Friday, October 30, 2009
Halloween special: Heterotopy, pleiotropy, and the origins of vampires (free dissertation topic/movie plot-line)
At first I thought it had to do with Hollywood logistics--putting the fangs in the canine position would be too far over for them to be easily visible normally. Putting them in the lateral incisor position, the fangs are now easily visible at all times. But wait! Could there be another, more complex answer?
What if, now stay with me on this, what if it turns out that this isn't just Hollywood--what if this is an example of heterotopy (development of a feature in a novel location). That is, something about vampirism also causes the canine to develop in the the position of the lateral incisor. Could this be due to pleiotropy? That is, perhaps genes underlying vampirism are also involved in dental development, or vice versa. Could the pleiotropic effects of this heterotopic change in canine development result in vampiric symptoms like immortality, bloodlust, and garlic aversion? Or could selection for vampirism (bloodlust?) have the added (pleiotropic) effects of disdain for garlic and crucifixes, and a heterotopic shift in canine development?
So here's a free dissertation topic: untangling the genetic architecture of vampirism. I'd do it, but I have other, actual things to work on. Incidentally, if anyone wants to use this idea for a book or movie, please ask first. I know you're all beating down my door for this one.
Sunday, October 25, 2009
What they have of the foot is nothing too shabby: a talus, medial and intermediate cuneiforms, a cuboid, the first three metatarsals (MT1-3), and some phalanges.
I mentioned a few weeks ago that one of the most striking features of the foot is the abducted hallux--Ardi had a grasping big toe, like apes. They can tell this from the orientation of the medial cuneiform's articular facet for the MT1. In addition, the joint surfaces of the two bones show that the MT1 could rotate about the joint. In humans and other bipedal hominins, these surfaces are more or less flat. Ardi's foot is a big deal because what we have of hominin feet up to this point suggest a more-or-less human-like ability to transmit forces from bipedal walking, and MT1 plays a major role in this (that's why our big toes are so big and stupid looking).
But Ardi couldn't have walked bipedally this way. Rather, the authors posit that walking-forces were largely transmited through MT2-3. They point out some interesting features to support this: relative to their length, the bases of these bones are fairly tall; the sphericity of their heads (especially on the superior surface) hint that the toes were hyperextended. Additionally, notches on the dorsal aspects of both the MT2 base and the intermediate cuneiform (which articulates with the MT2) were probably caused by habitual pressure caused by the tarso-metatarsal joint capsule, possibly from "upright walking and running" (Lovejoy et al. 2009, p. 72e9). Well, I don't know about running. One thing I noticed about the MT3 head is that, while it is fairly "domed" as in humans, with a transverse depression just behind the dorsal surface as in humans, it also has a large tuberosity behind the depression, which looks fairly similar when viewed from the side (but not totally) to the transverse ridges on African ape metacarpals. In apes, these ridges prevent the fingers from hyperextending--this similar-looking feature in Ardi's foot could also have prevented hyperextension of the toe (?), not very biped-like. Of course, who says there's only one way to be bipedal?
The talus--the bone sitting the center of your ankle--is variously ape- or monkey-like in how the joint contacts the tibia, suggesting the tibia was fairly obliquely oriented on the joint (in us bipeds it's more or less perpendicular) (DeSilva 2009). Ardi's cuboid also gets a lot of air-time in the paper. The cuboid is a little wedged bone sitting on the side of your foot, and because of this lateral position, the tendon of the fibularis longus muscle crosses over the side and inserts into the base of the MT1 (and of the medial cuneiform in humans). In humans and Old World monkeys, this surface also bears an articular surface for the 'os peroneum,' a small bone sitting within the tendon. Apparently this articular surface is lacking in apes. For Ardi, the authors discover a fairly monkey-like morphology for this surface. Ardi's cuboid, then, indicates a monkey-like ability to adduct the big toe, but also support the structural integrity of the middle foot.
In sum, like in the pelvis paper, the authors posit more bipedal functionality than I'd be comfortable making. The foot of Ardipithecus ramidus was certainly something interesting, capable of ape-like grasping, and probably a non-trivial amount of dorsiflexion at the ankle, as well (read: 'capable climber'). At the same time, it is not clear to what extent it was used for ape-like climbing and/or monkey-like quadrupedalism. And if it was walking bipedally, it was doing it quite differently from any human or other fossil hominin. Hey, maybe this is the foot you need to be bipedal in the trees?
Just one more thing: I'm not terribly familiar with what feet are available from Miocene apes--a decent outgroup for comparing the functional/phylogenetic morphology of fossil feet. But, again like in the pelvis paper, this paper doesn't mention Oreopithecus, which may be a good comparison to make. Oreopithecus was an Italian insular ape some 7 or so million years ago. It has also been argued, fairly recently, to be bipedal based on a lordotic lumbar spine, presence on the innominate of an anterior inferior iliac spine, and the architecture of the internal trabecular bone of the ilium (Rook et al. 1999; Kohler and Moya-Sola 1997). For the longest time, the enigmatic morphological convergences between Oreopithecus and hominins have been attributed to the former's unique insular habitat: absence of predators in this unique enviroment allowed for upright posture, and possibly even bipedalism, to evolve in this now-extinct ape. But seeing what all this shares with Ardipithecus--hominin or not--it may be that such upright, "arboreal bipedal" positional behavior is the ancestral hominoid condition.
DeSilva J. 2009. Functional morphology of the ankle and the likelihood of climbing in early hominins. Proceedings of the National Academy of Sciences 106: 6567-6572.
Kohler M and Moya-Sola S. 1997. Ape-like or hominid-like? The positional behavior of Oreopithecus bambolii reconsidered. Proceedings of the National Academy of Sciences 94: 11747-11751.
Lovejoy CO, Latimer B, Suwa g, Asfaw B, and White TD. 2009. Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus. Science 326: 72e1-72e8.
Rook L, Bondioli L, Kohler M, Moya-Sola S, and Macchiarelli R. 1999. Oreopithecus was a bipedal ape after all: evidence from the iliac cancellous architecture. Proceedings of the National Academy of Sciences 96: 8795-8799.
Thursday, October 15, 2009
Now, on to some more discussion of what the pelvis does (and does not) tell us:
Four questions that occurred to me while I read this article (in addition to the lordosis problems presented already by Zach):
1. Was Ardi bipedal based on the pelvis?
2. Wait, wasn't Ardi a female? What about her birthing process?!
3. Was platypelloid the ancestral condition for hominid pelves?
4. How come there's only four pages about the pelvis if it's so darn complete? Have you seen how many pages were written on KNM-WT 15000? That pelvis was way less complete than this one!
Here's my take:
The first question is by far the hardest (for me) to answer. Maybe? Sort of sometimes? I don't even want to get into terrestrial vs arboreal bipedalism. I'm satisfied with saying she was capable of being bipedal but also of climbing (what type of climbing, I don't dare try and suggest, I'll just embarrass myself). Problems with saying Ardi is bipedal: the lower pelvis is totally chimp-like! How much could this affect locomotion? I don't know. Most of the bipedal/not bipedal muscles would be attaching to the iliac blades, which are certainly more australopithecine-like than the lower pelvis. But that crazy curved medial portion of the ischio-pubic ramus has got to serve some functional purpose (unfortunately, for me to hazard a guess at this purpose, I would have to go study an anatomy text - something I'm just not going to do right now, so you'll have to keep wondering). So maybe I can't answer this question satisfactorily at this point (sadly the femur wasn't a great help, and I don't believe in the mythical lordosis).
I was told Ardi was a female (I believe this is mainly based on the skull, which I'll admit I haven't read about yet). HOW can you write a paper about a female primate's pelvis and NOT discuss how she was having babies??? The only mention of this is at the very end of the pelvis section, where it's pointed out that the differences we see between Ardi and Homo are probably due to "optimization of birth-canal geometry." That's lovely, but I want to know how Ardi gave birth - was it like a chimp or an australopithecine? How did that work with a mosaic pelvis? The birth canal seems awfully circular compared to Lucy, despite the flaring iliac blades that make it platypelloid. What does that mean? The lateral image of Lucy and Ardi shows that Ardi's outlet was either gigantic (doubtful) or, because her sacrum would be positioned differently than Lucy's, positioned antero-inferiorly compared to the australopithecine. Not to mention that the ischio-pubic region on Ardi would make for a much longer birth canal than what Lucy's progeny had to deal with.
The platypelloid pelvis: Ardi is said to have one based on how the iliac blades were straightened out (something that could have been done differently, I'm sure). It was based on Lucy and Busidima (a Homo erectus), both of which were female hominids with platypelloid pelves. This is a great set up for this being the ancestral condition, and makes sense when we consider that a wider pelvis will biomechanically make bipedalism easier (human females have narrower pelves only because of obstetric constraints). So this is a great story. A word of caution though: we can't assume Ardi's pelvis is wide because she's bipedal, AND that she's bipedal because her pelvis is wide - that's called circular logic! Assuming that her pelvis should be morphologically similar to other bipedal hominids in ways that make them bipedal, and then saying "ta da, Ardi is also bipedal!" doesn't work.
This should have been a much longer paper. It's ridiculous that the paper overall was so short, and that the pelvis had to share space with the (albeit almost non-existent) vertebrae and the femur. Get your own paper! Write more about the pelvis!! Address topics like obstetrics!!
This post refers to the same paper cited by Zach in the previous post - Lovejoy et al., 2009
Monday, October 12, 2009
Listing individual features is all well and good if you're into cladistics, but more interesting is the functional interpretation of the fossil and reconstruction. Here's what the authors have to say about the ilium:
[The] exceptionally derived ilium is striking. It implies an early adaptation to habitual terrestrial bepedality before any increase in the lumbar entrapment seen in the African apes. (Lovejoy et al. 2009, p. 71e3).Hold the phone! Why are the lateral flare and low height of the ilium necessarily adaptations to terrestrial bipedalism? One key word that follows from their reconstructions, but is not actually manifest in any of the preserved fossils, is lordosis. Lumbar lordosis refers to the frontward concavity of the lower spine, seen only in humans, fossil hominins, and bipedally-trained macaques (it's also why we're prone to lower back inuries).
Lordosis in Ardipithecus is entirely inferred. The (reconstructed, probably realistically) broad sacrum and shortened superior iliac blades suggest that Ardipithecus was capable of lumbar lordosis, because the lower lumbar vertebrae were not closely flanked by the adjacent ilia, as in apes. But to the best of my knowledge, they don't mean there was lordosis. Nevertheless, Ardi's lordosis is constantly referred to in the paper. The final word on the pelvis is that the ancestral condition of hominins (like the skull and other features, claimed to be manifest in the Ardipithecus ramidus fossils) "involved situationally dependent lordosis (during terrestrial upright walking)" (Lovejoy et al. 2009, p. 71e3). I'm not sure what exactly they mean by this, because animals can't just adjust the relative front and back heights of their vertebrae willy-nilly. If that were the case I'd have a straight spinal column when I sleep, and overly-lordotic when I'm pregnant (which is often; Whitcome et al. 2008), and random on Halloween.
It is an interesting pelvis, though I wouldn't be as cavalier about asserting that it belonged to an adroit terrestrial biped. Unfortunately, the partial proximal femur described with the pelvis mostly lacks any diagnostic morphology. It would be great to see what the thickness of the cortical bone in the femoral neck was like in Ardi, or the extent of articular surface on the femoral head, because these have been shown to have characteristic forms in bipeds. Hopefully future fossil discoveries will shed light on these in Ardipithecus, as well as pelvic morphology in the earliest Australopithecus.
Until then, I'm content to conclude that Ardi had a unique form of locomotion (arboreal bipedalism?), but I'm hesitant to call it a terrestrial biped.
Lovejoy CO, Suwa G, Spurlock L, Asfaw B, and White TD. 2009. The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking. Science 326: 71e1-71e6.
Whitcome KK, Shapiro L, and Lieberman DE. 2007. Fetal load and the evolution of lumbar lordosis in bipedal hominins. Nature 450: 1075-1080.
Wednesday, October 7, 2009
Last Friday, human paleontologists working in Ethiopia unveiled a partial skeleton and additional elements of Ardipithecus ramidus. Most of the material dates to around 4.4 million years ago. The discovery of the skeleton was announced in 1994, and for the past few years I've been pretty irked that it's taken so long to be published. But given the state of preservation of the fossils and the fact that the technology to carry out the studies' analyses just wasn't available until recently, I suppose the long prep time is alright.
I've only had a chance so far to read the papers on the skull, dentition (and peruse the monstrous supporting online material), and wrist. Let’s start with the skull. If I could summarize the paper with a question, this would be it: If Ardipithecus ramidus so typifies an ancestral condition (primitive compared to later australopithecines), and Pan species are variously derived relative to this condition, what’s keeping Ardi from being a Pan-Homo common ancestor instead of a hominin?
The skull was reconstructed using CT-scanned images of the fossils, much as was done for Sahelanthropus a few years ago. One cool thing they did was make a composite cranium from the ARA-VP-6/500 face and vault and VP-1/500 temporal-occipital fragment. I don't see any reasons to distrust the reconstruction. What does it look like? To me, the first fossil that came to mind was the AL-333 composite cranium (Australopithecus afarensis from Hadar, Ethiopia ~3 million years old). However, the lower face of Ardi is surprisingly short compared to what we have for later hominins, or really anything else I've seen for that matter. Also, the orbits are surprisingly large. Honestly I do not really see a strong similarity to the Sahelanthropus TM 266-1 cranium, even though the authors go to pains to point out similarities between the two (mostly it’s in the basicranium). One thing Ardi certainly lacks is Sahelanthropus’s massive supraorbital torus—Ardi’s appear more similar to Australopithecus afarensis frontal bones.
From the reconstruction, the brain was probably around 300 cubic centimeters (cc), with an estimated range of from 280-350 cc. This is about the size of a small African ape. We've known for a while now that increased brain size was not a hallmark of human origins. But what does seem different is that the cranial base is fairly flexed (the bottom of the brain was somewhat 'tucked under'); the authors argue that some kind of neural reorganization, different from other African apes, must have occurred early in hominin evolution. Sahelanthropus apparently shares with Ardi a relatively short basicranium, though I’m not sure about the flexion. While the authors argue this confirms Sahelanthropus’s hominid status, there’s no major reason why this can’t be an ancestral condition from which later apes are derived; I’ve never been convinced that Sahelanthropus is not just an ape.
While the canine teeth are not as projecting as they are in African apes, they project further above the other teeth than in Australopithecus. However, they lack the C/P3 honing complex that is expressed in apes and most monkeys. This arguably links Ardi with Sahelanthropus, although it was never clear to me that Sahelanthropus’s lacked some sort of a honing complex. Also like Sahelanthropus, the teeth and skull of Ardi do not display the heavy-chewing adaptations of the later australopithecines.
The authors tend to reach two conclusions about Ardi, which are not unequivocal. First, a common conclusion the authors reach based on comparative anatomy is that for most features, the probable morphology of the chimp-human common ancestor is represented in Ardipithecus and Sahelanthropus, among others. As a result, the common chimpanzee appears to be quite derived, both in terms of its large canine dimorphism and lower-facial prognathism. In fact, the authors attribute the latter trait to the former; Pan troglodytes is argued to be morphologically derived because of its high levels of male aggression. The problem that arises with this is that if so much of Ardi’s morphology represents the ancestral condition, these traits are symplesiomorphic, and not necessarily informative about its relationship to later hominins. That is to say, if Ardi so typifies the ancestral condition, there’s not a lot making it, say, a chimp-human common ancestor rather than a hominin.
A second common conclusion is that Ardipithecus was probably not very sexually dimorphic in terms of canine or body size. Recall from above that the authors posit that the chimpanzee is actually unique/derived relative to the chimp-human common ancestor, and this may be due to canine size, which is related to male aggression. That Ardi lacks such canine honing and dimorphism argues for low levels of male aggression. Then there’s this quote:
"...our scaling analysis shows that postcranially dimorphic species tend to exhibit a large cranial size relative to that of the endocranium, as well as a large degree of cranial size dimorphism. In this context, it is instructive that Ar. ramidus shares its relatively small cranial size with taxa that are weakly dimorphic both cranially and postcranially" (Lovejoy et al. 2009: 68e6).
I don’t know if this is what their scaling analysis shows. They regress log-transformed cranial length on log-transformed cranial capacity for several catarrhine taxa. There is a clear separation between great apes, on the one hand, and other anthropoids and Hylobates (the gibbon, the smallest living ape) on the other. This difference is due to great apes’ relatively larger brains, which in turn is probably due to their relatively larger body size. Ardi does fall below both male and female regression lines, indicating a relatively short (but not necessarily small) cranium compared to its cranial capacity. But then, so do two “African Apes” on the plot—this could be the highly sexually dimorphic gorilla or the less dimorphic chimpanzee. And I believe that both these apes display fairly high levels of male-male aggression. Furthermore, if separate regressions were made for the small-bodied anthropoids on the one hand, and large-bodied hominoids on the other, it looks like Ardi may actually fall above the regression line, indicating a fairly long cranium.
The point is that there are persistent assertions of low male aggression in Ardi. Some may recall Lovejoy’s 1981 paper in which he argues that low sexual dimorphism and a more monogamous reproductive behavior and male provisioning of female and offspring were responsible for hominin origins and bipedalism. While the Ardi material makes it unlikely that this reproductive behavior an unlikely cause of terrestrial bipedalism, it is interesting that this theme of reduced male aggression/sexual dimorphism and hominin origins emerges once again. Not that it’s incorrect or silly, just interesting. Of course, if this is the case, one should note that later hominins appear very sexually dimorphic.
Lovejoy CO. 1981. The Origin of Man. Science 211: 341-350
Suwa G, Asfaw B, Kono RT, Kubo D, Lovejoy CO, and White TD. 2009. The Ardipithecus ramidus Skull and Its Iimplications for Hominid Origins. Science 326: 68.
Monday, October 5, 2009
The major thing I've taken from what I've been able to read about Ardi is that it has an abducted big toe--unlike any other fossil hominin foot we've seen so far. More on this, other Ardi anatomy, and the implications of the fossils to come...