Why Do We Have Bad Backs?

MSNBC has an article up discussing that issue. The story concerns a new article published in Neurosurgical Focus by Aaron Filler – an M.D. and Ph. D. in anthropology.

Writing in the journal Neurosurgical Focus on Sunday, Filler said one main clue was a bone feature called the transverse process, which sticks out from the side of the hollow, round vertebrae, Filler said in a telephone interview. This is where muscles attach to the spine.
“The vertebra is transformed in a way that literally reverses the mechanics of the spine,” Filler said. “The bone lever of the vertebrae gets switched from bending the spine forward to bending the spine back.”
Most vertebrates are oriented forward, to walk on all fours. The transverse process is at the front of each vertebra, facing the animal’s belly. This is true of monkeys, too.
But in humans and in the 21 million-year-old fossil of a creature called Morotopithecus bishopi, a tree-dwelling, ape-like creature that lived in what is now Uganda, the transverse process has moved backward, behind the opening for the spinal cord.
Great apes, such as chimpanzees, share this feature.

I’ve tracked down the article and am in the process of reading it. But I am a bit skeptical. Below is a picture of a human lumbar vertebra.

vertebrae

At issue is the position of the transverse process. In humans the transverse process attaches to the dorsal part of the pedicle. This is similar to what is seen in apes and Morotopithecus bishopi. In macaques the transverse process arises from the vertebral body. Macaques also have a styloid process, unlike apes and humans. This leads Filler to suggest:

In his study and in a book published last week called “The Upright Ape — a new origin of the Species,” Filler argues that this common ancestor, and ancestors going back many millions of years before, walked upright. Homo sapiens, the human species, continued upright, while apes evolved back toward all fours, he argues.

One major problem, for me, is the picture below:

megaladap-post02

This is the lumbar vertebra of Megaladapis – a giant subfossil lemur from Madagascar. Just like in humans, the transverse process arises from the dorsal part of the pedicle. It goes without saying that no one has ever claimed that Megaladapis is bipedal…
Update: Karl wants to know what Filler’s explanation for bad backs is. Filler argues, correctly I think, that in humans the posterior superior iliac spine (pointed to by the arrow):

PSIS

is positioned more dorsally to the longissimus insertion in humans. This, combined with the higher placement of the lumbar transverse process, makes the longissimus a powerful extensor of the lumbar spine adding a lot more stress to it and making the muscle more susceptible to strain. This is one cause for bad backs, it does not support Filler’s contention that:

Morotopithecus bishopi therefore emerges from among the various proconsulid hominoids as a representative of a
new subgroup that can be called the “hominiforms.” The key difference between proconsulid and hominiform apes
of the East African Early Miocene is that the hominiforms have the same basic lumbar spine anatomy as that in modern humans (Fig. 1).
This modified vertebral pattern is quite unusual for a mammal and appears to have emerged fairly abruptly in a
limited period of time. From that point of view, it is almost possible to view this modification as a birth defect, makquadrupedal locomotion. The lack of any intrinsic mechanism to resist extension under the force of gravity in a horizontal pronograde position allowed the lumbar spines of these apes to function well only in upright or orthograde postures. The large modern hominiform apes such as gorillas and orangutans have independently evolved new types of osseous ridges that lock the lumbar facets against extension. 2 This finding suggests a continuous series of upright ancestral apes–with body plans more or less like those in
a modern human–from which other apes have descended to establish diagonograde (between upright and horizontal) postures during knuckle walking and other straightarm/ flexed-leg gaits.4

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12 Responses

  1. This is one of my pet peeves. Like childbirth pain and some other claims, where’s the evidence that bad backs are a specially human thing? And due to mechanics? How about due to the way we use our bodies, sitting for long hours then suddenly getting up and being very active, or alternatively doing hours of repetitive labor without rest? (And living longer, which is where we find many of our “uniquely human” ailments.)
    Frankly, I’d put a human up against most animals in a repetitive work contest — get a dog, say, to do something as “unnatural” as having a human dig a ditch for 6 hours. Which animal would come out the other side of that in better shape?

  2. To echo QrazyQat, anyone who thinks dogs don’t get bad backs should to talk to a vet some time. (And of course bad back are much more common in larger dogs … as in humans.)

  3. I’m confused. I thought that the answer to your question would be because the transverse process arises from the vertebral body and therefore our spines are not properly supported in the upright position. But, it seems to me, that Filler says, in humans “The bone lever of the vertebrae…bend(s) the spine…back.” Which sounds like an evolutionary adaptation to bipedal movement. And, therefore, we shouldn’t have bad backs.
    What am I misinterpreting?

  4. Is upright posture the same as being bipedal? for example lemurs show upright posture occasionally. But they are not bipedal. Even meerkats and praire dogs stand upright for long periods but are not bipedal. In the MSNBC press release Filler is quoted “That means that upright posture bipedalism goes back 20 million years, not just 5 or 6 million years,” I think his connection of upright posture and bipedalism is not very well supported.
    This would mean that reversion to diagonograde occurred independently in the ape lineages, not just in Orangs, Gorilla, and Chimps but also many other extinct branches of apes who show no evidence of being bipedal. If the ancestral condition was one of upright posture bipedalism as Filler puts it, a large number of speciation events lead to reversion to diagonograde. I would imagine most biologists will prefer the more parsimonious hypothesis that quadraped or diagonograde was the ancestral condition and bipedalism
    evolved more recently.

  5. Suvrat, it’s been explicitly mentioned by some at least (for instance, if I remember right, Alison Jolly did in a general book on primates) that primates are preadapted for bipedality because of their generally upright posture in sitting, which is what they do most of their waking time, often while grooming. I don’t think you’ll find many people suggesting that very early primates were bipedal, that is, the ancestors of monkeys or lemurs. But it’s possible that the ancestor of apes, or African ape (and hominids) was. It’s still one of those unknowns, but it does seem that as far as we get to go back in hominid fossils, we find pretty good bipedality when we find the bones that tell us their locomotion. So either we do have a common ancestor, probably just to African apes and hominids, which was bipedal, and from there probably just one, but possibly two, changes to knucklewalking, or bipedalism is extremely early in hominids, is actually the defining feature of our species, and therefore, we do have to “base Man’s dignity upon his great toe”.

  6. oops! Apologies for the previous multiple post. I thought my connection was on the blink and resubmitted.
    It is easier to accept that bidpedalism evolved in the common ancestor of the great apes and then one or two reversals to knucklewalking as QrazyQat posted, than what Filler is suggesting which is bipedality evolved around 20 my ago in the common ancestor of apes. I appreciate the argument of preadaptation for bipedality and the distinction between upright posture in sitting and bipedality but Filler does’nt seem to be making that distinction. His interpretation is that ancestral apes going back to 20 my ago walked upright, which I am finding hard to accept.

  7. suvrat – I agree with your interpretation. It sounds to me like Filler is saying that bipedality evolved 20 MYA.
    QrazyQat – I am inclined to be skeptical of the idea that the common ancestors of the African apes were bipedal. I tend to think that knuckle-walking and bipedality are derived traits relative to the the common ancestor of apes – but that is the topic of a future post.

  8. I’m also very skeptical that they were bipedal (or wouldn’t want to have to bet on them being knucklewalkers either). We do seem to find, though, that as far back as we find hominids, we find bipedal hominids. And I just always liked that “great toe” remark; it’s ironic that this is what it seems to be boiling down to in the question of what makes a hominid.

  9. Reading through the various responses I notice that all of those writing appear to be unaware that hylobatids are primarily upright bipeds. In fact in many substrates they are better at it than modern humans (see http://www.uprightape.net for images). Therefore the arguments that it might be OK to have bipedalism into the late or mid Miocence can accomodate hylobatid bipedalism, by pushing back to the early Miocene. Also, Morotopithecus is not alone. Oreopithecus bambolii from 15 million years ago appears to be an upright biped with a human-like spine. Also, Pieralopithecus catalaunicus appears to be an upright biped. Even Sahelanthropus – which appears one million years before the time of the genetic clock prediction for the chimp human split appears to many anthropologists to be an upright biped. Why is the possibility of upright bipedalism going back to 21.6 mya being ignored. Since all living apes – including hylobatids – have major components of upright bipedalism that far exceed what is seen in cercopithecoids (Old Wolrd Monkeys), since we share the anatomical changes (see “Axial Character Seriation in Mammals” – http://www.axialcharacter.com and “The Upright Ape: A New Origin of the Species”) and since a number of upright bipedal apes from across the Miocene keep popping up, it seems fair to consider the message the data is telling us. Yes, all this could have developed independently many times in parallel. Or it could reflect common descent with a hominiform group of hominoids having upright bipedality as ancestral from the early Miocene.
    As for the bad backs issue. My point is primarily that the sites of greatest modification are the sites of most common back pain pathology in humans. Adaptation or mutationist driven change need not achieve perfect results. Changes achieve new capabilities, but there may be compromises which become the focus of subsequent change. Since most severe back pain impacts in the late to post-reproductive period of our lives, it has a low profile from the point of view of natural selection.

  10. Dr. Filler, thank you for your comments. Before going further I should mention that review copies of books are always welcome. With that shameless plug out of the way, as I pointed out above and in a later post platyrrhines show both lumbar morphologies. In large bodied Cebidae such as Alouatta, Lagothrix, Ateles, Brachyteles and at least one species of the Lepilemuridae (Megaladapis). The above South American species tend to group with the Hylobatidae in analysis of the postcrania – particularly in the shoulder and forelimb region. Other Platyrrhini show the Cercopithicine lumbar morphology. Megaladapis, on the other hand, is considered to be a vertical clinger in the trees and quadrupedal on the ground. This indicates to me that the lumbar morphology (as it related to the transverse process) seen in the Hominidae (and Morotopithecus) is the primitive condition. While the hylobatids may be adept bipeds in some substrates they are highly derived brachiators (which is why the spider monkeys, etc, mentioned above group with them), chimps and gorillas are derived knuckle-walkers, orangs are derived suspensory locomotors and hominins are derived bipeds. A further point, because we see the same morphology of the transverse process in such a diverse group (in terms of locomotor behavior) this particular morphology does not necessarily indicate bipedalism.
    Having said all that, I totally agree with the last paragraph of your comment…

  11. Lumbar transverse process (LTP) position on the pedicle/lamina occurs in many mammals. This is why I have approached this in terms of an assessment of LTP origins and disposition across the entire mammalian order (Axial Character Seriation in Mammals, and in a forthcoming paper). In artiodactyls, for instance, this is associated with cylindrical locking facet joints. There are some features of this in Megaladapis. However, it may be the significant amount of orthograde posture in vertical climbing or vertically disposed leaping that was associated with the situation in Megaladapis. In hominiforms, the presence of the orthograde style configuration, together with presence of extensive bipedalism throughout the group is a basis for linking the two characters.
    I have also argued that the square/triangular cross section of the LTP in humans and in Morotopithecus reflects the role of muscle leverage in orthograde posture with lower extremeity weight bearing as opposed to the flat LTP embedded in the intertransverse ligaments as in Megaladapis and many other mammals.
    Phenetic (numerical morphometric) analysis rather than character based or functionally based analysis is also an issue as it suggests that generalized similarity is as important as character based analysis of evolution. However, there definitely is convergence with the functional and anatomic situation in Ateles.
    None of these considerations affects the significance of the distinction between Morotopithecus – with its human-like lumbar vertebrae and various other hominoid orthograde adaptations in the rest of its post-crania – and the lumbar vertebrae of proconsulid hominoids that resemble those of a more generalized cercopithecoid quadruped. The high level of correlation between a variety of lumbar vertebral character states in Morotopithecus and in humans has not been adequately dealt with in the literature because it simply does not fit well with the widely accepted model of human bipedalism originatin six million years ago from a knuckle-walker.
    The Morotopithecus anatomy – and the similar situation in Oreopithecus – are the basis of what I call a new “Humanian” model of hominiform evolution in place of the Troglodytian model (bipedalism from a knuckle-walking ancestor) or the Hylobatian model (bipedalism from a brachiating ancestor). The Humanian model posits an upright bipedal ancestor for the hominiforms – occuring in a series of species (including Morotopithecus, Oreopithecus, Pieralopithecus and Sahelanthropus) before the chimp-human split.

  12. “Pieralopithecus catalaunicus appears to be an upright biped”
    Any reference of this? Thanks.

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