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Curved spines and pregnant primates

Researchers highlight one more example of design, but give evolution the credit, as always. So far, ho hum, same old story. But there’s another issue …

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Photo by Brent Allison, www.sxc.hu

A pregnant woman

A recent (December 07) Nature research paper1 has had the media buzzing with reports of how human females had allegedly evolved a clever way of dealing with the problem of carrying a heavy baby while walking upright.

Human females, unlike apes such as chimps, walk habitually upright. So this creates a unique load/stress on their spines when pregnant. The researchers found that human females have a different spinal structure from males that makes it possible for them to avoid damaging their spines while still keeping their balance and walking upright, even when heavily pregnant.

As the baby gets heavier and heavier, the centre of gravity shifts substantially forward, so it is no longer over the hips. Females compensate by leaning backwards more, increasing the curvature in their lower back to shift the centre of mass back towards the hip line again.

The differences between human males and females in the lower back are as follows:

  • The associated ‘facet’ joints in women are larger relative to the size of the vertebral bones, and angled slightly differently. These subtle but significant differences mean that they can bear more load than would otherwise be the case.
  • The vertebrae taper backwards into a wedge shape that enables a greater curvature, especially given that women have three such wedge-shaped lumbar vertebral bones (L3, 4 and 5), compared to two in men (L4 and 5). That makes it easier for them to form a natural curve that reduces the ‘shearing stress’ in pregnancy. So women can bend backwards to realign their centre of gravity much more readily than men could if they had the same increase in abdominal mass.

Belief in human evolution demands upright-walking ancestors in our lineage, but evidence of upright-walking does not demand that a fossil creature be a human ancestor.

At first, it seemed doubtful whether it was worth responding to the news reports—after all, any time clever design is discovered in biology, especially when it comes to the human body, the cause is automatically assumed to be the blind forces of evolution. The story is then usually picked up by a willing media. And with all such discoveries, the mere repetition of the evolutionary assumption, linked to the discovery, then tends to give the impression that the discovery itself somehow helps to support the very notion of evolution.

All that is happening, of course, is a psychological reinforcement phenomenon. Logically, it has always been at least as reasonable (in fact, more so) to ascribe apparent design to real design. Especially when one is aware of the huge problems of getting the right mutations (random genetic accidents) appearing in just the right sequence and when needed for selection, let alone the information issues, including considerations of error catastrophe, and so forth. (This site has quite a few articles on the topic; see our Mutations Q & A page, and the articles Genetic algorithms and robotic folly and Weasel, a flexible program for investigating deterministic computer demonstrations of evolution.) Natural selection is helpless unless it is provided with the right raw material—all it can do on its own is to cull or eliminate information from populations.

But there’s more …

But rebutting the general ‘evolution did it’ claim was not the reason for finally deciding to comment. Within the report was an even more interesting claim by these researchers, namely that certain fossil specimens of Australopithecus africanus they studied, from the South Africa’s Sterkfontein caves, allegedly showed that australopithecines had the same set of spinal design features. Reports based on the Nature article (which was, as we shall see, commendably less dogmatic than this) claimed that, as with humans, australopithecine females have these, but not the males. It appears that of the two africanus specimens studied, one had three wedge-shaped lower vertebrae, one only had two.

The implication is clear for evolutionary enthusiasts, and is spelled out in many of the reports anyway; our ‘hominin’ ancestors were already walking upright 2 million years ago, so they needed to have this feature to help them cope with pregnancy. It ‘evolved early’.

Photo by Iwan Beijes, www.sxc.hu

A pregnant gorilla

But here is where confusion sets in, and where one is tempted to question the actual findings, not just the evolutionary interpretation. Because what is being said about the australopithecines appears to conflict with the findings and pronouncements of other evolutionists themselves.

Of course, if some extinct non-human creatures did walk habitually upright, this is not of itself an indication that they were ancestors of humans. At best, it would be merely consistent with that belief. Belief in human evolution demands upright-walking ancestors in our lineage, but evidence of upright-walking does not demand that a fossil creature be a human ancestor.2

However, the reality is that on the basis of quite convincing fossil evidence, the australopithecines that have been touted as our ancestors3 by many evolutionists (not all4) almost certainly did not walk upright, at least not in a human-like manner. And this includes A. africanus, the species about which these latest claims have been made. This species was among the fossil types studied by Professor Fred Spoor, a distinguished evolutionary paleo-anthropologist who used CAT scans to look at the anatomy of the semicircular canals, the structures in the skull that house the organ of balance. This is quite different in creatures that walk habitually upright as opposed to those, like chimps and gorillas, that do not. Spoor found that the australopithecines did not group with humans in that regard at all, but were rather chimp-like in the mode of locomotion indicated by this evidence (the semicircular canals are often very well preserved even in otherwise fragmentary skulls).5

To their credit, the authors ... make the very conservative comment [that the conclusion that these are sex differences in the australopithecines is just] ‘one possible explanation’.

Other evidence consistent with this is the conclusion by a prominent evolutionary anatomist6 that the australopithecines had a unique, rolling mode of locomotion, not at all like human walking (or ape locomotion, for that matter).

They also had the long arms, and curved hands and feet, characteristic of tree-dwellers. And consistent with all of this is the discovery that even the famous australopithecine ‘Lucy’ had the wrist-locking mechanism characteristic of knuckle-walkers, like chimps or gorillas (see Did Lucy walk upright? and New evidence: Lucy was a knuckle-walker. If Lucy were already walking around like humans, what was natural selection doing not getting rid of such a useless structure?)

What is one to do, then, with these latest reports, that the australopithecines are supposed to show these same sorts of spinal design features in females (but not males)? There seem to be only a few options.

Either:

  • The previous conclusions by prominent evolutionists about australopithecines not walking upright are all just plain wrong. The copout by the majority of evolutionists, who desperately need these creatures to be ‘ancestors’, that they sometimes walked upright, mostly not, won’t really cut it in this case. Such a detailed feature as this spinal one would not be needed at all for pregnant australopithecines if they had upright walking as only an occasional option.

Or:

  • These latest reported conclusions about the africanus fossil bones are themselves in error. Despite these two being the most complete australopithecine spines ever found, this may be a live possibility. No detailed diagrams or actual photographs of the fossil vertebrae were given in the article. Note that the researchers’ reasoning did not go: ‘Based on other anatomical considerations, this one is a female, and that one a male, and here are the differences between the female and male lumbar vertebrae.’ Rather, they seem to have had to use the number of wedge-shaped vertebrae to deduce the sex of the creature—which for one thing implies that the rest of the skeletons were not exactly intact, to put it mildly.
Human female spine

The human spine

Possibly complicating matters still further, australopithecines appear to have had six lumbar vertebrae, not five, as is the case in humans. For one of these Sterkfontein specimens (Sts 14), the researchers provide in their graph the associated joint angles of all six lumbar vertebrae, but for the other (Stw 431) there are only three, L3, L5 and L6—indicating something about the degree of preservation of the ones not mentioned. The only photo of Stw 431’s skeletal remains that I could find on the web at the time of writing (low resolution, with no closeup of the vertebrae) suggests that for some of the lower vertebrae of this one at least, the results given in the Nature article may owe something to reconstruction and interpolation, rather than purely observation and measurement.

To their credit, the authors (unlike some of the news reports) are appropriately cautious. Concerning the differences they report between the lumbar vertebrae of Sts 14 and Stw 431, they make the very conservative comment that the notion that these differences are there because one specimen is of a female and the other a male is ‘one possible explanation’.

When one is dealing with such a small sample size, pathology is a possibility, too. For instance, there is in humans a condition called Sheuermann’s disease that affects the development of the thoracic vertebrae in particular, resulting in a wedge shape—only in the opposite direction. Similar issues may be impossible to exclude, particularly if some of the vertebrae are poorly preserved.

Of course, supporting the authors’ comment that this is in any case ‘intriguing’ is their report that this parallel to the case of the human female is not just to do with the vertebral wedging, but also the stronger angling of the associated small joints. If these same features, associated with the same sexual dimorphism,7 were consistently able to be documented in a representative sample of future specimens, it would be a powerful indicator that the apelike Australopithecus africanus did walk upright—at least, while carrying its babies. But this would have to be weighed against equally powerful evidence that it was not a habitual upright walker. While such a confusing outcome is not impossible, it seems somewhat unlikely, and my hunch is that future well-preserved australopithecine spines, when found, will disappoint in this regard.

Perhaps, once the hype settles down, some evolutionists will themselves comment on this apparent inconsistency. Further developments are awaited with interest.

Recommended Resources



References

  1. Whitcombe, Shapiro and Lieberman, Fetal load and the evolution of lumbar lordosis in bipedal hominins, Nature 450:1075–1078, 13 December 2007. Return to Text.
  2. In fact, some creationist students of the fossil bones believe that e.g. Lucy’s pelvis was consistent with bipedalism. See These apes were made for walking. Return to Text.
  3. Chiefly A. afarensis (of which ‘Lucy’ is the best-known) and the later (by evolutionary ‘dating’) A. africanus, of which Raymond Dart’s Taung child was the earliest discovery. Return to Text.
  4. The ones who deny that the australopithecines were in the human line include Charles Oxnard, who was Professor of Human Anatomy at both the University of Western Australia and the University of California at Santa Barbara. Oxnard performed detailed multivariate computer analyses of the fossil anatomy of the creatures, comparing them to humans and living apes, to try to remove the ‘subjective’ element from such studies. Return to Text.
  5. F. Spoor, B. Wood and F. Zonneveld, Implications of early hominid morphology for evolution of human bipedal locomotion, Nature 369(6482):645–648, 1994. Return to Text.
  6. Charles Oxnard, already mentioned in reference 4. Return to Text.
  7. A term used to describe consistent differences of form in males and females within a given species. Return to Text.
Published: 19 December 2007(GMT+10)

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