Skepticism is good, but…

Well-informed skepticism is the best!

Earlier this week, Eric Michael Johnson drew my attention to a post by psychologist Christopher Ryan at his blog Sex At Dawn.  Ryan attacks Lovejoy’s monogamous humans model by citing many different lines of evidence.

I became so distracted by the reported testes:body mass ratio of 1/160 in humans that I couldn’t stop until I had some answers.  I am a female human, but even I thought that 1 kg of testicles would be an awful lot to lug around.  So I got out my books and my calculator and did some math, wrote in, and it was fixed.  Peer review in action!

But then I clicked through to the actual post, and realized that it didn’t get much better from there.  Apart from characterizing Ardi as mere “bits of bone,” Ryan displays many instances of ignorance.

First, a few things that Ryan gets right:

1. Yes, reduced male-male competition can happen in promiscuous mating systems (but I do think that characterizing the Chimpanzee as having reduced competition is dubious).
2. Humans are probably able to produce more sperm per ejaculate than the number that Lovejoy cites.
3. Reproductive anatomy and physiology is particularly labile, so postulating about reproductive physiology 4.4 million years ago is a risky undertaking.

But then…

Lovejoy writes that “Humans have the least complex penis morphology of any primate.” Unfortunately, he never defines what he means by “complex;” nor does he discuss the fact that the human penis is, by most measures, the longest, thickest, most prominently displayed penis among primates. No mention of the unusual flared head or the external scrotum—both strong indications of sperm competition in our species.

Lovejoy clearly states that humans lack “keratinous penile surface mechanoreceptors” (known to you and me as penis spines and ridges) and an os baculum.  So there’s your complexity.

Lovejoy also states in the footnotes:

Flaccid human penis length (13 cm) is unusually great for a hominoid. Length is ~4 cm in Pongo and 3 cm in Gorilla. Its erect size is greater in the multimale Pan (8 cm), but this reflects specialized adaptation to penetrate seminal plugs. Short notes that “(e)ven the pubic hair in the male [human] seems designed to draw attention to the genitalia, rather than to conceal them as in the orangutan and gorilla.”

Lovejoy states these things in support of a rather weird argument against hand-to-hand combat in human ancestors, but why not in the discussion of sperm competition?  Because a huge penis does nothing if you don’t have the testes to back it up!  Clearly a large, pendulous, prominently-displayed penis is something special in humans, but it is not about sperm competition. The external scrotum- by itself- may be indicative of sperm competition, but combined with the fact that human testes are smaller and have fewer seminiferous tubules, that the human sperm midpiece volume is low, and that we have lost the physiology for creating copulatory plugs indicates that there may be something else at work here.

Perhaps the most glaring mistake lies in Ryan’s argument about canine teeth:

For example, much of his thesis hinges on the absence of pronounced canines teeth (fangs) in the fossils found. He writes that we can assume that both males and females lacked these canines (even if the teeth were from a female) because “The SCC [sectorial canine complex] is not male-limited; that is, it is always expressed in both sexes of all anthropoids….” But this is wrong. Male bonobos have long canines, while females don’t (2, 3). Lovejoy also claims an association between reduced canines and pair-bonding, but as this photo of the skull of a monogamous gibbon demonstrates, even this claim is suspect.

When anthropologists refer to the canine complex, they are not merely referring to canine length.  The Sectorial Canine Complex, or the CP3 complex, or the “honing” complex all refer to the way that the top canine tooth fits in with the lower canine and premolar.  It fits in this way in order to sharpen the canine tooth so that it acts as a blade.  In contrast to what Ryan states, both male and female anthropoids (including the bonobo) WILL have this trait if it is present at all.  Yes, the tooth will be longer in males (in many cases, much longer), but the sexual dimorphism here is in size, not in the actual way that these teeth occlude.

Teeth from more than 35 individuals have been found, and not one of them exhibits this Sectorial Canine Complex, so I think it’s safe to say that Ardi and her pals didn’t have it.

And then there’s that beautiful gibbon skull.  Unfortunately, the canine monomorphism in gibbons actually supports Lovejoy’s whole thesis:  In monogamous species, the canine teeth are no longer under strict sexual selection, and natural selection can take over and do its thing.  In gibbons, this has meant that not only do males have large canines, but females have them as well.  Both males and females defend their territory, and so both have enlarged canines.  In humans, the argument is that the energy that goes into maintaining large canine teeth and getting into the fights that result when you have them could be better spent by gathering food for your partner and offspring.  As a general rule for primates, the canines don’t have to be reduced to indicate pair-bonding, but if they are the same in both sexes, something is up.

Lovejoy’s hypothesis is controversial.  It’s a Theory of Everything, and that should raise flags in everyone’s head.  But at the end of the day, it’s really stinkin’ weird that human males abandoned their canine teeth, and it’s really stinkin’ weird that human females abandoned their ovulatory advertisement.  I think monogamous pair-bonding goes further than any other hypothesis in explaining those two weirdnesses of the human.  However, my little red flag pops up when we connect those things to bipedalism.

Zingeser, M. (1969). Cercopithecoid canine tooth honing mechanisms American Journal of Physical Anthropology, 31 (2), 205-213 DOI: 10.1002/ajpa.1330310210
Lovejoy, C. (2009). Reexamining Human Origins in Light of Ardipithecus ramidus Science, 326 (5949), 74-74 DOI: 10.1126/science.1175834
Frisch, J. (1963). Sex-differences in the canines of the gibbon (Hylobates lar) Primates, 4 (2), 1-10 DOI: 10.1007/BF01659148
Leutenegger, W., & Kelly, J. (1977). Relationship of sexual dimorphism in canine size and body size to social, behavioral, and ecological correlates in anthropoid primates Primates, 18 (1), 117-136 DOI: 10.1007/BF02382954

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

Paleoanthropology on The Daily Show

The Daily Show With Jon Stewart	Mon – Thurs 11p / 10c
Human’s Closest Relative
www.thedailyshow.com
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The Daily Show weighs in on a paleoanthropological pseudo-debate.

“Does ‘interesting’ mean something different in the scientific world than it does in real life?”

Bicycling and Bone Density in the NYTimes

The WellBlog in the NYTimes Health Section recently had an post about low bone density in competitive cyclists.  The author of the post points out that high speed, high impact accidents are obviously the source of some bone breaks that occur, but also that bone mineral density is actually lower in competitive male cyclists than is average for men in their age range.  In the study cited, the cyclists were leaner, consumed more calcium, and showed no difference in testosterone levels- all things that are supposed to be good for bone health- but were still more likely to have a lower Bone Minderal Density (BMD).

This is strange because endurance athletes usually have much better BMDs than the rest of the population.  As the famous tennis player examples illustrate, performing weight-bearing exercise protects your bones from atrophy- especially when you start young.  New bone will always deposit itself on the outer surface of a bone (the perichondrium) which is why, as we get older, our bones get wider and have a larger cross-section.  This is a phenomenon known as “cortical drift.”  When you exercise, your bones aren’t getting denser- you’re just adding more bone along the outside, like everyone does.  What is different with exercise is that you aren’t losing bone from the inside as would happen if you were a couch potato.  Bone is an expensive tissue, so if you don’t use it, your body is going to want to lose it.  It’ll send osteocytes out to eat the bone away from the inside out so it doesn’t have to worry about it anymore.  But, if your body can justify the cost of continued maintenance of that tissue, osteocytes, osteoclasts, and osteoblasts will happily work at detecting microfractures, cleaning the “wounds,” and repairing them.

So why isn’t this happening with competitive cyclists?  Nobody’s quite sure yet, but there are a few clues.  The lightest cyclists have the lowest BMD, and we know that cycling is a relatively low-impact activity.  It builds lots of muscle without the continuous, repetitive stress of slamming your feet against hard pavement that happens when you run.  This could result in a lot fewer of the little microfractures in bone that kick the bone remodeling sequence into gear.  Without microfractures, maybe the bone doesn’t know that it’s being stressed.

Competitive cyclists also have very little body fat.  Bone has recently been discovered to be an endocrine organ that communicates with adipose tissue via leptin.  The adipose tissue secretes leptin, which binds to receptors in bone cell precursors and upregulates their differentiation into osteoblasts (the cells that deposit new bone).  At the same time, it downregulates differentiation of osteoclasts (the cells that “eat” old bone).  Of all of the negative health risks associated with obesity, it is actually very good for your skeleton, whether the person is active or not.  So maybe this lack of body fat is negatively affecting competitive cyclists as well.

The article mentions something about calcium intake and sweat, but I’m not sure if that really has anything to do with it.  Runners sweat a lot, too, but they seem to be okay.  They don’t provide a link or any names with the blurb about calcium and it’s not research that I’m familiar with,  so maybe I’m missing an important detail, but this avenue doesn’t sound very promising to me.

At any rate, most of us don’t have to worry about biking away our bone.  The casual bicyclist, and even the average person who uses bicycling as exercise, is not in danger of losing BMD because they don’t spend 8 hours a day burning calories and avoiding microfractures.  And those of us who spend 8+ hours in front of a computer screen per day usually have a little bit of adipose tissue protecting us!

News from Zhoukoudian

Peking Man Exhibition Site, Ca. 1930

Zhoukoudian is famous for being the cave system near Beijing in which some of the very first human fossils were found.  These fossil are sometimes referred to as those belonging to “Peking Man,” but most of us now just call them Homo erectus. The original fossils found at this locality were lost at sea during World War II, but since then, many new fossils have been discovered here.  Previous estimates have dated the hominin-bearing layers of the Zhoukoudian cave sites to between 230,000-500,000 years old, but new research indicates that these sites may be as old as 770,000 years.

The new technique used here is called cosmogenic 26Al/10Be burial dating, and is an aluminum/beryllium radiometric technique.  Aluminum and Beryllium exist in quartz grains found in the sandy bottoms of the caves, and each decay at known rates from exposure to cosmic radiation (The half-life of 26-Al is 730,000 years, and that of 10Be is 1.51 million years).  Scientists can compare the ratio of these isotopes to find out when the quartz sediments were buried under other sediment- an event which stops the isotopes from decaying since it halts the exposure to cosmic radiation.  Previous estimates have been based on magnetostratigraphy, in which the known dates of reversals of the Earth’s magnetic field are used to date sedimentary rocks and the associated artifacts.

Fossil evidence indicates that the first Homo erectus lived in Africa around 2 million years ago.  They had made it to Dmanisi, Georgia by about 1.75 million years ago, and Java by 1.6 million years ago.  Homo erectus was certainly a species capable of surviving in many different environments as it made its way across the world.  At Zhoukoudian, other mammalian fossils indicate that while Homo erectus was living there, there were periods of warm weather alternating with periods of cold weather, with an overall trend toward cool, dry grasslands. These grasslands were very rich in mammal species which Homo erectus could hunt (or at least take advantage of after other species had “hunted” them), making Ice-Age China a very comfortable place.

Shen G, X Gao, B Gao, and D Granger.  Age of Zhoukoudian Homo erectus determined with ²⁶Al/¹⁰Be burial dating. Nature.  458: 198-201.

Image Credit:  Bernard Becker Medical Library, Washington University in St. Louis.