The Human Fossil Record, Part 2: Bipedality

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January 5, 2011 Tags: Human Origins

Today's entry was written by James Kidder. Please note the views expressed here are those of the author, not necessarily of The BioLogos Foundation. You can read more about what we believe here.

This is the second part of a series by James Kidder on the human fossil record. The first part can be found here.

One of the most fruitful and exciting areas of research in palaeoanthropology is the search for the last common ancestor to the higher apes and humans. This question is inextricably tied to concepts of what separates humanity from the animals around us. This is a question that has spiritual as well as physical ramifications. In this set of posts, we are dealing with what makes us human from a biophysical perspective.

Traditionally, paleoanthropologists have considered the hallmark of humanity to be habitual bipedalism. While we share many characteristics in common with the higher apes, this trait alone is practiced by no other animal. Some animals practice facultative bipedalism, allowing them to go short distances on two legs when necessary, but only humans use it as their only form of locomotion. Put a man on all fours and even a squirrel can outrun him. Bipedalism is a skill that we learn early in life, before we are sentient and even understand what makes us different from the animals around us. It is programmed into us.

Bipedalism is marked by a number of anatomical modifications to the standard primate body. These center on the pelvis and involve changes in the head (cranium) and the rest of the body (postcranium), reflecting a shifting of the center of balance from the abdominal cavity to the hip. In mammals, the hip is composed of three mirrored sets of bones: the ilium, the ischium and the pubis (Figure 1). The top part of the leg fits into the bottom-rear portion of the ilium, into a round socket called the acetabulum. It is one of two ball-and-socket joints in the body, the other being where the arm fits into the scapula at the top of your shoulder.

Where the two pubis bones fit together in the front and the two ilia meet in the back with the sacrum forms the birth canal. In chimpanzees and gorillas, the ilium is narrow and tall (Figure 2). Consequently, the connection to the upper leg bone, the femur, is straight up and down. In humans, the ilium is flat and flared, creating an outward bowing of the top of the femur, which allows for the balance necessary for walking upright (see Figure 1). This, in turn, creates what is known as the valgus knee, where the bottom of the femur meets the top of the large lower leg bone, the tibia, at an angle. The fact that the two bones meet at an angle provides for a better balance of the body mass for upright walking. In contrast, when higher apes such as gorillas and chimpanzees stand, the femur and the tibia are both perpendicular to the ground, resulting in a straight knee joint. Consequently, when chimpanzees walk upright, they swing side to side in an ungainly fashion to simulate the balance that is inherent in human walking.

Other changes are present above the hip, as well. Because they are quadrupedal, chimpanzees, gorillas and orangutans have a straight backbone, or vertebral column (Figure 3). In humans, the vertebral column resembles a double “s” shape, which balances the torso above the hip (and creates the back problems we suffer later in life). At the top of the spinal column, the top vertebra, the atlas, has facets that balance the head and the second vertebra, the axis, has a prong that fits directly into a hole in the skull. This hole, which is called the foramen magnum, is at the back of the skull in higher apes, as in all quadrupedal animals. This allows the animal to keep its head up while it is trotting along the ground. In humans, the foramen magnum is at the base of the skull, allowing us to look forward as we walk. It also makes it hard to look forward when we crawl on all fours. Each of these modifications is diagnostic of humans and easily recognizable in the fossil record in specimens for which these anatomical areas are present.

Ardipithecus ramidus and the Origins of Bipedality

The origins of bipedality have traditionally been understood as having evolved at the end of the Miocene epoch, around 6 to 8 million years ago (Crompton, Sellers and Thorpe 2010) when the climate began to dry out and cool. Unfortunately, there are only scattered presumed hominin remains from this time period, all of which are taxonomically controversial and fragmentary and none of which have diagnostic postcranial remains. It has also been thought that the transition to bipedality likely did not happen all at once but in mosaic fashion (as evolution often proceeds) and this has recently supported by the fossil record. Up until a few years ago, the most widely-accepted model was that bipedality originated among a group of large-bodied hominoids that had adapted to the savannah-jungle fringe. The jungle, itself, was ceded to the precursors of the modern chimpanzees and gorillas and the savannah to the precursors of the modern baboons and other terrestrial monkeys. As a result, some workers (Crompton et al. 2010) suggested that both arboreal (tree-dwelling) and terrestrial locomotion might have been present in our earliest ancestors. Recent evidence has corroborated some parts of this model, but not others.

In 1994, the remains of a remarkable hominin, dated to 4.4 million years ago, were unearthed in the Afar Triangle of Northeastern Ethiopia. Examination of the surrounding deposits, however, yielded a conclusion that this hominin lived in a woodland environment, rather than a savannah/forest fringe environment (White et al. 2009a). Requiring over ten years of extrication from the surrounding rock and painstaking reconstruction, this fossil form, Ardipithecus ramidus (now represented by 110 individuals) yields diagnostic parts of the pelvis (Figure 4), as well as sections of the arms and skull (Figure 5) (White et al. 2009b). Although the base of the skull is not preserved, one striking aspect of humanity is present in the teeth. The canine (eye tooth) does not extend beyond the tooth row. Humans are the only hominins for which this is the case. In all other ape species, fossil and extant, the canine projects well beyond the tooth row.

Biomechanics specialist Owen Lovejoy and colleagues (Lovejoy et al. 2009) write about this species:

“The gluteal muscles had been repositioned so that Ar. Ramidus could walk without shifting its center of mass from side to side. This is made clear not only by the shape of its ilium, but by the appearance of a special growth site unique to hominids among all primates (the anterior inferior iliac spine). However, its lower pelvis was still almost entirely ape-like, presumably because it still had massive hindlimb muscles for active climbing.”

Figure 6 shows the intermediate nature of the pelvis of Ardipithecus ramidus compared to later hominins (Homo sapiens, Au. Afarensis) and chimpanzees (P. troglodytes).

Ardipithecus, then, represents a shift away from the primitive locomotion employed by the last common ancestor of our line and that of modern chimpanzees. Here is a hominin that maintained a link with its tree-dwelling past and yet had progressed toward the bipedal future. This evidence is striking because it firmly demonstrates that a species had arisen that was advanced in the human direction. Whether or not it led to the hominin forms that followed is not known but it clearly represents a phenomenal example of a transitional form in the human fossil record.

From this point on, the forms become noticeably more human in appearance, leading eventually our own species some four million years later. In his infinite wisdom, God had set us on a path toward our eventual communion with Him. That this path took such a long period of time and through so many varied forms of humanity is a testament to His creative power and patience.

Next, the successors to Ardipithecus and true human walking.

Sources

Crompton, R., W. Sellers & S. Thorpe (2010) Arboreality, terrestriality and bipedalism. Philosophical Transactions of the Royal Society B: Biological Sciences, 365, 3301.

Lovejoy, C. O., G. Suwa, L. Spurlock, B. Asfaw & T. D. White (2009) The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking. Science, 326, 71, 71e1-71e6.

White, T. D., S. H. Ambrose, G. Suwa, D. F. Su, D. DeGusta, R. L. Bernor, J.-R. Boisserie, M. Brunet, E. Delson, S. Frost, N. Garcia, I. X. Giaourtsakis, Y. Haile-Selassie, F. C. Howell, T. Lehmann, A. Likius, C. Pehlevan, H. Saegusa, G. Semprebon, M. Teaford & E. Vrba (2009a) Macrovertebrate Paleontology and the Pliocene Habitat of Ardipithecus ramidus. Science, 326, 67, 87-93.

White, T. D., B. Asfaw, Y. Beyene, Y. Haile-Selassie, C. O. Lovejoy, G. Suwa & G. WoldeGabriel (2009b) Ardipithecus ramidus and the Paleobiology of Early Hominids. Science, 326, 64, 75-86.

Figure 1: http://www.bikemonkey.net/2010/01/3-series-tune-up-checking-the-pelvic-girdle/

Figure 2: http://www.boneclones.com/KO-303-P.htm

Figure 3: http://www.fixscoliosis.com/entries/13-The-Human-Spine-and-Idiopathic-Scoliosis

Figure 4: Bone Clones

Figure 5: National Geographic Images

Figure 6: from Lovejoy et al. 2009


James Kidder holds a Ph.D. in Biological Anthropology from the University of Tennessee (UT). He currently employed as an instructor at UT, and as a science research librarian at Oak Ridge National Laboratory. He has been involved in the Veritas Forum at UT and runs the blog "Science and Religion: A View from an Evolutionary Creationist/Theistic Evolutionist."

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Jon Garvey - #47111

January 13th 2011

Hmm - I seem to have posted my last comment as “ha”. What a strange alias!


Jimpithecus - #47115

January 13th 2011

Sorry, I was away for a day or so.  This is my blog post about the debate between Albert Mohler and Darrel Falk.  It pretty much sums up my viewpoints on the matter:

http://scienceandcreation.blogspot.com/2011/01/albert-mohler-on-biologos.html


Jimpithecus - #47116

January 13th 2011

Chris wrote in #46982: “The problem with any history that involves only 2 founding humans is that the genetic diversity of modern human populations rules out a bottleneck of 2 at any time in our past.”

That is the tip of the iceberg.  Who built the pyramids?  Who constructed the fantastic civilizations in India and China?  Who built the Minoan civilization?

Closer to home: who did Cain build the city for if there were no people around to build it for?

Even if we are to take the Genesis story literally, THERE ARE GAPS IN THE STORY.


Jimpithecus - #47120

January 13th 2011

Over on my blog, a gentleman wrote, in response to my post: ”

We know the Scriptures are reliable because of history - we received them from our spiritual fathers, who received them from theirs, etc.

I think a lot of the problem is the definition of the word “science”.

If you mean “repeatability and observability” then there would be no problems.

If you mean “assumptions of naturalism”, then you run into problems.

I wrote in response:

We know that the scriptures are reliable, but are they a scientific account of creation? Were they ever meant to be?

You write “If you mean “repeatability and observability” then there would be no problems.”

You failed to mention “predictability,” which involves reconstruction. This necessarily assumes that things that behave in a certain way today behaved the same way in the past.

When I use the term “naturalism,” I do not mean “philosophical naturalism,” I mean “methdological naturalism.” Reverend Mohler conflates the two.


nedbrek - #47122

January 13th 2011

Hello Jim,
  How do differentiate “philosophical” and “methodical” naturalism?


Jimpithecus - #47132

January 13th 2011

“Methodological naturalism” is an understanding that we can use scientific methods to uncover the secrets of the universe because the universe behaves in an ordered fashion and always has.  Whether or not that universe was created by God is an interesting question and we are certainly free to do so, but it is not necessarily germane to the immediate question that we are trying to address regarding whether or not there really was a supernova in NGC somethingsomethingsomething.  “Philosophical naturalism” states that, in Carl Sagan’s words “The universe is all that is or ever was or ever will be.”  There is no God and it is silly to consider the question.  This is the viewpoint of Richard Dawkins, Jerry Coyne, Christopher Hitchens and P.Z. Myers, to name a few.


Martin Rizley - #47133

January 13th 2011

John Garvey,
You write, “Gene scattering would take longer than that. For animals it would require death or, at least, shedding live cells about.”  This is true, of course, as long as we are think that “the present is the key the past.”  By that rule, the whole process of God creating the biodiversity of life by means of genetic engineerring—creating one new kind of creature from the genetic material of an earlier one, and that from the genetic material of an earlier one, etc.—would indeed take a very long time.  But this only highlights the whole problem of trying to apply uniformitarian thinking to miraculous works of God—to rationalize them.  But miracles cannot be explained by human reason at all, so it is silly for us to try.  Moreover, when God performs true miracles, they don’t always need the time frame that our rational minds tell us they would surely need.  If God wills, he can cause a mature almond plant to grow on dead stick overnight; he can cause a tumultuous sea to become calm instantaneously, and a fig tree to wither in a day (continued)


Jimpithecus - #47134

January 13th 2011

Correction: That clause should have read “Whether or not that universe was created by God is an interesting question and we are certainly free to do believe in Him if we like,” 

That’ll teach me to fire off something quickl


nedbrek - #47135

January 13th 2011

How does methodical naturalism handle miracles?


Martin Rizley - #47136

January 13th 2011

So if he willed, He could surely make the whole biodiversity of life through genetic engineering in a single day.  How could He do that?  Through HIs unlimited power and wisdom as God.  My whole point in presenting this scenario is to show that there may be a way of explaining the transfer of genetic material from one creature to another apart from naturalistic evolution.  You have admitted as much when it comes to the origin of Adam, but you say that ‘creation by genetic engineering’ could not apply to the whole biodiversiy of life.  You may be right.  But at the same time, I know that God’s power and wisdom are so trancendent, it seems increasingly foolish to me for human beings to try to explain how God did it all.  There are too many variables about which we have to say, “We don’t know.”  Therefore, I believe that the course of biblical wisdom is to say, “God knows how He did it,” and to continue affirming by faith without full understanding those truths which we know about the past by divine revelation, such as the historicity of Adam and Eve as the progenitors of the human race and the historic fall of mankind into sin through the sin of Adam.


Jon Garvey - #47137

January 13th 2011

@nedbrek - #47135

How does methodical naturalism handle miracles?

That’s easy: if I’m using the existence of fixed rules (“laws of nature”) to investigate the universe, then I can’t possibly use them to investigate events that break those rules. I just have to say, “wrong tool for the job.”

A scientist can, of course, believe in miracles, but not “as a scientist”, just as a plumber could write music, but not using spanners and not as a plumber.

Historians, of course, can pretend they’re bound by “scientific laws” too and use methodological naturalism to say that miracles are ahistorical, but in my view that’s pretentious: history isn’t a science, but a weighing of human sources. That includes weighing the effects of intuition, imagination, emotions and all kinds of other soft targets as well as the supernatural.


Jon Garvey - #47140

January 13th 2011

@Martin Rizley - #47136

Yes it’s possible - everything’s possible for God.

But, as you well know, the problem is not that we have simply the fact of existing things to explain, but the evidence of process.

Were the pyramids build by ordinary people, or did aliens levitate the blocks into place for mystic purposes a la Erich von Daniken? When you find blocks marked with “This way up” and “For the Royal Tomb”, when you find bits that were bodged and covered over, when you see progress in construction technique, the houses of workers etc, etc (and when there is a complete absence of alien artifacts) then to maintain the interstellar connection you have to say the aliens wanted to cover their tracks. Or that aliens wrote on the stones etc for reasons we cannot understand, them being so advanced and all.

I met a physics student once who believed von Daniken (and should have known better). “Nobody knows how it was done”, he said. I said, “They do, but you don’t want to listen to the obvious explanation.”

The only reason to propose genetic engineering is to accommodate to genetic findings. If miracles are beyond understanding, then why bother?


Jimpithecus - #47145

January 13th 2011

There are several schools of thought about that.  Carol Hill argued, in her article on the Mesopotamian hydrological basin, that the miracle of the quail falling for the Israelites in the desert can be explained by God simply shifting a regular wind pattern a tad to the south, since quail normally make their migration over the southern Mediterranean Sea every year anyway.  Was it miraculous?  Yes, but you had to be paying attention to see that.  All Christians who are scientists agree that Jesus seems to have been a special case, with nothing like Him before and nothing like Him since. 

There seems to be a general consensus on the economy of miracles in that they are few and far between.  Miracles are often visited on a few people in a specific situation and, once again,  are not obvious to a lot of people.

In truth, most scientists sort of shy away from miracles since they represent a grey area of creation.  As Christians, we know that miracles occur and that they are from God.  But they are rare and cannot be figured into models concerning observed phenomena

The catch is that science works.  We understand a heck of a lot about our universe and our observations bear out.  As I mentioned on my blog,

—next—


Jimpithecus - #47147

January 13th 2011

the reason things seem to work well is that we have tested them ad nauseum and found that they do, in fact, work.  No airplane that has ever crashed has ever yielded an absolutely inexplicable cause.  One of the reasons that we like shows like The Twilight Zone is because it isn’t like life as we know it.  Things seem to be orderly and behave in a uniform fashion in our world yesterday, today and tomorrow.

So we are stuck with this notion that, yes miracles occur but that on average, not very often and, when they do, God often uses known forces to effect them.  Not so helpful but that is all I have on the topic at the moment.


nedbrek - #47152

January 13th 2011

Jon, “I can’t possibly use them to investigate [miraculous] events that break those [naturalistic] rules”

So, if there were miracles involved in Creation and the Flood, science cannot investigate those events, right?

Jim, would you agree?


Martin Rizley - #47162

January 14th 2011

John Garvey,  “If miracles are beyond understanding, then why bother?”  Because a Christian shouldn’t simply ‘close his eyes’ to what is found in the natural world when it seems to challenge traditional understandings of biblical teaching.  Rather, he should look at the data honestly and ask this question:  within the framework of a robust, biblical worldview, what might possibly account for this data?  A Christian should ask himself, is it possible I am misunderstanding the Bible’s teaching about a particular subject?  Or is it possible that the scientific establishment, owing to the inherent limitations in its methods of research, is being too dogmatic about something that does not allow for dogmatism?  So I am asking the question, first, is there really a genetic link between all life forms on earth?  If so, is the only possible explanation of that link slow, stepwise evolution through the unguided natural processes of mutation and natural selection, leading to a gradual diversification of life through descent plus modification?  Or given the doctrine of providence, could God have created life new forms through genetic engineering?


DBB - #47177

January 14th 2011

Dear Dr. Kidder:  I was surprised to see that comments were still being posted. I earlier had asked in comment 46109 about the biomechanical advantage of the anterior inferior iliac spine and about the actions of the gluteal muscles in the great apes.  Your reply in comment 46237 was that “the anterior inferior iliac spine demonstrates that the gluteal muscles had become repositioned to help stabilize the trunk…”  The muscle that attaches to the anterior inferior iliac spine is the rectus femoris ( see Grant’s Atlas of Anatomy, eleventh edition, page 362) which joins with the vastus muscles to form the quadriceps muscle which attaches to the knee cap and is the prime extensor of the knee.  In humans there are three gluteal muscles on each side of the pelvis. The gluteus maximus is the strongest extensor of the hip and is most important when we rise from the seated to the standing postition ,or when we go up and down stairs.  In ordinary walking the gluteus maximus plays only a small role.  The gluteus medius and minimus act to abduct the hip (that is move the leg away from the body in the sideways direction) and to medially rotate the hip. (continued)


DBB - #47178

January 14th 2011

When we walk and put our left foot on the ground (stance phase) and lift our right foot off the ground (swing phase) and swing our right foot forward, the gluteus medius and minimus stabilize the left hip and pelvis so we don’t fall to the right side. What I would like to know is what is the exact nature and actions of the gluteal muscles in the apes(chimpanzees,gibbons,gorillas,  and orangutans) or their earlier ancestors and what changes had to occcur in the pelvis, the spine,  the hip and the gluteal muscles to make bipedalism possible.  I realize that this is a big question and it might be necessary to have another whole blog to really answer this question but I think that the readers of Biologos would really love to have a detailed explanation of this question.


Jon Garvey - #47181

January 14th 2011

@ Martin Rizley - #47162

“...is the only possible explanation of that link slow, stepwise evolution through the unguided natural processes of mutation and natural selection, leading to a gradual diversification of life through descent plus modification?  Or given the doctrine of providence, could God have created life new forms through genetic engineering?”

Or, given the doctrine of providence, that God was as involved in the naturalistically speaking “unguided” processes of mutation and natural selection as he was behind the millions of apparently random or humanly choses events that led, infallibly, to the fulfillment of prophecy in the birth of Jesus.


Argon - #47204

January 14th 2011

Martin Rizley - #47136 So if he willed, He could surely make the whole biodiversity of life through genetic engineering in a single day.

Yep. And perhaps last Tuesday He did just that.

The trouble with the genetic engineering model of Adam/Eve is that if one assumes a species bottleneck of two individuals, you don’t generate the pattern of genetic diversity found. For humans, the first literalist bottleneck is Adam and Eve but it is followed up by a second bottleneck with the Flood. Many animal species should also experience a severe decrease in genetic diversity, post-Flood. But the data doesn’t point that way and as Jimpithecus suggests, you’re left positing millions of other uneconomic, ‘special interventions/miracles’ that just happen to fall in a consistent pattern that looks unlike tinkering or special intervention. And most of that happending *after* the creation week. Last Tuesdayism is more parsimonious, in comparison.


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