The Seed-eaters

If someone stuck a gun to my head and told me to pick my favorite primate, I think I’d have to go with the Gelada (Theropithecus gelada). Just look at him!  Have you ever seen anything more handsome in your whole entire life?

Gelada Yawn

Gelada Yawn

Come on! He’s a freakin’ beatiful monkey, that’s what he is. I dare you to tell him otherwise.

Perhaps my fondness for the most beautiful monkeys on Earth is why Clifford Jolly’s The Seed Eaters has a special place in my heart.  Around the blogosphere, we’ve been talking quite a bit about human origins modeling for the past few months, and while we’ve disagreed about a lot of things, I think the discussion has been interesting and thoughtful.  Jolly’s model has since been falsified, but it shows us that even if a model is incorrect, it can be incorrect in an interesting way.

In The Seed Eaters, Jolly summarized many of the origins models that had been prevalent at the time:  That tool and weapon use led to feminized canines in males, bipedalism, and encephalization; Holloway’s hypothesis that canine monomorphism was the result of selection for hormonal monomorphism and cooperation; that bipedalism was the result of a display posture; and the “Man the Hunter” idea that once humans moved to the savannah, meat-eating and hunting took on special significance and acted as selective forces on bipedalism, and allowed brain size to increase.

Each of the models above tries to answer at least one question about humans:  Why don’t we have any canine dimorphism?  Why are our brains so much larger?  Why are we the only ape that habitually walks upright?

At the time that Jolly wrote his paper, the only fossil humans that we had came from the savannas, so paleoanthropologists figured that one thing that differentiated us from the other apes was that our ancestors had left the wooded forests and lived on the savanna.  We now know that bipedalism evolved before we left the forest, so life on the savanna doesn’t explain very much about our origins.  However, Jolly didn’t know that.

Jolly’s model was a comparative model that made use of geladas, mandrills, and baboons.  Mandrills and many baboons live in wooded environments, but the gelada has become remarkably committed to life in the open grassland.  Jolly asked, What if the primary dietary shift in human origins wasn’t from fruit to meat, but from fruit to grasses and seeds as happened in the gelada?

Jolly made an impressive list of characters shared between humans and geladas but not baboons or chimps, including

  • extremely opposable thumbs
  • relatively non-opposable great toes
  • dimorphism in the hair around the face and neck
  • female epigamic features on the front and back
  • short and broad cranial bases
  • anterior migration of the temporal muscle
  • many features of the teeth, such as small incisors, crowded cheek teeth, and early canine eruption relative to the molars

Of these, the ones that I find the most interesting relate to the teeth, because we *do* see parallelisms between geladas and the robust australopithecines in many of these features, particularly in canine size.  Canines and incisors are extremely reduced in the more robust species of human.  Jolly points out that in geladas, the canines have not been reduced because of any behavioral changes, but that their reduction is probably the pleiotropic result of selecting for massive molars.  You’ll still see this argument made for the reduction of canine size in humans.  However, with Ardipithecus, we see that the canine reduction is not accompanied by an increase in molar size, so it probably cannot account for the initial shift.

(Aside- I know from the picture above it doesn’t look like the gelada has reduced or monomorphic canines at all!  But compared to fossil Theropithecus species, they actually are.  In male geladas, the largest individuals have the smallest relative canines.)

Jolly also points out that, in many of the savanna-dwelling species, the typical one-male/multi-female group lives within a larger “troop” composed of many of those one-male groups.  The males within the troop cooperate with each other when defending their groups against extra-troop males.  This idea is relevant to the recent discussions of Ardi as well.

Jolly concludes his paper by outlining what he thinks a good origins model should be:  It should have testable predictions (which can be falsified by new fossils), and it should be able to account for data from different fields.  It should not only be plausible, but convincing.  As I said above, Jolly’s model fails now based on his own criterion of falsifiability, but I think that’s what makes it stand out in my mind.  Many of the origins models that we discuss today have no way to be falsified, so they stick around because they are plausible.

The Seed-Eaters: A New Model of Hominid Differentiation Based on a Baboon Analogy, by Clifford J. Jolly

© 1970

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9 responses to “The Seed-eaters

  1. Eric January 10, 2010 at 10:21 pm

    It’s quite an interesting point you’re mentioning here, since falsifiability is nothing special in other natural sciences.
    The question we should ask now: Why do we even bother with “models” on human origins which aren’t falsifiable?
    Ok, maybe they are plausible, but without the possibility of falsifying them, they have no scientific value. In the end, they are just stories.

    Oh and you’re right, the Gelada surely is a beautiful Monkey.

    • zinjanthropus January 11, 2010 at 11:50 am

      I guess we always run the risk that something that can’t be falsified might actually be *true,* so we keep it around. As much as I love paleoanthropology, I think some of its practitioners have a habit of being rather religious about these sorts of things. Oh well… we’re all only human.

      • zinjanthropus January 11, 2010 at 11:54 am

        And just in case I’ve got some creationists lurking- those paleoanthropologists (even the crazy ones) are not religious in their acceptance of human evolution- that is an indisputable fact.

  2. Beast Ape January 11, 2010 at 12:32 am

    Geladas are awesome. Can I recommend you include this on the upcoming 4-stone hearth?

    A quick aside, could canine reduction be the result of selection on something other than molar size? Granted diet exerts strong selection on teeth, but I’m thinking here mostly about multi-modal signaling and sexual selection. For example, canines are not only effective weapons in male-male competition; they also are used in displays between males. If males instead used some other cue to indicate fighting ability and/or status in displays (i.e. gelada chest patches, see Bergman et al. 2009), having canines (for display purposes) may become less important. This is admittedly a speculative idea, but I thought it might be an interesting avenue of research. Sadly, most ‘bright male’ primates are OWM and tend to be pretty closely related (mandrills, drills, and geladas), thus not a very good comparative set.

    • zinjanthropus January 11, 2010 at 10:20 am

      I wonder if anyone has looked at this problem in birds? Or another animal perhaps. Birds might not be good because I can’t think of any that really “fight” like primates do for mates, so it would be hard to compare… but maybe they don’t fight because they’re so colorful!

  3. rich lawler January 12, 2010 at 2:16 am

    Steve Leigh, Jo Setchell et al., have a cool paper looking at male canine length and male fitness in mandrills. They find canine length to be directly and positively associated with male fitness, and when canines begin to wear down so does male reproductive output. This paper is in JHE in 2008.

    Following up on Beast Ape’s comment, there is some new sexual selection literature that focuses on traits pertaining to agility (where overall size doesn’t necessarily matter) and not fighting; most of this research is on birds. It’s possible to have strong sexual selection acting on a male trait such as canine size, but the type of selection is stabilizing and not directional selection, hence no increase in male canine length relative to females, despite lots of variation in male mate-getting ability that is causally associated with canine variation.

  4. zacharoo January 12, 2010 at 1:08 pm

    I’m intrigued by Jolly’s idea about reduced canines as the result of selection on molars. I’m very new to developmental biology, but it’s my understanding that each tooth class are fairly independent modules? That is, I don’t know that one would necessarily expect selection on the posterior dentition to necessarily affect the canines, at least not due to pleiotropy (instead, rather, perhaps correlated selection?). Of course, almost everything I’ve read about dental evolution and development has been done on mice, who are lame in only having incisors, a huge diastema, then molars. Now that I’ve raised that issue I should probably do some reading to address it . . .

  5. Pingback: Giant’s Shoulders « The Renaissance Mathematicus

  6. bob December 18, 2011 at 4:56 pm

    I was wondering what part of thy genetic code separates humans strength from apes strength?

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