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Liza Shapiro: Primate Locomotion


Q: What can living animals tell us about the behavior of extinct animals that we otherwise only know from fossils?

A: When you look at the fossil record, all you have really is a pile of bones. It's a non-moving entity. There's not much you can know about it unless you look for living analogs. So if you look at living animals, you've got the bones, but you can also look at how they're moving, and you can piece that information together. Then, when you go back to the fossil record and you look at the bones, you're able to make a lot of inferences about how that animal would have been moving in the past.

Q: What sort of things do we need to know about living animals in order to be able to interpret fossils this way?

A: If you can look at the shape of their bodies and study in detail what kinds of movements they have, you can make a link between the body shape and their movements. Then when you go to the fossil record, you see the body shape and you can reconstruct what kinds of movements that animal would have had. Most of what we do relies on our knowledge of what living animals are doing.

Q: Your field of research is primate locomotion. Why are you particularly interested in the spine?

A: Most of the work that's been done on the primate body is focused on the limbs -- which are, of course, very important in locomotion -- but it's very important to study the spine. It's a crucial component of the locomotor system, a very primitive structure that has been around a long, long time in evolution, and you can see the modifications in it very easily for different kinds of behavior. It's very basic to your posture. So it's important to understand the differences and the similarities among primates in their spine.

I'm also interested in determining whether the length of the spine really does make a difference in flexibility, because there's this basic assumption that long backs give you more flexibility, [but] short backs give you more stability. That's a great assumption, but you really need to test it by actually measuring the range of motion in the spine.

Q: Tell us about your experimental work using video and computerized motion analysis.

A: I've done a lot of work looking at the vertebrae of different primates -- taking measurements on the vertebrae, trying to understand how the shape of the vertebrae differs among primates. But I can't really understand what those differences in shape mean unless I can look at how the shape of the spine relates to how the animals actually move. This experiment allows us to see how the spine moves in different planes. I will be able to compare different kinds of primates in terms of their spinal movements. You can actually measure the movement itself compared to the anatomy and complete the story of the spine and its function.

Q: And do you have any clues so far? Do you have any sense as to whether one kind of vertebrae dictates a specific kind of movement?

A: I have seen differences in the structure of the vertebrae that have translated into differences in the emphasis on different kinds of movements. That some primates have vertebrae that have joints that permit them to move more side to side. And you also see, when you measure the movements, that they emphasize more side to side movement than fore-aft movements.

[When you compare] a human vertebra, a chimpanzee vertebra, [and] the vertebra of a lemur, you can see that the human and the chimpanzee are much more similar to each other when compared to the lemur. That's because the human and the chimpanzee have adaptations for upright posture. The human and the chimp vertebrae are very short. They also have fewer vertebrae, which makes for a short spine, which is very stable in upright posture. In comparison, the lemur has elongated vertebrae and more vertebrae, which gives it a lot of flexibility and a wider range of motion for their quadrupedal movements.

Q: How do these three -- human, chimp, and lemur -- fit into the primate family?

A: Primates are divided into three basic groups. There are prosimians, such as the lemurs who live on Madagascar, who are primates but they preserve a lot of primitive mammalian features. When you first look at them, you don't necessarily think of them as a primate, but they share features with all other primates [T]heir brains are bigger than other mammals. Their eyes are in front of their face instead of on the side of their face. They have nails on their fingers and toes instead of claws.

Another group of primates are monkeys, and they are different from prosimians. They share features that are not found in prosimians. ... Some of them live in Central and South America, some of them live in Africa and Asia. They're all closely related as monkeys.

Then there are apes and humans, who are very closely related to each other.

There are features that separate monkeys, apes, and humans from all prosimians. Their brains are bigger than prosimians, relative to body size. The other features are ... [more] obscure, like they have bone behind their eye socket. But they also have less emphasis on smell, and more emphasis on vision.

And then there are features that separate ape and humans from all other primates, including monkeys. All apes and humans share the loss of the tail and adaptations in the spine that we've been talking about -- a short, stable back, and all of these adaptations for that upright posture. We have broad chests; we have very mobile shoulder joints and hip joints compared to monkeys.

Q: Why did apes and humans lose their tails?

A: I don't know the answer to that one. ... It probably has something to do with upright posture. When you lose the tail, the tail musculature gets incorporated into the pelvic floor, which can form a support for your organs, because now that you're in an upright posture, your organs are going to tend to move downward. So you need that support on the pelvic floor.

Q: What do we mean when we say bipedalism, and why is it important?

A: Literally, the word means "two feet" ... walking on two legs. It's important because it is one of the main distinguishing characteristics -- one of the most important things -- that sets humans apart from other primates. It's also probably the first thing that we see in the fossil record in human evolution. Humans were bipedal before they had big brains and before they were using stone tools. So, if you look at living primates, and you look at humans compared to other primates, humans are the only fully bipedal primates. It's important for us to know what are the aspects of the body that allow us to walk like that.

Q: Is there some idea that bipedalism led to big brains and tools?

A: The initial reasons for bipedalism are unknown. There are lots of different hypotheses for why bipedalism evolved in humans. It was the best way to travel from one tree to another, whether or not it gave you an advantage in thermal regulation. But we don't have a clear answer on that right now. But once you are bipedal, it frees your hands; it allows you to do things with your hands you couldn't have done otherwise. And once you start using tools, then it would be advantageous to have a larger brain. And so a cycle could start that way.

Q: So bipedalism led to humanness?

A: Right. To me bipdalism is humanness.

Q: What had to happen for us to become bipedal?

A: If we can understand what makes us look different from other primates [and] allows us to be bipedal, we can then understand what kind of changes had to occur in the body as humans evolved to allow us to evolve bipedalism. So you can try to figure out what our pre-bipedal ancestors looked like, and then understand the anatomical changes that had to occur to transform that animal into a human being.

We need to understand the anatomical features of bipedalism. We need to know how humans look different from other primates so we can trace the anatomical changes that occurred to get to that point.

Q: What can lemurs tell us about the evolution of human bipedalism? After all, the last common ancestor of humans and lemurs lived more than 50 million years ago.

A: One reason it's interesting to look at lemurs is because the earliest primate ancestors were probably very lemurlike. This could represent the kinds of movements you would see in an early primate ancestor. It also can just represent basic movements of a primate. We don't know anything, really, about how the spine works together with the bones in a primate -- nobody's really been doing work on that. So you can look at the details of how the spine moves in relation to how the arms and legs move. How do primates incorporate their spine into their movements? Does the spine help them move their limbs in a certain way? So the limbs and the spine work together as a unit.

Q: Why do they think the earliest primate ancestor was lemurlike?

A: We have a great deal of fossils from the earliest primates. And although they were not directly ancestral to the lemurs, in terms of their adaptations they were probably most similar to the living lemurs.

Q: What sort of adaptations do lemurs have?

A: The lemur's spine is a very different shape from the spine of a primate that keeps its back upright a lot. A lemur spine looks very different from the back of a chimpanzee. All monkeys or prosimians who walk on all fours most of the time have much longer spines, more flexible, and they have a harder time holding their back upright. Lemurs do a lot of different kinds of locomotor behaviors. They do walk quadrupedally, but they're also very good at leaping. And in some of the lemurs, they do their leaping from a horizontal position. And their spine looks like that of a primate that walks quadrupedally. Other lemurs leap from a vertical position, and they spend almost all their time either clinging to the trunk of a tree vertically, ... then they leap. So their posture almost all the time is vertical. And you see adaptations in their spine that are similar, on some level, to that of an ape.

Q: What can we infer from that?

A: You can see evolution working on the spine in that way. You can see that when you've got a particular kind of posture, you have a particular kind of body shape. There's a selective advantage in having that kind of a spine -- an upright spine -- if you're going to be keeping your back upright all day.

It's important to realize that even though we have specializations in our spine that allow us to walk bipedally, on a more basic level there are many similarities between our spine and the spines of other primates that tend to keep their back in an upright position often in their locomotion.

Q: Like what?

A: All primates that tend to keep their backs in an upright position have a shorter back. And the way their back musculature is arranged. The parts of the vertebrae where muscles attach are oriented in such a way to give the back muscles an advantage in keeping the spine upright.

Q: You're basically saying there's a lot of similarity between us and other primates.

A: Right. A human biped doesn't have to come from an animal that kept its spine completely horizontal all the time. That we could have easily evolved from an animal, and we probably did evolve from an animal, that kept its back upright in other kinds of locomotion, like arm swinging or climbing. And its spine would share those similarities.

The similarities in the trunk and the spine of humans and apes have been known for a long time. The traditional focus was on apes and humans being similar, but I've looked at a lot of other primates who also keep their back upright, and I find a trend to the same kind of anatomy in other primates. Not just apes and humans, but other primates converge on that anatomy and behavior.

My focus is trying to separate what are the really truly unique human features from the basic blueprint for an upright spine ... [W]e started out with a basic adaptation for upright spine, but what are the extra features we have to add on to that to make it bipedal? If you can really understand those functional features and separate them, then you can look [for them] in the fossil record.

Q: What are the differences between humans today and our closest living relatives, chimps, that allow us to be bipedal while they really aren't?

A: If you compare the human body to that of a chimpanzee, there are a lot of similarities in the upper body: the arms, the chest, the upper trunk. But there are many changes from the waist down, many differences between humans and chimpanzees that allow us to walk bipedally all the time. For example, in the lower back humans have a curve, a forward curve called lordosis, that you don't see in any other primate, which allows us to balance our upper body over our legs. The vertebrae themselves are much bigger in humans relative to body size than in any other primate, which allows us to bear the loads which are coming from the upper body through the lower back all day.

There are changes in the balance of the skull on the spine. You can see that in the position of where the spinal cord enters the skull is more directly underneath in a human.

There are many differences from the waist down. The human pelvis is reoriented, which changes the function of the hip musculature to give us better balance when we're walking. Our lower back is balanced [over] the hips, the knees, the ankles, and the feet because we have a very small base of support on out feet. So everything has to be completely in line and in balance. We also see differences in the knee, the ankle, [and] the foot. Our foot is completely restructured -- we don't have a grasping big toe anymore. The foot is very robust, which allows a lot of weight bearing in the foot, as well. We have more stable ankles. So there's stability throughout the joints, and a completely different shape to most of those joints in humans compared to chimpanzees.

Q: What's your personal sense of our connectedness to animals like chimpanzees?

A: I don't think anyone could look in the eyes of a chimpanzee [and] come away not thinking we're very closely related to those animals. When you look in their eyes, they look back at you in a way that most other animals don't, and you know it as soon as it happens. It's a feeling that they are thinking on a level that's very close to the way you're thinking.

Interacting with a young or baby chimpanzee is almost identical to interacting with a human child. If you tickle them, they laugh; they like to be chased; they like when you act silly. They like to play, in the same ways that children play.

Q: Do you teach the evidence for evolution? Or is evolution just a given at this point?

A: I wouldn't call it a given. I don't think anybody should accept something on faith, to begin with. This is the evidence for evolution, and that's what we're studying.

I'm trying to educate students on two levels. One is showing them the evidence for evolution, so if there is any question in their mind, there won't be after they look at the evidence themselves. Once they get to that level, then they can start to look at the details of how evolution works. And to answer really important and exciting questions, like "How do humans come to be the way they are today?"

Most people are interested in humans, because we are humans. When you start getting into the details of studying primate evolution, you realize that there are other interesting questions to ask besides just about humans. But that's usually what pulls us in.

Q: What is the evidence that you see, in the living world, for evolution?

A: There are two things you can look at as evidence for evolution: what makes us different, and what makes us similar.

When you look at primates, and you look at humans compared to other primates, you see a lot of things that make humans different from other primates. You see a lot of things that make humans similar to other primates. They're both evidence for evolution. You could focus on one or the other: You could be interested in what makes us so special, and you can look at that from a scientific viewpoint, or you can look at what makes humans like all other primates. How are we all different from other mammals? That, to me, is just as interesting as what makes a human different from a chimpanzee.

Everyone wants to know who we are and where we came from and why. There are many different points of view that people can take to answer that question. You can answer it from a religious point of view; you can answer it from a scientific point of view. To answer it from a scientific point of view, you look at evolution. The study of evolution can give you insight into who humans are -- what makes us different, why are we here, and how do we fit into the natural world?

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