Ardipithecus ramidus Australopithecines Bipedalism Bones in the News Brains Eocene Euprimates Forensic anthropology fossils Genome sequencing Graduate school Holocene Hominoids Homo erectus Human Variation Lemurs Living primates Locomotion Miocene Monkeys Neandertals Osteology pelvis Physiology Pleistocene Plesiadapiforms Sexual selection Skeletal Biology Tarsiers Uncategorized
January 8, 2010Posted by on
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?
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