it would be like and why it would be scary.

Magnus from Testimony of the Spade reviews To Wake the Dead, a book about Cyriacus of Ancona and the birth of archaeology.

Julien from A Very Remote Period Indeed relates a call to us paleo-people to integrate all of our cataloged fossils to a better picture of the history of biodiversity.

Another post from Julien discusses some of the recent literature on the archaeology of modern human behavior in East Asia.

At Neuroanthropology, Greg Downey discusses a recent article about the anthropology of American mental illness and other culture-bound syndromes.  What a thought-provoking post!

Over at Anthropology in Practice, Krystal D’Costa was prompted by the Hadza to take stock of her essential items.  My essential item right now is a mug of hot chocolate.

Krystal writes another great post on gold as an internal currency in South Asian culture.

Kerim has a post about becoming expat teacher near the location of his old field site.

Finally, here is one last gratuitous picture of a gelada, because this is my blog, I just submitted a first draft of my thesis, and I do what I want!

Julien will be hosting the next edition of the Four Stone Hearth over at A Very Remote Period Indeed.  Make sure to submit some nice posts to keep us all busy.  I’m looking at YOU, linguists!

So… Did knuckle walking evolve twice?

Almost certainly.

We had lots of clues that this was the case before Ardi, but now that we’ve got Ardi- the palmigrade extraordinaire, we know that humans did not go through a knuckle-walking phase, and that chimpanzee knuckle-walking has evolved since the split with our last common ancestor with them.  Which would also means that it evolved after our split with the gorillas… which means that knuckle-walking evolved twice.

The Great Auk by James Audubon

As we’ve discussed before, knuckle-walking is a pretty weird thing to do, which is why the idea that it evolved only once is hard to shake.  But once you’ve got a particular body plan, there are only so many ways to accomplish a certain task.  For example, the now-extinct Great Auk was a flightless sea bird that hunted fish underwater.  It was white on its front, and black on the back, and had powerful rear feet and webbed toes.  Sound familiar?  The Great Auk was the Northern Hemisphere’s version of the Penguin, but the two were not particularly closely related.  It’s simply that, once you’ve got the body plan of a bird and you want to start diving for fish at high latitudes, you’ve got to rework the wing a little bit so that it’s no longer any good for flying in the air- but man, will it be good for underwater flight!  And then you can work on your body shape a little so that you’re like a little avian torpedo.  And then you’ve got to put some body fat on so that you can withstand the frigid ocean temperatures.  And voila!  You’ve got two almost-identical ocean birds separated by an entire planet and a couple of hundred thousand years, if not more.

The apes have done a very similar thing.  The general Miocene ape was an above-branch quadruped.  In order to get a flexible shoulder, the ape had to move its scapulae so that the glenoid fossa, where the humerus articulates, was facing laterally instead of ventrally.  The glenoid fosssa also became a little shallower, so that it could accommodate a wider range of motion.  When monkeys, or squirrels, or dogs land on the ground with their front feet, the forces are allowed to travel all the way up the humerus until they reach the glenoid fossa.  But think… if you’re an ape with a fancy new flexible shoulder, and you try to land on your front feet, what happens?

Well, you’ll dislocate your shoulder.  That’s a very bad thing for an ape to do!

Monkey Thorax. From Aiello and Dean's Human Evolutionary Anatomy

Ape Thorax (in this case, a human). From Aiello and Dean's Human Evolutionary Anatomy.

Another thing that some of the apes have done is make their elbow more flexible, as well.  If you look at a monkey’s (or squirrel’s, or dog’s) ulna at the elbow, they have a huge chunk of bone on the end of the olecranon process that we don’t have.  Also, the little U (U for ulna!) that cups the trochlea of the humerus points to the side in all of those quadrupeds- more like a C than a U.  But in apes, it points straight up.  All of that makes for an extremely flexible elbow- and gives us the same problem with dislocation that we had with the shoulder.

Olecranon processes (proximal ulna) in a human (left) and a cat (right)

So, you’re a Miocene ape, and you’ve evolved this flexible shoulder and flexible arm, but you’re also starting to get quite large.  So large, in fact, that sometimes you have to leave the trees because the terminal branches that connect one tree to another just aren’t large enough to support you.  So, you come down out of the trees and walk on the ground.  And man, are your muscles working overtime to keep those bones from dislocating!  Each step you talk with your palms on the ground sends a huge amount of force up through your wrist to your elbow, and then up through your shoulder.  Your muscles are concentrically contracting, trying to keep everything in its right place- but the problem with concentric contraction is that you’re already contracting as hard as you can, so one misplaced step, and your muscles can’t do anything to help you avoid injury.

But if you walk on your knuckles, it doesn’t do that!  Because you are such an excellent climber, you have these massively strong flexor tendons in your fingers.  If you flex them and put your weight on your knuckles, those tendons eccentrically contract and are able to absorb some of the force coming at you from the ground.  That way, your arm muscles don’t have to work so hard simply to keep your bones from dislocating.  Plus, you’ve got those long fingers for vertical tree climbing and suspension, and tucking them under your hand gets them out of the way.

Gorillas and chimps have both figured this out (anatomically speaking- I don’t know if any of them have put any thought into it!).  And orangutans are fistwalkers- but I know that I’ve seen one on a really hard concrete surface using his knucles.  It’s a classic case of convergent evolution- but you can’t really tell from the genetics or the living anatomy of the animals- we needed a fossil to finally be relatively certain.  Much like the Auk and the Penguin, the Chimp and the Gorilla faced similar problems brought about by their environment and anatomy, and ended up solving them in similar ways.

Lovejoy, C., Simpson, S., White, T., Asfaw, B., & Suwa, G. (2009). Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive Science, 326 (5949), 70-70 DOI: 10.1126/science.1175827

Ward, Carol (2007).  Postcranial and Locomotor Adaptations of the Hominoids.  Handbook of Paleoanthropology.  DOI: 10.1007/978-3-540-33761-4

NPR: The invention of osteopenia

All Things Considered ran an interesting story about the birth of osteopenia as a treatable disease.

Bone densitometry becomes increasingly available. And women start wanting it, and they hear their friends have had a measurement of bone density, and their friend was told that they have osteopenia, and they want to know whether they should be treated. And so, it’s almost viral.

I can’t offer any commentary from a medical or clinical perspective, but as an evolutionary anthropologist, I wonder how many of these women could benefit from weight-bearing cardio (walking and running) and strength-building exercises?  As people age, their bone density naturally decreases unless they actively work to maintain it.  It’s a shame that I only very rarely see women over 40 in the weight room doing deadlifts and squats.