Ardipithecus ramidus and Human Evolution

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Ardipithecus ramidus and Human Evolution

Postby Paralith on October 24th, 2009, 5:08 pm 

Introduction

Almost everybody has heard about "Ardi," the new hominin fossil species, on which many scientific publications and a Discovery Channel documentary have been recently released. At the moment, however, all this information is presented to us and interpreted by the group of researchers who discovered Ardi, all of whom have a pretty much unified viewpoint on the implications this species has for human (and, indeed, all ape) evolution.

Scientists are people too, and everybody has their own preferred hypotheses. Fortunately, the process of science involves multiple people with different viewpoints, all attempting to falsify each other's hypotheses. It will be a little while, though before rebuttals to some of the Ardi group's ideas are published, so I thought it would be worthwhile to post a summary of a discussion about Ardi that members of my anthropology department had recently, in order to provide a broader perspective on Ardi and what her discovery means to the field of paleoanthropology.

I will also provide links to a few posts from around the Anthropology Blogosphere on Ardi, to add more viewpoints. Some are definitely far more detailed than what I'll write here, so if you want more details without digging through the primary publications in Science, definitely check out some of these blog posts. My goal here is to provide an easily readable synopsis of one department's take on Ardi.

I. How Ardi Moved

The basic picture painted by the Ardi authors is that Ardi was an obligate biped on the ground and a monkey-like quadruped in the trees. Meaning, when she was on the ground, bipedal walking was the most efficient way for her to move, but when she was in the trees, it was easier for her to move quadrupedally.

Living monkeys and apes move in significantly different ways. Monkeys are quadrupedal, with arms and legs that are about the same length. When in the trees they walk on top of branches, on the palms of their hands and feet - aka, palmigrade. Apes are not exactly quadrupedal. Their arms are longer than their legs, so when they walk on the ground their bodies are not parallel to the ground, like a monkey, but at a diagonal. This is called orthograde. In the trees apes are brachiators, which means they hang underneath branches that they grasp with their hands. (Imagine swinging on the monkey bars at recess; funny enough, only apes can actually swing on monkey bars.) Apes also have feet that are a lot like hands, with a grasping toe that sticks out to the side like a thumb to help them clamber through the trees.

So, yes: this means the authors think that when Ardi was in the trees, she moved more like a monkey than like a modern ape. This is not completely unreasonable if you know a little about the very first apes to ever evolve (of which we have the fossils of several species) - these animals were a lot like monkeys in the way they moved through the trees. So this means that Ardi is like a very primitive ape in the way she moved through the trees. Based on the anatomy of her wrists and the fact that her legs and arms are very similar in length, no one in my department thought this was an unreasonable conclusion.

The authors concluded that Ardi was a biped based on her pelvis and her feet. Her pelvis looks like an intermediate between a chimpanzee and Australopithecus afarensis (Lucy, who many different scientists agree was undoubtedly bipedal). But Ardi's feet don't look very much like a biped's feet - they have a grasping toe, like the feet of orthograde apes like chimpanzees. Usually a foot with a grasping toe is not a very efficient foot for walking. Bipeds, like Lucy and us modern humans, have our big toe lined up parallel with our other toes, to help provide push-off force when we're walking. All of this brings into question how exactly Ardi may have walked bipedally, and how efficient that walk might have really been. One of the graduate students in my department wondered how the musculature would work in a foot that supposedly has a fully functional grasping toe and muscles arranged for obligate bipedalism. She was not convinced by the Ardi authors' descriptions that it could work.

Lastly, the authors state that because Ardi is bipedal (at least, they think so) and because she is found in woodlands (more open than rainforest/jungle, but still closed canopy), that this disproves the hypothesis that bipedalism evolved in open savannas. However, that hypothesis has already been disproved by Lucy and other representatives of her species, who are also often found in woodlands, as well as some areas that are mixed woodlands and open grassland. Lucy was certainly not a dedicated savannah denizen, but she is definitely bipedal.

II. Ardi's Social System

One of the Ardi authors, Owen Lovejoy, has been pushing the "food for sex" hypothesis for Ardi's social system. A lot of people in the anthropology community have serious doubts about this. It's also interesting to note that Lovejoy proposed this same hypothesis thirty years ago, but applied it to Lucy. The anthropology community found no evidence to support it, and essentially dropped it.

The "food for sex" hypothesis is both a hypothesis about a social system and a hypothesis for the evolution of bipedalism. The authors claim that Ardi stood up in order to free the hands to carry food; specifically, males would carry food from some distance to a female, and in exchange the female would choose to mate with him. They say that further support of this hypothesis is a lack of sexual dimorphism in Ardi's canine size. Often, primate males that fight with each other over access to mates have long canines that they posture and fight with, longer than the canines of females of the same species. This system results in a few males mating with most of the females. Because Ardi's canine sizes are the same for both sexes, the authors argue that males did not fight with each other for mates. Instead, males offered gifts of food in order to convince only one or a few females to mate with them.

The problem here is depending on canine size alone to be a reliable indicator of male on male aggression. Canines are teeth, after all - they are subject to selection both from sexual selection of fighting for mates, and from natural selection on the requirements of the diet. Studies of living primates show that dimorphism in body size is a more reliable indicator of mating system than dimorphism in canine size. So, you may ask, is there body size dimorphism in Ardi? There is - but the authors pass it off as "minimal" body size dimorphism "similar to chimpanzees." Chimpanzees may be less dimorphic in body size than gorillas or orangutans, but their dimorphism is still significant. And this dimorphism is most likely caused by the sometimes very violent interactions between males as they vie for more dominant social positions, those being the positions with the most access to fertile females. Thus it is pretty unreasonable to completely rule out male-male aggression playing an important role in Ardi's social system.

When it comes to understanding the social structure of primates, diet is a very important consideration, and explains large amounts of the variation between primate species' social systems. So the next question is, what did Ardi eat?

III. Ardi's Diet

Ardi's teeth appear to be an intermediate between chimpanzees and Lucy. Chimpanzees have very large incisors (front teeth) and relatively thin enamel, the protective hard coating over teeth. This is thought to be an adaptation to eating a lot of fruit with thick rinds. The large incisors help the chimps bite into these fruit, but the meat of the fruit is not itself very hard to chew. Lucy has smaller incisors, much larger molars, and thicker enamel. This leads us to believe Lucy was eating less fruit, and more tough and/or hard foods like nuts and tubers that required some serious crunching and chewing. Ardi has neither particularly large incisors nor particularly large molars, and what seems to be an intermediate enamel thickness.

Carbon isotope analysis of teeth also tells us about diet, and in particular the relative amount of C4 versus C3 plants the animal in question consumed. C3 foods are from trees and bushes - things like fruits and leaves. Chimpanzees are considered to be almost pure C3 feeders. C4 foods are from grasses - the grass leaves themselves, and the roots and tubers of grasses. Lucy and other members of her species have a range of evidence of C4 eating, between 30 and 80% C4. Again, Ardi is somewhere in the middle with what appears to be 10 - 25% C4.

You may be thinking, OK, but that paints a pretty vague picture, doesn't it? It does. Ardi appears to be somewhat more of a generalist feeder than chimps, though it's very likely that, like most living apes, fruit of some kind made up the majority her diet. We're quite certain she wasn't folivorous, aka eating a lot of tree leaves, because animals that eat large amounts of leaves require specialized shearing crests on their teeth that Ardi definitely lacks. Overall, the evidence from Ardi's body and teeth suggest a social system broadly similar to that of chimpanzees and bonobos.

IV. Ardi, the Last Common Ancestor, and Ape Evolution

So: what does all of the above say about the last common ancestor (LCA) between modern chimpanzees and humans? Currently, most researchers accept that the LCA was similar in many ways to modern chimps. The Ardi authors claim that Ardi proves this isn't true.

Firstly, it must be made clear that Ardi herself is not THE last common ancestor. Current estimates for the last common ancestor are between 6 and 8 million years ago, and Ardi is quite solidly dated at around 4.4 million years ago. (The authors themselves comment on the remarkably consistent time period in which all the Ardi specimens were found.) And even then, some of the papers published by the Ardi authors claim that the time of the LCA should be pushed back even farther than 6 - 8 million years ago. Clearly this would leave a lot of time between the LCA and Ardi, so why do they continue to insist that the LCA looked more like Ardi than a modern chimp?

They claim this because they also claim that Ardi makes implications for the evolution of the entire ape lineage. Remember I mentioned earlier that Ardi appears to resemble very ancient apes in some ways, apes that lived around 20 million years ago. Because of this, the authors claim that most members of the ape family tree must have maintained these primitive traits all the way from 20 million years ago, until Ardi herself 4 million years ago. Thus, of course the LCA was primitive this way too. And all the apes that are alive today, that are orthograde brachiators? All these apes evolved these similar traits independently. Orangs, gorillas, chimps, bonobos, even other extinct brachiating apes like Dryopithecus. All of these apes are examples of parallel evolution, according to the Ardi authors.

Most of the people in my department found this to be a very unreasonable claim. One of our post-docs described it as claiming approximately four primitive traits to be maintained down through evolutionary history, while claiming that nearly fifteen traits common to the living apes all evolved independently multiple times. Though not impossible, this is simply very unlikely, and to support such a hypothesis would require a pretty heavy burden of proof. The Ardi authors claim that this scenario isn't so unlikely because all apes share a similar gene structure, so similar selective pressures will easily result in similar evolutionary results. A primatologist in our department pointed out that the selective pressures on the different ape species are not all the same - orangs spend nearly 100% of their time up in the trees, and use a four legged scrambling technique to move through them; gorillas on the other hand spend large amounts of time on the ground; chimpanzees are more intermediate. The selective pressures acting on these different animals are not all exactly the same; the most logical explanation for their sharing so many traits is that they inherited them from their common ancestor that developed them long ago.

This does of course mean that Ardi appears to represent something of a reversion to a more ancestral state. Neither parallel evolution nor reversions are particularly parsimonious explanations (those that require a minimum number of evolutionary changes, usually considered the most likely to be true barring strong evidence to the contrary), but the Ardi authors are suggesting a really incredible amount of parallel evolution in many different species. In comparison, a few reversions in one species seems much more likely.

Stepping aside from Ardi's ancestral looking features, however, a recurring point in our department's discussion of Ardi's other features was how they much looked like an intermediate between chimpanzees and Lucy. In many ways, Ardi resembles an animal we might expect to see that was evolving from a chimp-like state to a Lucy-like state, which is much more supportive of the more prevalent view of the LCA being similar to a chimpanzee.

V. Conclusions and Links

In the general opinion of my anthropology department, Ardi is certainly an important paleoanthropological find, but at the moment it doesn't seem like she is going to drastically change our understanding of human and ape evolution. But of course, a lot of work remains to be done. Other experts will need to examine Ardi and her characteristics and weigh in, but unfortunately it may be quite some time before such independent analyses can be done. The Ardi authors did not publish any actual raw measurement data; certainly no casts of the bones have been made available to other research groups yet, let alone access to the actual fossils themselves. Unfortunately one of the lead researchers of the Ardi group, Tim White, has a reputation with being very stingy about fossils he discovered and letting other researchers have access to them. It is my sincere hope that this is not a trend that continues with Ardi, and that the paleoanthropology community as a whole will be able directly assess Ardi in the near future.


Anthropology.net's List of Ardi References

John Hawks' Ardi FAQ

Ad Hominin on Ardi's Pelvis

John Hawks' Video Interview on Ardi
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