Bipedal: The First Steps toward Becoming Human

In Intro to Anthro 2022 after thinking about the Extended Evolutionary Synthesis we begin talking about “Human Biological Evolution” by reading the first of three parts in Through the Lens of Anthropology. This section concentrates on becoming bipedal.

Next up: the Homo species

Bipedal Steps

The big points are: First, that humans have always been on the move. We’ve always been a migratory species. That’s one of the hallmarks of our humanity. Second, that we’ve always been diverse, learning, adaptable and flexible. That’s how we spread out to all parts of the habitable world. Finally, the idea that humans come in these chunks of groups that we call races is an incorrect way to view humanity. It simply does not work as a biological division of our species. Those will be the three big lessons that I’d like to keep in mind as we investigate some of these excruciatingly detailed accounts. I always like to have the big picture stuff in mind.

Now, when it comes to human evolution and this study of ancient stuff there’s still a lot of things we don’t know. There’s a lot of things that are complicated. There’s a lot of debates. I want to say from the outset that these debates are often very heated, but they’re within the field of evolution. They’re within the general framework, or accepted theory of evolution. It’s simply about the details of what happened when.

In a lot of these debates there’s discussion about teeth and dental formulas and what you might learn from the creature’s teeth. . . . Why are we always talking about teeth?

They preserve the best over time. It would be wonderful to be talking about all kinds of other parts of the skeleton, but in many cases those parts are simply not there. As we know from contemporary disasters, we have the teeth. The dental records are often what we use to identify people. The general idea is that the fossil record is extremely fragmentary. When you’re a creature who is alive, one of your biggest goals in being alive is not to become a fossil. You don’t want to die and be fossilized. If that happens to you, that’s not good. You’re trying to not become a fossil. If you do die, it’s often the case that scavengers and other creatures are going to get your bones and take all the stuff away.

[The result is that] there are certain regions that are more productive than others. One of those regions that we know a lot about or has been the most productive is the Great Rift Valley in the eastern part of Africa. Most of the good stuff of human evolution happens in Africa. We know a lot more about what happens in the eastern part of Africa simply because that’s the region that gives us fossils. We’ve come to know that there were probably other things going on in western Africa and southern Africa and northern Africa, but they have not been fossil producing regions.

The other problem is that there are time gaps. Even in a productive region, you might be missing a hundred thousand years or a million years and you just don’t know hardly anything about what happens during that time. So, even if you find something . . .

[You need to do] “taphonomy.” What is that?

Taphonomy: the study of what happens to organic remains after death. Trying to figure out if what happens to them after death is a result of something that they might have done or is something that happens as a natural process. There’s a huge debate [about the] Homo naledi find in South Africa. . . . It is a wonderful find, great preservation of bones. The people who discovered these are arguing that they were deliberately buried there. To be able to argue for burial in an ancient population would be a huge deal. But other people say “no, hyenas dragged them into this cave,” or they got swallowed up in a cave. There are lots of arguments about what happens and how what happens to creatures after they pass away.

The other huge problem here is determining what a species is. . . . or what the definition is. This isn’t just a problem for anthropologists; biologists have some issues with this too. Part of it has to do with what . . . happens with creatures, if they’re naturally separated from each other, they can form a species because they’re not in contact with each other. But sometimes if you bring them together, you find out they’re all a species. In fact, . . . coyotes, dogs, and wolves, all perfectly members of the same species. We think of them as quite different creatures most of the time, although sometimes they get crossed together in funny ways. So, species boundaries are often unclear. What’s the classical definition of the species? . . . We usually talk about horses, donkeys, and mules to illustrate. What are species supposed to be able to do with each other? Reproduce and have reproductive offspring. So, horses and donkeys are each their own species. If we put them together and make mules, apparently mules don’t reproduce with each other. They don’t get to be their own species. . . . The classic definition of a species: creatures that can interbreed together and have interbreeding offspring. As I said, even in today’s world we’re not always sure where those boundaries lie, but it’s pretty difficult to do when you’re talking about fossils that you can’t put them together in real life and see if they can interbreed or not.

There are some people who favor the broad definition and [are sometimes called] the “lumpers.” Then there are the “splitters.” The lumpers, every time you find a bone they say, “ah that’s just a variation on this species, I’m sure if they got together, they could reproduce. So, no reason to get excited here, no new species names.” Whereas the splitters if they find a bone or a tooth or something, they’re like, “aha a new species. In fact, let’s give it a name, let’s give it my name.” This is in part because who wants to just find a bone and be like, “oh that’s just part of this species.” Much better to find a bone and be like, “aha, a new species, we can name it!” Everybody likes to name things.

Some of you were perplexed, as I sometimes am as well, about the debates and the definitions and will they ever settle these things. It is true in this field there have been some huge and rancorous debates. I’m not sure if more than others, but quite a lot of debating. Again, part of it is the fame of being able to name things. Part of it is the limited area in which some of these things are found. There have also been some outsized personalities in this business the Leakey family, for example, had several generations of people doing these things. . . .

We’re going to try and stick to the big stuff. The big lessons that we know about human evolution. If you really want to get into the debates, you can.

How do you determine interbreeding fossils? How do you distinguish a male pelvis from a female pelvis? . . . What is the huge defining feature that we can all see that females have, and males don’t?

True, tends to be wider. But as Muckle, González, and Camp tell us, that’s a subtle thing (85). It tends to be wider or more basin shaped. My point is that there is nothing that pops out that you’re like, “aha, there it is, we know that all the males have this, and all the females have that.” It will depend upon whether the female has given birth or not, or how many times. There’s a lot of indeterminacy in the classification of skeletal remains. In the old days we just lumped them all into males. There was an over-classification of males in the archaeological record. These days we’re slightly more careful about this. We’ve reclassified some of those skeletons and left a lot of them as indeterminate, because there is no single feature that you can point to. There’s quite a wide variation, both in the male/female and in-between categories. There’s nothing about the human skeleton that necessarily tells us exactly what we sometimes believe about what’s going on down there.

Before we get into what we know now about human evolution, there are some things that we used to think, some of the old ideas that got us into trouble. When we look back at human evolution in the old days, we were looking for big brains. Because some human beings like to believe we are really the smartest creatures on earth and our intelligence is what marks us off as human beings. We always went searching for creatures that had larger brains. We were also searching for what we talked about with the Great Chain of Being: missing links. The idea that between every creature there had to be other creatures. This was the idea of gradualism, or that everything had to be along a nice smooth line from one creature to another. It could only change over a very long period. Nice smooth and straight lines of evolution. . . . The idea that things proceed in a very linear fashion from one creature to another. This got us into trouble because since people were looking for big-brain creatures and missing links, there were some clever people who put a chimpanzee bone together with a human jawbone and said, “aha, here I found the missing link.” It was accepted for a long time and later finally proven to be a hoax. Piltdown Man was a hoax, but people were prompted to believe it because they were looking for those big-brained creatures.

One of the reasons these hoaxes were more viable is because only the fossils were available. There was only fossil evidence. That’s all you had to go on. Another idea was that biological change came first, or physical changes and then after that, once we had big brains and physical change, we get tools and learning. What we call culture. These were the old ideas . . . of what people were looking for in human evolution. As I said, it led to some hoaxes, some dead ends, and some bad ideas about human evolution.

What we think now will be open to revision, but I think we have a better picture of what’s going on in evolution. We are not looking for missing links, but common ancestors. Creatures that from those ancestral creatures diverge into various types. We know that our most recent common ancestor is probably like chimpanzees and probably like bonobos in some ways, but it’s not going to be exactly the same as either one of those because those creatures have also been evolving. . . .

The first thing that distinguishes, or the first element of divergence between the hominins and the other creatures who would eventually become more like chimpanzees and bonobos, is those creatures that became habitually bipedal. Walking around more and more. That was their primary difference: mobility. The brains of these creatures were still very much within the ape range. There was no noticeable cranial expansion for millions of years. This divergence is probably around 6-8 million years ago. Again, it is not big brains which separate out the species, it’s bipedalism.

We now know that there are a lot of different bipedal species. How many exactly is up for debate. We know there’s a range of variation. Some people say 30 bipedal species, some people say it’s more like three, but all the same there’s quite a wide range of diversity in what we call “hominins” or bipedal apes . . .

It’s oftentimes easy to project back into the past and talk about Homo sapiens and “us” and what “we” were like 6 million years ago. If you ever hear me saying that just yell out and say, “No, that’s not ‘us.'” We might say “us Homo sapiens” back to about 300,000 years ago. But when we’re talking about bipedal creatures in Africa, I’m not sure if we should say “us.” A lot of those creatures went extinct and didn’t make it.

We now know that hybridization of species is a very important part of evolution. Subspecies that drift off and then come back together and then re-hybridize. This results in sometimes dramatic genetic changes. . . . This is part of the model of punctuated equilibrium . . . that not all changes are gradual. Some can be rapid in evolutionary time, perhaps in 100,000 years. They result in what some people call “mosaic evolution,” which is to say that different traits arise at different times and are combined and recombined in different creatures. There’s not one gradual species ascent over the time of the species.

We now have a little bit more, or hopefully a lot more, than just fossil evidence. We get stuff from genetics, geology, and about the environment. People have become much more able to reconstruct ancient environments and ancient climate patterns. That’s gotten a lot better. The genetic stuff has been surprising, because we thought genetics would sort everything out, and we’d finally figure out exactly what happened, and that it would make everything simpler. It turns out that the more we’re learning about genetics, the more and more complicated the human story appears. We’re seeing hybridizations events and species that we used to think were distinct turn out to have genetic evidence that they interbred more than we thought. Genetics has been a surprisingly rich field for revealing the complexity of human behavior and our human ancestors.

Finally, in contrast to the idea that biology comes first and then tools, intelligence, culture later. We now know that being able to learn how to do things, to learn how to walk, to learn how to use tools, to learn different cultural traditions, are extremely ancient and interact with our developing biology. We have been evolving together with stone tools for over 2.5 million years, and surely there was tool use even before those stone tools that creatures were developing and using and transmitting from generation to generation. . . . Culture is part of biology and part of our developing organism.

I think this helps us to interpret what was going on better in the existing fossil record. What we now look for in fossils, the defining feature that results in these divergences between what we call hominins or bipedal hominoids, starts at around 5-10 million years ago. Again, this is a long period of time because these things don’t just happen all at once. We used to think they happened all at once. We used to imagine that suddenly, these creatures stood up, took their big heads out of the ground, and strode out onto the African savannah with a spear in hand ready to hunt some wild animals. What would happen to our big-headed spear person striding out onto the savannah? What’s the first thing that’s going to happen to that person?

Gonna get hunted, going to get knocked down. Because in fact these first creatures that were bipedal were about as tall as preschoolers. They weren’t going to be doing any hunting. They were going to get knocked off as soon as they stood up. We also know that the grasslands were emerging in Africa, some of the great African forests were dying back, but it was not as grassy and savannah-like as we once thought. Probably there was a varied terrain that the creatures were trying to get across and navigate different environments. We used to have this wonderful idea of Man the Hunter striding out there, but that doesn’t seem to be exactly right.

Why do we go bipedal? . . . [One idea] is to use your hands to use tools or freeing the hands . . . But if you look at chimpanzees and bonobos, when they’re not walking around, they can just sit back and use their hands too. I like the “carrying the babies” idea, that seems to me more likely if you must carry a baby over distance. Carrying babies is probably important. Although again, if you look at chimpanzees or bonobos, they can put that baby on their back and walk off, or the baby is clutching its tummy. . . .

[Another idea]: thermoregulation. This was an idea that came about when we were thinking about the African grasslands and that hot African sun beating down on our backs all day long, and we’d be like, “aha, if you stand up it just hits your head, and you don’t have your back all sunburned.” Again, maybe. It does seem to regulate body temperature, but it might not have been the grasslands that we thought.

We talked about being able to view prey over the grasslands, maybe spot those predators before they got to you. Being able to see things. Some people have thought about scavenging animals. Many people like to think of early Homo species as hunters, but probably in the great lands of African predators what they did was take the bones and get the bone marrow out with some of those tools, after other animals had their fill. . . .

The African terrain was becoming more varied, and although walking is not a very efficient way to get around over short distances (you can run faster if you’re quadrupedal, as many animals do when they’re out-running us), over time you can walk longer periods than you can if you’re using all four limbs. It might not be the most efficient over a short burst, but if you need to navigate varied terrain and varied environments in one day or several days, walking can be a more efficient way to do that.

Muckle, González, and Camp mentioned display. That certain primates make themselves seem larger and scarier if they stand up. We see this happening and that could be part of why these creatures went bipedal. I would say that none of these has been definitively proved. Maybe there’s a combination of these. Maybe there’s no one factor. Somebody asked: . . . what’s my favorite one. This is not my field, but I’ll tell you one that I don’t like.

“Some others have suggested that the major advantage of bipedalism was for display. There are two aspects of this. . . . Many animals, including other primates, make themselves look larger as a show of dominance or aggression. . . . Another view of the display hypothesis reasons that those who stood upright exposed their genitalia more, likewise leading to more sex and more babies who carried the trait. An interesting, but not likely, scenario” (92).

I say: not even an interesting scenario. It’s not even nice to think about it. I’m sorry that I just read that, or that they even included that in the book. This is not something that happened in human evolution. Don’t use it to justify anything we do in today’s world: there is no reason to do that. I don’t like that one.

I’m a little bit partial to scavenging animals. I think that was a huge part of human evolution. I’m also partial to endurance walking. I think those were big back in the day, but that’s my take. This isn’t my field.

The most important part about this is that we eventually did start to go habitually bipedal.

Now, when we think about walking, Muckle, González, and Camp tell us that some people have estimated there were as many as, or more than, 30 bipedal species, and then most of them went extinct. . . . There are disadvantages of being bipedal, and when 30 species go extinct that are bipedal, you might wonder if that wasn’t such a great idea after all. Maybe we just got lucky. . . .

Also, humans learn how to walk. It’s not something that we come out doing when we come out of the womb. There are creatures that move quite well when they come out. Humans take time, and they need the assistance of other caregivers. We’re very dependent on others to learn how to walk, and to learn how to walk in different styles. Now, certainly the anatomy changes. It does change over time, but there’s been some speculation recently that maybe the anatomical changes aren’t as big as we thought and if you have chimpanzees in the right environment, and you teach them how to walk from a young age: look at that cute little chimpanzee, that’s from today’s world, just walking along. Other primates: gibbons do a lot of bipedal locomotion. So do bonobos. There are anatomical changes, but those might not be as necessary as we think.

Ardi is a great find. It’s the shorthand for a creature that is called Ardipithecus ramidus. It was discovered around the year 2000, but then took a long time before it was announced. They reconstructed the skeleton, one of the great reconstructions of a skeleton. . . . If we have the first bipedal creatures emerging 6-8 million years ago Ardi is seen as a creature between some of the older and some of the newer. So, between the early hominoids, the bipedal hominoids, and the Australopiths. . . . One of the interesting things about Ardi is that it has an opposable toe. . . . The question has been, if Ardi seems to be bipedal, does it mean that they were grasping up into the trees? Is it supporting the so-called “walking in trees” hypothesis? This idea is that bipedalism didn’t develop on the ground as much as it was for getting around in the trees. The people who discovered Ardi were very excited–obviously this was a big find. There’s a decent amount of skeletal stuff, but then some people were like, “ah it’s just a creature who went off on its own and it’s not even on the main line of human evolution.” So, then they got mad at each other for a while. It’s hard to say. . . . I think what we know is there were a large variety of creatures that were becoming bipedal, and maybe becoming bipedal in different ways, but whether this skeleton was exactly part of the line that would lead to humans is less clear.

Then we get to what are called the Australopiths, or technical name is the Australopithecus afarensis, from about 4 million years ago to about 2 million years ago. Don’t be confused by the “australo.” All of this happens in Africa, but these are known as the Australopiths. The most famous of these is the skeleton of Lucy, which is so named because on the night that it was discovered they were playing the Beatles song “Lucy in the Sky with Diamonds.” This became a very famous skeleton and toured the US.

With Lucy, we have more secure skeletal evidence of bipedalism. . . . There’s a lot of them, not just Lucy. You have toes that are going in the right direction, you have limbs that look more adapted to bipedalism. We have wonderful hard evidence of habitual bipedalism, which comes to us from Laetoli in Tanzania at about 3.7 million years ago, where we have a layer of volcanic ash. We get this wonderful thing where we have skeletal evidence and we also have a layer of volcanic ash, which we can date, because volcanoes erupt at certain times. We have a good dating, and then we have a line of bipedal footprints with the toes in the right position. Little creatures walking next to bigger creatures. It doesn’t get much better than that. We’re quite sure that about two million years ago there are several different creatures–there is still variation and diversity. Whether these are part of a single species, or we have multiple species of habitually bipedal creatures, they are making their way in Africa in different forms at about 2.5-3 million years ago. That is when we start to see out of the Australopiths the development of various lineages that have been assigned to Homo.

We see that mobility, being able to migrate, move, and be bipedal is at the very center, the very heart of human evolution. If anybody wants to make laws to keep people from moving around and migrating and doing all these things–humans are great at moving around. We need to be able to do that to survive. That’s been part of our human heritage.


In Intro to Anthro 2022 after thinking about the Extended Evolutionary Synthesis we begin talking about “Human Biological Evolution” by reading the first of three parts in Through the Lens of Anthropology.

Next up: the Homo species

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