An interesting thought

“… it is illogical to invoke the behaviour of living apes to explain the origins of something that they themselves have not developed…”

The Seed-Eaters: A New Model of Hominid Differentiation Based on a Baboon Analogy, by Clifford J. Jolly © 1970

The feet of Ardipithecus ramidus

We’ve already covered Ardi’s hands here, so let’s move on to what is possibly the most interesting aspect of her skeleton: The feet.

As humans, we have pretty special feet. They’re good at dissipating all of the force that comes with walking on only two feet. They’re also good at propelling us forward, since we’ve lost the mechanism that most quadrupedal animals use.

Chimps and gorillas have feet that are essentially hands. Compared to animals like Proconsul, the back of the foot was shortened, and the joints were modified to allow more flexibility at the ankle joint.  They also got rid of a teeny tiny little sesamoid bone embedded in the fibularis longus tendon called the os peroneum, which acts in old world monkeys to adduct the big toe and prevent twisting and buckling of the ankle joint.  Because the great apes have gotten rid of the little bone, their tendon can now grasp with the big toe, while the plantar surface of the foot can conform to whichever substrate they choose: Tree trunk or ground.

Most humans still have the little os peroneum, or at least a cartilaginous mass in the right place.  We clearly don’t use the associated muscle to adduct our big toe since it’s permanently in-line, but fibularis longus stabilizes our longitudinal arch.  We’ve moved fibularis longus to the outside and above the position of where it is in monkeys to aid in this function, and since we don’t need it for toe adduction anymore, the little os peroneum has been allowed to become almost vestigial.

In Ardi, the big toe is opposable, and the os peroneum is big.  In contrast to both the human morphology and the great ape morphology, it is comparable in size to what is seen in many old world monkeys, and probably Proconsul as well.  The fibularis longus muscle and tendon adducted the toe and flexed the foot as in OWM, but also added just a little bit of stabilization to the area of the foot that would become the arch in later hominids.

Now, to the lateral foot.  A second metatarsal was preserved, from which we can reconstruct the plantar base.  The second metatarsal articulates with the ankle bones at the tarsometatarsal joing.  In Ardi, there are small little dents in the proximal joint surface of the second metatarsal.  The proximal joint surface second metatarsal is mostly cartilage throughout development, and as such will preserve loading patterns from that time.  The indentations probably show us that these joints were experiencing regular pressure from the joint, which would happen during bipedality.

The third metatarsal has a big “gutter” at the junction of epiphysis and physis.  This gutter reflects loading during dorsiflexion at the ankle, and again, because it is mostly cartilage during development, can preserve important information about loading patterns.  The head shows that it had been consistently rotated during development, which would have occurred during bipedality.

The talus and tibia/fibula of Ardi form an angle (measured from the talus) that is within the range of OWM.  In hominids, this angle is considerably lower because of the angle of the valgus angle of the knee.  The talus in Ardi also preserves the flexor hallucus longus groove, which is shaped like a trapezoid tipped on its side and shows that the FHL approached the talus at an angle.  This is in contrast to the condition of Lucy and other hominids, where the FHL groove is a vertical parallelogram.  This means that, while Lucy was able to keep her knee rather straight during the stance phase of walking, Ardi’s knee was probably rotating.

Ardi's foot

These features, as well as some stuff in the pelvis that we’ll get to later, suggest that when Ardi was on the ground, she placed her foot along the midline of the body, with the big toe pointing inward and the front of the foot pointing outward.  In humans, most of the body weight rotates around the ball of the foot when we “toe-off,” but in Ardi, this pivoting occurred around a diagonal line drawn through the foot, with weight balanced both by the big toe and the fibularis muscle on the lateral edge of the foot.

Much like in the hand, the foot of Ardi suggests that, instead of modifying a foot that looks essentially like a chimpanzee foot, humans, chimpanzees, and gorillas have all taken a generalized Miocene ape foot and done our own special thing to it.

Lovejoy, C., Latimer, B., Suwa, G., Asfaw, B., & White, T. (2009). Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus Science, 326 (5949), 72-72 DOI: 10.1126/science.1175832

Did knuckle-walking evolve twice?

Knuckle-walking is a pretty special mode of locomotion. Amongst primates, only the African apes do it habitually, and anteaters are the only other mammal who does it. It would seem, then, that the most parsimonious explanation for such a specialized form of locomotion would be that the African apes all share a common ancestor who was also a knuckle-walker. An addendum to this explanation would be that humans, since they fall within that nested African-ape clade, also share an ancestor who was a knuckle-walker. The thing about parsimony, though, is that when a “parsimonious” explanation is met with conflicting evidence, it is no longer parsimonious!

The idea that knuckle-walking had independent origins in Gorilla and Pan is not a new one.  Dainton and Macho visited the subject in 1999, and Richmond and Strait have been arguing against that interpretation ever since.  Everyone can agree that gorillas put more weight on the ulnar side of their hands and wrists than chimps do.  The controversy is over whether they do so simply as a result of being larger, or as the result of a distinct evolutionary history.

Chimps and gorillas begin their lives in a similar manner.  They cling to their moms, and then once they are big enough, they begin swinging from branches.  But gorillas get bigger more quickly, and have to switch to quadrupedal behavior earlier than chimps.  Since this is the time when their bones are taking shape, this earlier shift may result in bones which are super-adapted for knuckle-walking.  In this scenario, chimps and gorillas have the same structures, but the gorillas display them to a greater degree.  They would also develop them earlier, since they adopted those positions earlier in life.  However, if the differences between chimps and gorillas are the result of a different evolutionary trajectory, and not simple allometric scaling, we would expect convergences in some aspects of the wrist, but some things that were just plain different.

When Dainton and Macho did their analysis, they found a mix of similarities and differences which they interpreted as support for a knuckle-walking as homoplasy hypothesis.  They examined two wrist bones on the ulnar side of the wrist- the hamate and capitate, and found that they were shorter throughout ontogeny in gorillas.  They also found that the articular surfaces of the hamate, capitate, and triquetral bones (again, from the ulnar side of the wrist) were larger and allowed more flexibility.  Both of these observations are consistent with the idea that the ulnar loading is a distinct biomechanical behavior in gorillas and not a side effect of gorillas being bigger animals.

That sets the stage for a new paper, released this week by Tracy Kivell and Daniel Schmitt.  In it, they examined a larger sample of African apes, and also included some arboreal quadrupeds as well.  They explored three wrist bones in search of features which are usually discussed as adaptations for knuclke-walking.  In the scaphoid, there is a concavity on the top and a “beak,” which together “catch” the radius as it extends dorsally.  This would act to limit the flexibility at this joint to only what is needed, while allowing the wrist to be stable when weight was applied.  On the capitate, there is a concavity and a “waist,” which catch the scaphoid and limit the flexibility at this joint.  The capitate and hamate both have a little ridge on their dorsal sides, which limit the flexibility between the first row of wrist bones and the second row.  And finally, the hamate is said to have a concavity which catches the triquetral and limits flexibility there.  So, the authors looked for these features in gorillas, chimps, humans, and cercopithecoids of all ages.

What they found was pretty intriguing.  Gorillas had miserable percentages of most of these supposedly knuckle-walking adaptational features.  The scaphoid concavity and beak?  Only 6% of them had it, compared with 96% of the Pan troglodytes and 76% of P. paniscus.  Even more surprising was that 80% of the arboreal palmigrade monkeys, 76% of the terrestrial palmigrade monkeys, and 57% of the digitigrade monkeys had these features!

The same pattern is exhibited in the other features examined, as well.  Gorillas display them less often, and to a lesser degree than in chimps, and sometimes even less than in other monkeys.  Gorillas, then, have a more flexible wrist, at least in terms of their skeleton.  It could be that their ligaments and muscles are doing more of the work to keep them stable, but we also know that gorillas are able to extend their wrists to around 58 degrees, while chimps can only extend it up to 42 degrees.

The authors suggest that, instead of supporting itself on an extended wrist like the chimpanzee, the gorilla is adopting more of a straight, neutral posture.  The chimp, because of its extended wrist, will experience more bending loads, and needs more bony reinforcement to keep the wrist in place.  The gorilla, because of its straight wrist, would not experience those bending loads, and would have no need for the reinforcement.

The authors discuss for a bit the differences in substrate use between gorillas and chimps.  Gorillas are large, and become that way quickly, so they spend most of their time on the ground.  Chimpanzees are a bit smaller, and can spend quite a bit more time in the trees.  Chimpanzees, while in the trees, are sometimes knuckle-walkers and sometimes palmigrade. During growth, they could develop bony growths which reflect those positions.  This is different than developing bony growths which limit the associated movements- an important distinction which the authors are very correct in making.

So, what we have here is some very interesting evidence that knuckle-walking in gorillas is different than it is in chimps.  We also have a good scenario for why it would be:  Different substrates require different biomechanics.  We even have good reason to throw out some features of the wrist which have been reported repeatedly as markers of knuckle-walking, when what they really reflect could be arboreality.  All of these things could very well indicate that knuckle-walking evolved twice- once in the gorilla clade and once in the chimpanzee clade- and that humans, therefore, probably didn’t go through a knuckle-walking phase.  It’s a good and interesting conclusion to make.

I’d like to play devil’s advocate, though.  I read a really awesome paper this year by Drew Rendall and Anthony Di Fiore about behavioral evolution.  The main gist of the article is that behavior can be just as powerful as genetics or morphology when constructing phylogenies.  Morphology and genetics are just as evolutionarily labile as behavior.  They lay down some really good evidence, so give it a read.  Given that behavior isn’t “special” in terms of evolution, what if knuckle-walking as a behavior evolved once, and gorillas and chimps (and humans!) went their separate ways morphologically?  Their morphological differences accrued as their ontogenetic trajectories differentiated from one another.

I’m not committed to either idea, because if there’s one thing that I’ve learned, it’s that homoplasy is quite common in Miocene and extant hominoids.  The problem with my behavior-as-evidence idea is that behavior doesn’t fossilize, so all we have to go on are the bones, and I can’t really think of a way to test it.

Kivell, T., & Schmitt, D. (2009). Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0901280106
RENDALL, D., & DIFIORE, A. (2007). Homoplasy, homology, and the perceived special status of behavior in evolution Journal of Human Evolution, 52 (5), 504-521 DOI: 10.1016/j.jhevol.2006.11.014
Dainton, M., & Macho, G. (1999). Did knuckle walking evolve twice? Journal of Human Evolution, 36 (2), 171-194 DOI: 10.1006/jhev.1998.0265

P.S.  Also check out Laelaps and Afarensis for more.

Bare Feet

Wired has an interesting story on barefoot running (hat tip to Gene Expression), with some special commentary from Mr. Long-Distance Running himself, Daniel Lieberman.  The article encourages people to cast off their thick-soled athletic shoes for a more natural alternative:  Going barefoot, or at least nearly so.

Running is a type of locomotion in which the animal has all of their feet off of the ground at the same time.  In humans – the bipedal apes – this consists of a fancy jump from one leg to the other in a forward direction.  As with walking, you can divide your gait into two main phases:  Stride and stance.  Stride is when your foot is up in the air, and stance is when it’s on the ground.  The stance phase is usually the one we’re worried about, because that’s when the foot hits the ground, and it determines how your foot leaves the ground as well.

Normally, when we run in our running shoes, we hit the ground with our heels.  Our shoes are designed to withstand all of the impact from that contact (which is something like 2-3x body weight!).  Sounds good!  The less impact on my bones, the better!  However, the barefoot running people are claiming that this is making our strides too long, and  our feet too lazy.  A more natural pattern is to hit the ground with the lateral side of your feet, and then let your weight roll inward.  This way, our arches get to extend and flex transversely and longitudinally the way they like to, instead of just on the transverse plane.

Our feet are designed this way, says Lieberman, because our ancestors were persistence hunters, much like the !Kung (video of a persistence hunt). The article says,

He’s sure that running barefoot or with minimal footwear is the way to avoid injury. After all, we evolved without shoes.

“If a third of runners had gotten injured in the Paleolithic with runner’s knee or plantar fasciitis, you can bet that natural selection would have weeded them out,” Lieberman says.

Right.  We’ve spent long stints of our evolution doing lots of different things though.  I mean, we evolved without clothing of any kind for a pretty long time.  But I’m not going to outside on a chilly day without my jacket, and I wouldn’t go out persistence hunting without my SPF 45 sunscreen, even though humans evolved without them!  Perhaps that’s an unfair analogy, since skin pigment is probably a more labile trait than foot morphology, but I still don’t think that we should swear off modern technology because we evolved without it.  And perhaps natural selection did weed out the guys with runner’s knee, and the ones who couldn’t keep up were left for dead on the Paloelithic Plains.  That left us modern humans with a population of flexible-footed, midfoot-strikers.  Or maybe they stayed at home waiting for it to heal, and that’s how a division of labor was born!  Or maybe humans just aren’t particularly well-adapted for running because very few of us have ever incorporated regular endurance running into our repertoires.  Perhaps there were a few elite hunters in a few populations who had adopted persistence hunting as a means to acquire supplemental protein, but my guess is that walking was the way to get around in the Paleolithic.

Either way, I’m usually very cautious about shaping my lifestyle to fit the needs of a paleolithic savannah-scape.  We’ve done a lot of evolving since then, after all!  If I push my lifestyle back to the Paleolithic, then who’s to say that I’m not even BETTER evolved for the Pliocene?

Reed Ferber shares at least a little of my skepticism:

Ferber is more cautious. His studies of the biomechanics of running show that a midfoot strike does reduce the initial peak loading force — the impact in the first 25 milliseconds after your foot touches the ground. But your foot sustains a second peak load of three times your body weight about 100 milliseconds later, regardless of whether you’re a heel-first or midfoot-first runner.

So, if either of those peaks was a selective force, why select against one and not the other?  Is it just that we’re anatomically committed to that second peak, while we could play around a little with the first one?  I can see that, I guess.

I think lots of people probably need better running shoes, and if those goofy toe-shoes work for some people, then they should have them!  But I also think that it’s wise to keep in mind that the average life span in the paleolithic was much shorter than it is now, and bad knees and arthritis might be a sign of subjecting our bodies to decades more wear and tear than they evolved to withstand.  We’re committed to having skeletons, which, remarkable as they are, do show some wear and tear after awhile.  Our bodies are laid out in such a way that we are able to engage in a wide variety of activities with very few catastrophic consequences, but any activity, no matter how flawlessly performed, is going to leave a mark at some point.  Humans have never been perfectly adapted to an environment or a particular lifestyle.  Evolution just doesn’t work that way.

Flip Flop Biomechanics

This is a post from an older incarnation of this blog.  I’ve imported it in honor of the summer months, and also as a companion piece to a post I’m working on about barefoot running.

A study focusing on flip flops and biomechanics was presented at the American College of Sports Medicine meetings recently, and it’s bad news, folks.  Now, I love a nice flip flop to go with my breezy summer dresses, but after stepping on a smashed beer bottle after a football victory on one Saturday in September, I realized that the cheap plastic ones are not the way to go.  So, I bought a sturdier number crafted of leather and cork and brought them on a dig in Poland.  One stress fracture of my fourth metatarsal later, and I still can’t give them up.  At least now I can wear them with science in mind…

Justin Shroyer and his colleagues analyzed gait pattern of a group of subjects wearing flip flops, and then of the same group while they were wearing athletic shoes. While wearing flip flops, it turns out that the people weren’t hitting the ground with as much vertical force. This can probably be attributed to something the authors term “toe-scrunch.”  Toe-scrunch is, apparently, what happens to your toes as you try to keep your flip flops from flying off of your feet. Toes are an important part of the “toe-off” and “swing” phases of walking, so when they’re all scrunched up, it affects the way you lift your foot, which in turn affects the way your foot hits the ground. Flip-flop wearers don’t lift their feet as high, and they don’t flex their ankle as much.  These awkward alterations to gait patterns result in the “flip-flop shuffle” that we all know and love.  We take shorter steps, and end up having to take more awkward, shuffley steps to go the same distance as someone else who is wearing a nice, sturdy athletic shoe or ballet flat.  Instead of hitting the ground with the nice vertical force which our bones have evolved to accomodate, we hit the ground at a weird, oblique angle.

More steps + weird steps = injury.

Since tendons and ligaments and all of the other connective tissues in our feet aren’t used to these weird steps, they have to work overtime to make sure that our bones are staying in place.  The result may turn out to be plantar fasciitis, an inflammation of connective tissue which is generally unpleasant.  Or, if I can add my own anecdote to the data, a stress fracture resulting from the fourth metatarsal continuously slamming against the cuboid!

Shroyer suggests that you get yourself some shoes with a few more straps for everyday wear, limiting your flip flop use to short outings and the beach. He also warns that “broken in” flip flops are no good, and they should be replaced every 3-4 months.

Now, if only I could find a study touting the ill-effects of wearing crocs…

The big hole in your head (Picture-heavy post!)

When an animal is walking around doing whatever that particular animal does for a living, its eyes are usually looking forward, toward the horizon.  The animal needs to see whatever it is that the animal needs to see- predators if you’re a prey animal; prey if you’re a predator- and the best way to do this is to position your head so that you can see in front of you without having to flex or extend too many muscles.  When the animal is positioned in the normal old looking-in-front-of-you position (norma lateralus in anatomy-speak), certain points in the head line up to form a plane which is horizontal to the ground.  These points are the bottom-most point of the eye orbit, called the orbitale, and the top-most point of the external bony auditory meatus, called the porion. So, you draw a line through the left and right porion and orbitale, slice through the skull in a horizontal plane, and you have a plane which is horizontal to the ground.  This plane is called the Frankfort Horizontal Plane, and the first step in most cranial morphometric studies is to put the skull in Frankfort Horizontal. (Some people like to use the horizontal semicircular canals to put the skull into a horizontal plane, but sometimes that requires expensive CT technology, or bones that didn’t survive millennia in the ground or centuries of being slammed around in big specimen drawers in museums.  The Frankfort Horizontal is the cheap and easy way, and in primates, it’s usually pretty accurate.)

There are many reasons why you would want to orient a skull so that it’s in the same horizontal position that it took to during life.  What we’re going to talk about here is the position of the foramen magnum- the big hole at the base of your skull where your spine meets your head.  In most quadrupedal animals, the foramen magnum is at the back of the skull, which makes a certain kind of intuitive sense.  Your eyes have to be at the front of your face so you can see, and if your spinal column is a horizontal line, it makes sense to have it coming out of the back of your head because where else would it be?  Here, look at this Mexican Wolf skull.  The first picture is the bottom of the skull, and you can’t see the foramen magnum.  That’s because it’s at the back of the head, as shown in the second picture.

Mexican Wolf Skull, bottom view

Mexican Wolf Skull, back view

Human Skull

In humans, the foramen magnum is tucked under the head so that the head is right on top of our vertical spinal column.  This movement of the foramen magnum forward and under is actually, in developmental terms, a lack of movement.  All baby apes, including humans, start out with foramina magna in about the same position.  As the rest of the apes develop into adults, their foramina magna migrate toward the back of the head.  But ours stay in the same, baby-like position.

Giraffe

So, is the foramen magnum at the bottom of our heads because we need to balance our head on top of our erect spinal column?  Or might it have to do with something else?  Let’s think about some other animals.  Giraffes are quadrupedal, right?  But they also have really long, erect spines.  So where is their foramen magnum?  Pretty much at the back of the head, as it turns out.  Same with kangaroos, llamas, birds, and some other guys.  So, it might not have as much to do with posture as we thought.

What else might it be?  Maybe it’s the weight of the face.  When you have a heavy face, you need a lot of muscle at the back of your head and neck to balance it and hold it up. The spinal column is basically a fulcrum over which the front and back sides of your head are balanced.  If you move the fulcrum more toward the center of the weight, the weight will be more evenly distributed and you won’t need to use as much muscle to keep your head up.  Sounds pretty good.  The only problem is the gibbon.  Gibbons have a heavier head relative to their body weight than chimps or even humans, yet their foramina magna are more posteriorly positioned than either one.  And, since they’re suspension brachiators, their posture is very upright as well.  And yet, that foramen is still pointing out the back of the head instead of the bottom.

That leaves us with brain size and encephalization.  As the cerebral cortex (the “thinky” parts of the brain) gets bigger and more complicated, it tends to not only get longer and taller, but it changes shape as well.  Alligator brains are basically arranged in a straight line with the forebrain at the front and the hindbrain at the back.  As brains have expanded, the brain curves around so that it forms a reflex angle (greater than 180 degrees).  Now, instead of the hindbrain being behind the forebrain, it’s under it.  Sounds a lot like a certain hole I know…

The jury’s still out on what exactly causes this shift in the position of the foramen magnum, but it’s an important question for paleoanthropologists to ask.  Some of our most famous fossils have been described as bipedal based on the position of the foramen magnum alone- fossils like the Taung Child and Sahelanthropus.  In the case of the Taung Child, we now have lots of corroborating evidence that members of the Child’s species were bipedal, but that’s not the case with Sahelanthropus.  We need to make sure that we know what exactly an anteriorly positioned foramen magnum means before we use it as a defining character of members of our family!

In parting, I leave you with these images of a cat and a macaque.  Both are quadrupedal animals, but the differences in foramen magnum position and brain size (by way of skull size) are striking.

Cat

Macaque

Human Ichnology: Turkana-style

About 1.5 million years ago, a Homo erectus went for a walk, pausing to observe her surrounding landscape.  A river stretched out beside her, and the fine-grained sediments left behind from the last flood squished between her toes.  A flock of birds was feeding on small fish and invertebrates that had found themselves in a rivulet on the banks of the bigger river.  As she slowly resumed pace, a hippopotamus poked his head out of the river for air, and a distant volcano belched smoke and ash.

Okay, enough tableau.  A group of scientists working at Illeret in the Turkana Basin has described two sets hominin footprints which date to about 1.5 mya.  The volcanics that were associated with the footprints had been fluvially altered, but since each eruption has its own geochemical signature, the tuffs can be matched with a corresponding deposit with a known date.  The first set of prints has three footprint trails-one with seven prints- and several isolated prints, and the second set has one trail and one isolated print.  Footprints are remarkable, rare fossils.  Together with the actual foot bones of an animal, they can give us a rich picture of how an animal moved, what it’s social habits were, and the environment in which it lived.  Our first clues that dinosaurs weren’t tail-dragging, sluggish, and solitary reptiles came from footprints, for example.

When we walk along a soft substrate, like at the beach, or in the yard after a hard rain, our footprints leave more information behind than we might suspect.  They leave behind a record of how a person distributes the weight of their body over the platform of their foot.  Most people’s footprints are going to look very similar: Since your heel reaches the ground first and sustains high amounts of pressure, it’s going to leave a deep mark in the substrate.  The heel bone is bigger and spongier in humans for exactly this reason- larger size allows more space for the pressure to be absorbed, and spongier bone allows for greater elastic compression.  Next, the pressure rolls from the heel to the outside of your foot.  It rolls along the outside here because of the longitudinal and transverse arches of the human foot.  The arches act to provide a maximum amount of weight-bearing combined with maximum flexibility, sort of like the Roman aqueducts.  From the outside edge of your foot, the pressure then rolls to the ball of your foot, and then to your big toe.  Throughout the course of the whole step, the highest concentrated amount of pressure is going to be underneath the ball of your foot, and then underneath the big toe.

Apes don’t have a single, stereotypical footprint, because they are adapted to be able to sustain pressure from many different directions.  They usually make contact first with their heel and midfoot, but their liftoff is highly variable.

The new footprints show remarkable similarity to the modern style of walking, with the stereotypical pressure distribution and resulting footprint.  The authors used digital imaging to record the relief of the footprints and then compare them with those from modern humans, some prints left during the Neolithic and Bronze ages at Sefton Coast, and the ones left at Laetoli, which is trail of footrpints left 3.7 mya, presumably by Australopithecus afarensis.  They analyzed 13 landmarks on each foot and analyzed them using Procrustes analysis (the best name for a statistical method ever, btw).  While the feet at Illeret were a bit narrower than those in modern humans, they had perhaps a more defined longitudinal arch than the ones at Laetoli.

The authors also made some inferences about stride length and speed based on the 7-print trail.  They estimated, based on a hip height of 830 mm, that the individual was moving at about .63 meters per second, which might be consistent with someone who had stopped walking, and then began again.  The stride lengths were uneven, and consistent with how a person would move when presented with muddy, uneven terrain.

These footprints don’t necessarily provide any surprising information.  Given the modernity of the prints at Laetoli, I would expect that a hominin with modern body proportions would leave footprints resembling those of modern humans almost exactly, just as these do. They are also completely consistent with what we have from skeletal morphology.  As much as we all love paradigm-shifting discoveries, it’s also nice to find evidence that fits in perfectly with the prevailing ideas and theory.
Matthew R. Bennett, et al.  (2009)  Early Hominin Foot Morphology Based on 1.5-Million-Year-Old Footprints from Ileret, Kenya.  Science 232:1197.

DOI: 10.1126/science.1168132

Love for Lucy

I don’t know why I do it to myself.  Perhaps I’m a glutton for punishment and frustration.  Every so often, I’ll feel the need to go to one of those Intelligent Design/Creationism blogs and get myself all angry and riled up.  This morning I went over to Evolution News and Views and saw that Casey Luskin has been to the Pacific Science Center’s Lucy exhibit, and he’s soooooo not impressed.  That’s okay though, because I’m not impressed with his critique.

Luskin says,

The first thing my friends and I noticed when seeing Lucy’s bones was the incompleteness of her skeleton. Only 40% was found, and a significant percentage of the known bones are rib fragments. Very little useful material from Lucy’s skull was recovered. (This seems to be common: many of the replica skulls of early hominids at the exhibit were clearly based upon extremely fragmentary pieces.) And yet, Lucy still represents the most complete known hominid skeleton to date.

I’m not sure if this is just a confusion of terms or just glaring ignorance, but Lucy is not the most complete fossil hominid known to date.  Meet Nariokotome Boy.  If you’re looking for complete skulls, let me introduce you to the Taung Child, Little Foot, Mrs. Ples, or KNM-ER 406.  Or, open a book and introduce yourself to any number of the other skeletons that are comparatively or more complete than Lucy.

Next, he says:

If the next rainstorm could wash Lucy away completely, what happened during the prior rainstorms to mix-up “Lucy” with who-knows-what? How do we know that “Lucy” doesn’t represent bones from multiple individuals or even multiple species?

Well, you see, a person doesn’t get to be a paleontologist unless she knows her anatomy.  She has to know where every single little muscle attaches onto every single little bone.  It’s her job.  All of this anatomical knowledge makes it really easy when someone comes into a forensic anthropologist and says, “I think I’ve found a human skeleton behind my house, and I suspect murder!”  A forensic anthropologist can go to that site, look at a single bone fragment from the tibia or a medial phalanx and tell the person, “No, don’t worry, this is just a dog.”  She can do this because she is intimately familiar with anatomy, and knows how, in the dog, the tibial plateau will be shaped quite differently than in the human because of the different mechanical requirements.

Paleoanthropologists can do the same thing with Lucy’s pelvis or femur.  The pelvis and femur don’t look like anything we see in any quadrupedal animal at all.  And wow- that COMPLETE sacrum is just screaming “BIPEDAL ANIMAL HERE!!!”  We can look at muscle attachment sites and say, “Gee, whoever this was, she had a really huge gluteus minimus!”  We can then compare the size of different gluteus minimus muscles across the animal kingdom and see that only animals who walk upright have such a large gluteus minimus.  So, it’s not merely that we’ve counted up our bones and we don’t have any duplicates.  We can look at the functional anatomy of these bones and determine that we don’t have a quadruped.

Regardless, seeing the broken scraps of old Lucy laid out under the protective glass, with full skeletal and full-flesh reconstructions of Lucy abounding throughout the exhibit, I could not help but recall the words of the famed physical anthropologist Earnest A. Hooton, who in 1931 wisely counseled that “alleged restorations of ancient types of man have very little, if any, scientific value and are likely only to mislead the public.” (Up From The Ape, pg. 329.)

It’s important to note the date that Earnet A. Hooton said this.  It was 1931.  That was before we had anything to base our restorations on!  Of course restorations in 1931 were misleading and unscientific.  That didn’t stop the very same Earnest Hooton from concluding that black babies were closer to primitive man than white babies, though, so take his wise counsel with a grain of salt.

Luskin then talks about how the Lucy’s curved phalanges, long arms, and funnel-shaped chest detract from Lucy’s position as a transitional hominin:

Lucy did have a small, chimp-like head, but as Mark Collard and Leslie Aiello observe in Nature, much of the rest of the body of Lucy’s species, Australopithecus afarensis, was also “quite ape-like” with respect to its “relatively long and curved fingers, relatively long arms, and funnel-shaped chest.”

Collard and Aiello’s article also reports that we now have “good evidence” that A. afarensis (including Lucy) “‘knuckle-walked’, as chimps and gorillas do today.”

First, a nit to pick:  Of course A. afarensis was “quite ape-like.”  It’s an ape!  Humans are also “quite ape-like” with respect to our Y-5 molar pattern.  Our opposable thumbs are rather monkey-like.  Our five digits are rather tetrapod-like.  Our digestive system is rather deuterosome-like.  The nucleus in our cell is rather eukaryote-like.  And the fact that we have DNA is rather prokaryote-like.  Hopefully you see where I’m going with this, but just in case you don’t- “ape-like” doesn’t mean, “resembling modern apes.”  It means “resembling the primitive ape-like condition.”

Okay, now let’s look at this evidence for knuckle-walking.  It includes a tiny ridge on one of the metacarpals that indicates a wrist-locking mechanism.  Now let’s look at that sacrum again.  It’s completely restructured, and unlike any quadruped we know.  Why is Luskin so quick to accept a tiny ridge on a carpal bone, and so willing to throw out the sacrum completely?  Is it because the carpals reinforce your preconceptions, while the sacrum blatantly flies in their face?

Further on, Luskin is feeling generous:

Let’s assume for the moment that Lucy was a fully bipedal ape: should that necessarily qualify her as a human ancestor? Given that the much earlier fossil record from the Miocene yields bipedal apes that supposedly evolved upright-walking completely independently from the line that supposedly led to humans, it would seem that the answer is clearly no.

That Miocene ape he’s talking about isn’t Oreopithecus, is it?  I can only conjecture, since he won’t come right out and say who he’s talking about, but a bipedal construction of Oreopithecus is based on some horribly deformed lumbar vertebrae.  Again, we have a case of someone latching onto a fringe interpretation because it supports his preconceptions.

Okay, how ’bout this:

It doesn’t seem very advantageous, and therefore likely, to use bipedality as your primary mode of locomotion if you can’t use it to quickly run away from predators.

And what if, instead of running away from them on flat Earth, you were retreating to trees to get away from them?  Wouldn’t we then expect not only adaptations for bipedalism, such as a wide sacrum and a short ilium, but also adaptations for arboreal life, such as curved phalanges and long arms?  Let’s go back to our friends, the knuckle-walkers.  Knuckle-walkers only knuckle-walk when they’re on the ground foraging.  When they’re in the trees, they swing.  No one would look at a chimp skeleton and say, “Oh, they were knuckle-walkers, so how could they get into a tree?”  The two aren’t mutually exclusive.  And depending on the amount of swinging A. afarensis was doing, bipedalism and climbing trees aren’t mutually exclusive, either.  In fact, it’s just what you would expect from a creature that was “transitioning” from an exclusively arboreal existence to an exclusively terrestrial one, don’t you think?

Further, we know that Australopiths lived in forests.  Forest animals are not known for their speed, but for their agility.  Being a short, wide-hipped animal isn’t good for speed, but it is pretty good for agility.  Humans didn’t evolve for speed until they left the forests.

Luskin then throws around some old quotes from 1995 and 1981 to show that we need more fossils.  Well, guess what?  We’ve found more!  Each one of them makes our perception of human evolution a little clearer, but you know what?  We still need more!  Does this mean that paleoanthropology is a dead science?  Of course not!  It means that it’s a healthy, growing science that it still in its childhood, if not its infancy!  So yes, we should all take Luskin’s advice and “keep an open mind” about Lucy and our origins.  But we should also let the evidence shape our conclusions instead of forcing it into something it’s not.

The Bipedal Pelvis

Humans are unique among mammals in their exclusive use of a bipedal gait.  While many other mammals may use a bipedal posture on occasion, humans and their ancestors have restructured their postcranium so drastically that they must use this posture permanently.  Many of the most striking differences between bipeds and quadrupeds reside in the pelvis.

In a bipedal animal the quadriceps muscles become the primary muscles involved in propulsion.  These muscles act to straighten, or extend the knee while the limb is still touching the ground.  The quadriceps is divided into four different heads.  Rectus femoris is the most superficial of these, and originates at the front and bottom of the ilium as well as from a groove situated above the acetabulum.    It attaches to the patella.

The hamstrings acts in direct antagonization to the quadriceps by acting as a knee flexor.  The most important activity of the hamstrings is to control the deceleration of the limb as it reaches the final stages of the swing phase of walking.

Gluteus minimus and medius act as pelvic stabilizers while the body is being supported by only one limb, and Gluteus maximus stabilizes the trunk upon that same limb.  It also controls forward rotation of the trunk when making ground contact by drawing the pelvis backward.  Gluteus maximus originates at the back of the superior-most spine of the ilium, as well as from the posterior and inferior portions of the sacrum. It inserts into the iliotibial band, which is a tract of fascia which runs down the leg, as well as onto the gluteal tuberosity of the femur, which runs from the linea aspera to the base of the greater trochanter.  Gluteus minimus arises from the outer surface of the ilium and the sciatic notch and inserts onto the anterior surface of the greater trochanter of the femur, and Gluteus medius arises from the outer surface of the ilium and inserts onto the lateral surface of the greater trochanter.

We can look to the fossil record to see how the locomotion of our ancestors compares to our own and to that of modern apes.  The “Lucy” specimen of Australopithecus afarensis bears an almost completely intact left innominate bone.  Lucy exhibits a broad, low, and flared ilium, which shows that the anterior abductor muscles (Gluteus maximus, medius, and minimus) had a very active role in supporting the torso while supported on one leg.  The position of the ilium allowed the gluteus maximus to exert control over trunk rotation as the heel was hitting the ground. The shortened shape of the pelvis also allowed the spine to curve enough so that the center of gravity of the animal was now positioned directly above the hip.

The upper pelvis of A. afarensis is almost certainly indicative of a modern bipedal locomotion, but the lower pelvis retains some ape-like features.  This is because many of the changes that occured in this part of the pelvis reflect the contraints and demands of giving birth to large-brained infants rather than of locomotion.  A. afarensis was still giving birth to relatively small-brained infants, so this selective pressure was not yet acting.  Thus, the pelvis of A. afarensis doesn’t represent “functional intermediacy,” but rather, it shows that mechanical advantage in the human lineage has actually decreased as the result of parturition.