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 third trochanter and gluteus maximus of Ardipithecus

The femur can be an extremely informative bone when reconstructing the locomotor behaviors of fossil primates. The head and neck are particularly informative. The morphology of the head can tell you how flexible the hip joint is. If you can get a good CT scan, the distribution of cortical bone at the neck can reflect how the primate applied force to the skeleton during locomotion. If you look to the other end of the femur, the condyles can also tell you how a primate loaded the skeleton while moving around by way of the famous bicondylar angle.

Unfortunately, none of those characters were preserved in Ardipithecus. What we do have is a shaft, and as we’ll see, the shaft can give us some information about muscle attachments- especially the ever-important gluteus maximus.

First, a little background: In great apes, the insertion for the gluteus maximus is positioned further down and toward the midline of the femur relative to what we see in humans. A different muscle, the vastus lateralis, attaches to the femur a bit further up.  The two muscle attachment sites are separated from each other by a bump called the lateral spiral pilaster.

In humans, the gluteus maximus inserts onto the lateral aspect (outer edge) of the femur.  What we see is a third, very small trochanter that is part of the greater trochanter.  Immediately below that third trochanter is a bumpy depression in the bone  (a hypotrochanteric fossa).  Both of these characters are associated with our enlarged gluteus maximus. Apes never have either of them.

I bet everyone can guess that Ardi shares a more human-like femur morhpology (why else would we be talking about it?), and indeed it does have a third trochanter, as well as the depression below it.  What is surprising is who else has similar morphological characters.  Nacholapithecus has a third trochanter, as does Dryopithecus, and even primitive Proconsul! This is in addition to Orrorin tugenensis, Australopithecus afarensis and A. anamensis.

The femora of Australopithecus afarensis (A- the "Maka" femur) and Ardipithecus ramidus (B). The arrow indicates the third trochanter and the bracket indicates the hypotrochanteric fossa. (From Lovejoy et al. 2009)

Because the back of the African, large-bodied suspensory apes has been so drastically modified since Proconsul, the gluteus maximus also had to be moved around.  The gluteus maximus insertion diverged distomedially away from the vastus lateralus, and the gap was filled in by the lateral spiral pilaster.

…In both lineages!

The gluteus maximus in humans has changed, too.  By the time we get to Australopithecus, the insertion has moved back and to the midline (posteromedially).  The authors suggests that this is because the quadriceps has gotten bigger, while the hamstrings became smaller.

Clearly, we shouldn’t draw any conclusions about bipedality in Ardi from a third trochanter and hypotrochanteric fossa if even Proconsul had similar characters.  Enlargement of the quadriceps hasn’t happened yet, but if we’ve got an animal who was participating in bipedality only some of the time, I’m not sure if we’d expect it yet.  The take-away message here is that we have yet another anatomical detail that supports the idea that humans are more primitive than chimpanzees and gorillas in many aspects.  Using chimps (or bonobos) as an analog for early human behavior- locomotor or otherwise- is becoming less and less defensible.

Lovejoy, C., Suwa, G., Spurlock, L., Asfaw, B., & White, T. (2009). The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking Science, 326 (5949), 71-71 DOI: 10.1126/science.1175831

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

Climbing on the branches of the family tree: The hands of Ardipithecus ramidus

As everyone has already heard by now, the long-awaited Ardipithecus ramidus has finally been published, and boy is she a beauty!  So many “anatomical surprises”! There are so many strange and new things about this skeleton that it took an entire issue of Science to describe them.  Clearly then, it will take a few blog posts.  The first post will be about the hands.  Feet will be next, and we’ll move on from there.

But first, a few basics.  The papers report on over 35 individuals represented by various parts of the body.  Lots of teeth.  They were found in Aramis, Ethiopia and date to 4.4 million years ago.  For reasons that I’ll discuss below later, we think that Ardi was a facultative biped who spent most of her time in the trees.  When she was up there, she walked above the branches with her palms and plantar surfaces in contact with the branch.

That Ardi is a palmigrade, above-the-branch walker is surprising. Based on the apes that we have around today (chimps, humans, gorillas, and orangs), most anthropologists have always constructed the common ancestor to chimps and humans as an animal who was suspensory when in the trees (that is, hanging from the branches), and a kunckle-walker when on the ground.  There have been plenty of people who have called this scenario into question (in fact, I talked about one of them here a few weeks ago), but that hasn’t stopped many anthropologists from using the chimp as an analog for what the last common ancestor roughly looked and acted like.  Ardi shows us that the LCA probably looked nothing like either the chimpanzee or the human, but was more likely a generalized Miocene ape which lacked the specializations of either.

(As an aside, I’ve seen a rather questionable quote by Owen Lovejoy mentioned a few times– something like, “Humans didn’t evolve from apes.”  It is a very unfortunate thing for him to have said from the standpoint of science journalism, but taken in the context of the 11 articles that he and the rest of the team have written, as well as numerous other quotations, he clearly means that humans didn’t evolve from anything that looks like any of the extant apes.)

Anyway, on to some anatomy!

Because the extant African apes are knuckle-walkers, they have stiff, inflexible hands and wrists that allow them to support their body weight in sort of a weird position.  Because they also have to climb trees for food and protection, their hands are very long and powerful.  Humans, on the other hand, have pretty mobile hands and wrists which allows us what we call a “power grip.” We are very good graspers, and this has allowed us to become the dextrous tool-wielders that we are.  Because of our close genetic similarity to chimps, and the close morphological similarity between chimps and gorillas, it has been argued that certain features of the Australopithecine wrist- and even the human wrist- were “hold overs” from the period of time when we, too were knuckle-walkers who required a stiff wrist and hand.

Ardi's hand. The inset includes the capitate from a chimpanzee, Ardipithecus, and a human.

However, Ardi’s hand more closely approximates the human hand than the knuckle-walker hand.  It is very flexible.  The midcarpal joint in particular is striking in its flexibility.  Ardi could lay her palms flat on the tree branch and support her entire weight this way.  In this respect, Ardi resembles many generalized Miocene apes, such as Proconsul. Because this wrist is so generalized and resembles the wrist of so many other Miocene apes, it is probably the ancestral condition.

If we look at it that way, then human hands are only moderately derived when compared to those of chimps and gorillas.  Humans don’t enjoy quite the degree of flexibility at the midcarpal joint that Ardi does, and we’ve enlarged our thumbs and shortened our fingers a little.  Chimps and gorillas both had to modify their hands substantially to be able to move around the way that they do.

[Edited to insert link and fix a typo]

Lovejoy, CO., Simpson, S., White, T., Asfaw, B., & Suwa, G. (2009). Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive Science, 326 (5949), 70-70 DOI: 10.1126/science.1175827

What microcephalics can tell us about human evolution

Microcephaly is a disease in which the brain is smaller than normal.  A small brain can result from several different developmental conditions.  Babies can be born with normal-sized crania and brains which then fail to develop with the rest of the head, in which case it’s indicative of a neurodegenerative disorder that may be caused by environmental exposure to some sort of toxin (toxoplasma or alcohol) or some sort of genetic condition which causes deterioration of the brain.  The type of microcephaly that is important to us in this discussion is termed “primary microcephaly,” and is a microcephaly in which infants are born with crania and brains which are already smaller than those of their peers.  It is an autosomal recessive condition with seven identified genetic loci which may or may not be involved in each individual case.  In primary microcephaly the brain is small, but architecturally normal. (That is, they possess the same parts of the brain that humans with developmentally “normal” brains possess, but scaled down and in different ratios.  The cerebral cortex is usually the area of the brain which is most reduced, in terms of both white matter and gyrus complexity.)

Though having microcephaly is strongly correlated with mental retardation, microcephalics exist on a wide spectrum of cognitive abilities.  Retardation is usually only mild.  Social, motor, and speech development may be delayed, but they can progress and function quite well.

A sample of microcephalics taken by Weber and his collleagues in 2005 shows that cranial capacity ranges from 280 to 591 cubic centimeters (cc), with a mean of ~400 cc.  Humans have a pretty wide range, but we usually place it between 1000 and 1800.

Now, I bet you’re thinking that I’m going to talk about Hobbits now, but I’m not!  Right now, I’m going to talk about apes.  Bonobos have a cranial capacity ranging from around 280 to 380 cc.  Chimps range from 280-450.  Orangutans clock in at 280-500cc, and Gorillas at 350-780 cc.

But wait.  None of those guys can talk.  None of them functions normally within a human social group.  None is capable of our fine precision grip.  And yet, many of them have larger brains than microcephalics who can do all of those things.

“Of course!” you may say.  “They are from a different species than us!”  But keep in mind that the accepted knowledge of only a generation ago was that human brains were remarkable because they were big.  Big brains made us smart, especially when scaled for body size.  With our big brains, we conquered tool-making, fire-making, and even monogamous love-making.  The human brain was a chimpanzee brain writ large, as the chimpanzee was a monkey brain writ large, and so on down the line.

The observation that there were humans- humans who could talk and play sports and read and write- with smaller brains than some chimps was a rather profound observation to make.  That observation implies something very important:  Our brains are not special because they are big, they are special because of how they are big.

In terms of simple anatomical repartitioning, human brains are special primarily because of an enlarged and super wrinkly neocortex.  The neocortex became larger while most other parts of the brain either stayed the same or were reduced when scaled for body size.  Our brains are also more spherical, which may reduce the distance that electrical signals need to travel.

While microcephalics generally have a smaller portion of their brain devoted to cerebral cortex than other humans, the proportion is still greater than in the apes.

Paleoanthropologists are often tempted to use brain size as a proxy measure for intelligence, and intelligence as a proxy for “humanness,” but the case of microcephaly should give us a good reason to stop using this measure.  Indeed, most researchers use some sort of “encephalization index” in their work, so that absolute size is no longer the primary reported measure, but a measure of brain size scaled to body size.  However, even this may be inadequate if we consider the vast gulf between a microcephalic human and an ape with a similar body size.

And yet, many researchers still report cranial capacity!  It is, admittedly, only used as a last resort (to my knowledge, at least) when there are no postcranial remains from which an approximate body size can be calculated.  Even if there is a postcranium, calculating body weight for those fossils comes with a rather large margin of error.  So perhaps reporting this number can be forgiven, even if it tells us very little about the individual to whom it belongs.

The next logical step would be to skip reporting the simple size of the brain cavity, and look at the actual brain itself.  Fossilized brains, or endocasts, may be useful for gleaning information about anatomical structure of the brain, but these are very, very few, very far between, and usually very difficult to interpret (see the debate over the lunate sulcus of the Taung child, for example).  For these reasons, comparative neurology may be the best way to find out about the neurology of fossil hominids.  Figuring out the patterns and structures present in different taxa, or even within one taxon as in the case of the microcephalic, can lead us to information that we simply can’t get from rocks and bones.

WOODS, C., BOND, J., & ENARD, W. (2005). Autosomal Recessive Primary Microcephaly (MCPH): A Review of Clinical, Molecular, and Evolutionary Findings The American Journal of Human Genetics, 76 (5), 717-728 DOI: 10.1086/429930
Sherwood, C., Subiaul, F., & Zawidzki, T. (2008). A natural history of the human mind: tracing evolutionary changes in brain and cognition Journal of Anatomy, 212 (4), 426-454 DOI: 10.1111/j.1469-7580.2008.00868.x
HOLLOWAY, R. (2004). Posterior lunate sulcus in Australopithecus africanus: was Dart right?*1 Comptes Rendus Palevol, 3 (4), 287-293 DOI: 10.1016/j.crpv.2003.09.030
Falk, D. (1983). The Taung endocast: A reply to Holloway American Journal of Physical Anthropology, 60 (4), 479-489 DOI: 10.1002/ajpa.1330600410
Weber, J. (2005). Comment on “The Brain of LB1, Homo floresiensis” Science, 310 (5746), 236-236 DOI: 10.1126/science.1114789

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.

A New Archaeology

No, not the kind that Lewis Binford would want us to adopt.  Julio Mercader and a bunch of other primatologists and paleoanthropologists have just published a paper about Primate Archaeology in Nature.  When we find stone tools in the fossil record, we usually attribute them to humans and their ancestors, but this may not always be the case.  Humans, after all, are not the only species who use tools.  The first site to be identified as a non-human archaeological site is a 4,300-year-old site in Cote D’Ivoire attributed to chimpanzees.

I am out traveling and gathering data for my thesis right now, so I don’t have access to my electronic journals right now.  When I get home, I’ll be very interested to see how the authors address the issue of how to identify a tool made by an early chimpanzee versus that made by an australopithecine.

Last week I was complaining about having to learn archaeology instead of insect sociality and cuttlefish biology, but this week I think I’m back on board with archaeology!  This new field serves as a reminder of why anthropologists should adopt a holistic, interdisciplinary approach and quit bitching about it!

Knuckle-walking anteaters!

I’ve been working on a post about Grehan and Schwartz’s new orangutan paper, but I’ve been getting  sidetracked by teaching and doing my own research along the way.  To make matters worse,  a friend was telling me about an anteater that he saw at the zoo and mentioned that it was a knuckle-walker!  I wondered if “they” knew about this fascinating discovery (“they” being my senior colleagues),  so I went straight to the internet and found this paper (which I think is open access.  Let me know if it’s not).

The paper sets out to examine the specific morphological traits that are shared by all knuckle-walkers, regardless of phylogeny.  There has been lots of debate about whether or not humans evolved from a knuckle-walking ancestor, but comparing morphology in such closely related organisms as gorillas, chimps, and early humans may result in a false positive for knuckle-walking detection.  We could share certain morphologies not because of functional similarities, but because of phylogenetic closeness.

When chimps and gorillas knuckle-walk, they bear their weight on their middle finger bone, or phalanx.  When in this position, the joint between the phalanx and the metacarpal (the bone that makes up the palm of your hand) is hyperextended.  If you position your hands palm-side down and then bend your finger up, that is hyperextension.  Gorillas and chimps both do this, but the molecular data suggests that chimps and humans are more closely related than chimps and gorillas.  This means either that knuckle-walking evolved twice- once after the gorilla-chimp/human split and again after the chimp-human split- or that all three shared a knuckle-walking ancestor.  Parsimony would suggest the latter scenario, but if there’s fossil evidence which contradicts the “simplest” answer, it’s no longer the simplest answer! To figure out which scenario is more likely, we look at the fossil evidence and compare it to living primates.

Let’s say, for example, that in gorillas the distal radius (the part near the wrist) has an extra ridge of bone on the dorsal surface.  We could say that that ridge of bone is there to provide support for the wrist so that it can stand vertically while knuckle-walking without buckling.  If we find that same ridge of bone in human or early human radii, then we might infer that early humans were also doing some knuckle-walking.  But what if that ridge of bone really is just evolutionary baggage from when gorillas and humans shared an arboreal or digitigrade ancestor, who needed a rigid wrist for something else?  That’s where the anteaters come in- they act as an outgroup comparison.  They shared a common ancestor with primates a very, very long time ago, so any convergence in the wrist is likely to be a functional adaptation rather than the result of being closely related.

Giant Anteater at the Santa Barbara Zoo

Giant Anteaters (Myrmecophaga tridactyla) have long claws which can be cumbersome if they’re walking around on the ground, so they tuck them under their hands, just like chimps and gorillas do with their long fingers, which are retained so they can swing around in the trees.  The anteates bear most of their weight on the fourth and fifth digits (ring and pinky), and on the middle phalanx (though in the fifth digit, the distal phalanx has been lost, “middle” isn’t really the middle one).

Two features which are shared by chimps, gorillas, and anteaters are 1) an expansion of the top side of the capitate on the end closest to the wrist and 2) a wide dorsal surface relative to palmar surface on the fingers and metacarpals that are the most weight-bearing.  Australopithecus afarensis also seems to share the shape of the capitate with these animals.  In the radii of all animals who use a vertical hand when walking (digitigrade and kuncle-walkers), there is a ridge of bone which projects off of the dorsal end and acts to stabilize the hand.  Some early humans share this morphology, and since we can be fairly certain that they weren’t walking on their digits, we might use that as evidence of knucle-walking.

The paper is far from a final word on the topic, but it is nonetheless a very creative and interesting way to test our assumptions about adaptation, function, and morphology.

Orr, C. (2005). Knuckle-walking anteater: A convergence test of adaptation for purported knuckle-walking features of african Hominidae American Journal of Physical Anthropology, 128 (3), 639-658 DOI: 10.1002/ajpa.20192

Miocene Ape: Anoiapithecus brevirostris

Salvador Moyà-Solà and his colleagues describe a new Miocene Hominoid in this week’s PNAS.  They’ve dubbed it Anoiapithecus brevirostris, and it hails from what is now Spain.  Like many other Miocene apes, it’s a mix of the primitive and the derived, the unique and the shared.  It’s commonly said that apes were as diverse in the Miocene as monkeys are today, so picking out which one of these groups of apes was ancestral to the surviving apes is quite a task.  So is deciding whether the African apes (gorillas, chimps, and humans) evolved from a different group than the Pongids (Orangutans).  The authors suggest that this fossil helps to settle the score on some of the unanswered questions of ape evolution.

Anoiapithecus brevirostris, From Moya-Sola et al.

First, let’s start with a little morphological description.  The most remarkable feature of A. brevirostris is an incredibly flat, or orthognathic, face.  The jaws don’t prodrude very much, and the angle formed by the face is very high.  This face is the flattest face we get until we get to fossil humans!  Not only is it flat, but it’s moved forward.  If you draw a vertical line down from the eyes through the jaw, the eyes line up with the premolars.  In most apes, this line falls further back, with the molars.  There are two muscle attachment sites on each side of the head (the parietal bone) called the temporal lines, because they are where the Temporalis muscle  and the associated fascia attach.  In A. brevirostris , these lines lie on top of each other, suggesting that the fascia and the muscle originated at the same line, and that he had a sagittal crest.  The nose was apparently relatively wide compared with other apes.  The enamel on the teeth of A. brevirostris was thick and the cusps low.  The canine teeth were relatively small compared to other apes.  The lower jaw, or mandible, was V-shaped rather than U-shaped, which means that the two sides of the jaw diverged from each other rather than running parallel. A. brevirostris also had a “simian shelf,” which is sort of like a palatine bone for the mandible.  It serves to reinforce the jaw from bending moments, and is common in apes.

The authors use a phylogeny in which the Miocene apes are divided into a few different Families: The Proconsulids, the Afropithecids, and the Hominids are the big ones.  The Afropithecids are divided into the Afropithecines and the Kenyapithecines, and our new fossil A. brevirostris fits in here.  Humans, gorillas, and chimps, as well as many other extinct apes, are in the Hominid clade (I may be using the improper anglicization of the Latin Hominidae here.  Someone correct me if I am!).  Miocene ape systematics is a pretty hotly contested subject, and theirs is one of many possible arrangements.  Systematics isn’t my forte, so I’ll leave any critique here to someone else!

So, with that flat face, people might jump at the chance to say, “Hey, it’s a human ancestor!”  But other features of the teeth and the frontal sinus also ally it with primitive apes.  None of those features were used for phylogenetic analysis because either everybody had them or came from someone who had them, or nobody else had them, came from someone who had them, or looked like they were evolving them.  Yes, humans have flat faces, but we have many more recent ancestors who had more prognathic faces than A. brevirostris.

We can’t use the flatness of the face in any useful way, but what about the position of it?  It was shifted forward and shortened, remember?  And remember that simian shelf in the mandible?  Those two features ally it with the Kenyapithecines.  Kenyapithecines had short, anteriorly-positioned faces, and A. brevirostris just took those features to the extreme.  With the hominds, A. brevirostris shares a high face, a nose that is widest at its base, and a deep palate.  The authors suggest that all of this means that the African hominids evolved from the Eurasian kenyapithecines, and the two are sister clades.  So, the kenyapithecines evolved in Africa (Kenya), then some of them went to Europe and/or Asia, then some of them evolved into hominids, which then went either back to Africa (chimps, gorillas, us) or over to Asia (orangs).

Alternatively, we could be seeing a whole lot of homoplasy and convergence.  It’s not a parsimonious explanation, but it might be the correct one.  As apes became less diverse ecologically and converged on similar niches, their morphology may have converged as well.

This is where paleontologists usually say, “More fossil discoveries will help us decipher the phylogeny more accurately,” but with Miocene apes, it’s still pretty much a crapshoot.

Moya-Sola, S., Alba, D., Almecija, S., Casanovas-Vilar, I., Kohler, M., De Esteban-Trivigno, S., Robles, J., Galindo, J., & Fortuny, J. (2009). A unique Middle Miocene European hominoid and the origins of the great ape and human clade Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0811730106

Is a new adapid a “Missing Link”?

I know I said I’d be on a brief hiatus, but a friend sent me this article from the Daily Mail.  Apropos of my previous post/exam question, I thought I’d clear up a few of the misconceptions hidden (or not so hidden) in the article, which just so happens to feature a species of adapid that was recently discovered in the Messel Shales of Frankfurt, Germany.

A good starting place is the term “missing link.”  The term dates back to the medieval concept of the Great Chain of Being, or the Scala Natura.  The Great Chain represents a hierarchy, with each “link” in the chain being higher than the one that preceeded it.  Rocks are down at the bottom, and humans are at the top.  Angels and God are even further up.  “Missing links” are the links in the chain that have gone extinct.  So why don’t modern evolutionary biologists like to talk about “mssing links”?  Apart from the idea of the Great Chain being atiquated, it implies that we are linking some known, lowly form with a known, higher form.  But that isn’t always the case when we find fossils.  We don’t always know the animal that preceeded our new find, and we don’t know which animal succeeded it.  We can know that an animal represents a transitional phase between two different, general kinds of animals, though, so we usually use the term “transitional form” nowadays.  To illustrate my point a little more clearly, I’ll use the famous whale example.  We know that the ancestors of whales were once terrestrial animals, and that they evolved into the aquatic animals that we all know and love.  We know that Ambulocetus is a transitional form between the animals which were fully terrestrial and those which are fully aquatic.  What we don’t know is which particular terrestrial animal evolved into Ambulocetus, and whether or not Ambulocetus eventually evolved into, say, an Orca.

Okay, on to something a bit less general.  As I said before, the article is about a species of adapid which was recently discovered in Germany.   The generally accepted theory is that the adapids are the precursors to modern-day strepsirrhines (lemurs, bushbabies, lorises) because of certain shared anatomical traits.  They have long, projecting snouts like lemurs, smallish eyes like lemurs, and shared a number of features of the wrist and ankle with lemurs and their close cousins.  Happlorrhiness such as ourselves, the other apes, monkeys and tarsiers evolved from a different primate that was around at the same time.  This primate was probably an omomyid, and we can say that because omomyids share a certain number of features with modern happlorrhines: they have short, broad faces, huge eyes, and some even have a fused tibia and fibula like modern tarsiers.

The actual journal article hasn’t been published yet (or, I can’t find it if it has been!), but it seems that they are suggesting that their new fossil, dubbed Darwinius masillae, may be a stem Happlorrhine, even though it’s an adapid!  They say this because the fossil lacks a tooth comb, which is a highly specialized set of lower incisors used for grooming, and a also lacks a toilet claw, which is a retained claw on the first digit that is also used for grooming.

Hmm.  Absence of Strepsirrhine traits doesn’t a Happlorrhine make.  I will have to wait for the journal article before I can say anything more on that subject.

It sounds like they’ve got an exciting fossil, but not quite for the reasons stated in this Daily Mail article.  It’s not that this fossil is possibly a human ancestor- it’s much, much broader than that.  This fossil might be the common ancestor to monkeys, apes, humans, and tarsiers- or it might not be.  A rather annoying graphic shows our new little fossil evolving directly into an ape, skipping all the really interesting and diverse animals in between.  Animals like Aegyptopithecus, Eosimias, Proconsul, and Oreopithecus.

Annoying graphic from the Daily Mail

I understand the emphasis on the human connection, but in the effort to make sure that that angle of the story was represented, the rest of the story became convoluted and confusing, and in many places inaccurate (Humans did not evolve from tarsiidae!  We just have a more recent common ancestor with tarsiers than we do with lemurs.).  I have no doubt that David Attenborough will present the story much more elegantly and accurately.  And I REALLY want to read the article by Phillip Gingerich and Jorn Hurum!